Chapter 5 The Working Cell.

Chapter 5 The Working Cell

Biology and Society: Harnessing Cellular Structures
Nanotechnology is the manipulation of materials at the molecular scale. Researchers often turn to living cells for inspiration. Let’s consider one example of cell-based nanotechnology and see how it relates to three of the main concerns for working cells: energy, enzymes and the plasma membrane. © 2013 Pearson Education, Inc. 2

Researchers at Cornell University are attempting to harvest the energy-producing capability of human sperm cells. Like other cells, a sperm cell generates energy by breaking down sugars and other molecules that pass through its plasma membrane. Enzymes in the cell carry out a process called glycolysis. During glycolysis, the energy released from the breakdown of glucose is used to produce molecules of a special energy molecule called ATP. © 2013 Pearson Education, Inc.

The ATP produced during glycolysis and other processes provides the energy that propels the sperm through the female reproductive tract. In order to harness this energy producing system, the researchers attached three glycolysis enzymes to a computer chip. The enzymes continued to function in this artificial system, producing energy from sugars. The hope is that in the future that a larger set of enzymes can eventually be used to power small robots. © 2013 Pearson Education, Inc.

Perhaps such nanorobots could use glucose from the bloodstream to power the delivery of drugs to body tissues. This example of cell-based technology highlights the three main topics of this chapter. We’ll explore how a cell uses energy, enzymes and the plasma membrane to carry out the work of controlling its internal chemical environment. © 2013 Pearson Education, Inc.

Figure 5.0 Figure 5.0 Cellular structures

SOME BASIC ENERGY CONCEPTS
Energy makes the world go around – both the larger world outside and the internal cellular world. But what is energy? Our first step in understanding the working cell is to learn a few basic concepts about energy. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students with limited exposure to physics may have never understood the concepts of “energy” and “the conservation of energy” or distinguished between potential and kinetic energy. Understanding such broad and new abstract concepts requires time and concrete examples. 2. Although typically familiar with the concept of dietary Calories, students often struggle to think of Calories as a source of potential energy. For many students, it is not immediately clear how energy is stored as Calories in food. Teaching Tips 1. In our daily lives, we rely upon many energy transformations. On our classroom walls, a clock converts electrical energy to mechanical energy to sweep the hands around the clock’s face (unless it is digital!). Our physical (mechanical) activities walking to and from the classroom rely upon the chemical energy from our diet. This chemical energy in our diet also helps us maintain a steady body temperature (heat). Consider challenging your students to come up with additional examples of such common energy conversions in their lives. 2. Some students can relate well to the concept of entropy as it relates to the room where they live. Despite cleaning up and organizing the room on a regular (or irregular) basis, the room increasingly becomes disorganized, a victim of entropy, until another energy input (or effort) is exerted to make the room more orderly again. Students might even get to know “entropy: as the “dorm room effect”. 3. All too often we hear or read that some thing or reaction creates energy. We might hear or read that a power plant “produces” energy or that mitochondria “make” energy. Even in our classroom conversations, we may occasionally slip into this error. When discussing the first law of thermodynamics, consider emphasizing the inaccuracy of such statements. 4. The heat produced by the engine of a car is typically used to heat the car during cold weather. But is this same heat available in warmer weather? Students are often unaware that their car “heater” works very well in the summer too. Just as exercise can warm us when it is cold, the same extra heat is released when we exercise in warm conditions. A car engine in the summer struggles to dissipate heat in the same way that a human struggles to cool off after exercising when weather is warm. 5. Here is a question that might make cellular respiration a little more meaningful to your students. Ask your students why they feel warm when it is 30°C (86°F) outside if their core body temperature is about 37°C (98.6°F)? Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that help release our extra heat generated in cellular respiration. 6. Here is a calculation that has impressed some students. Depending on the size and activity of a person, a human might burn 2,000 dietary Calories (kilocalories) a day. This is enough energy to raise the temperature of 20 L of liquid water from 0° to 100°C. This is something to think about the next time you heat water on the stove! If you can bring in ten 2-L bottles you can help students visualize how much liquid water can be raised from 0° to 100°C. (Note: 100 Calories raises about 1 L of water 100°C; but it takes much more energy to melt ice or to convert boiling water into steam.) 7. The same mass of fat (about 9 kcal per gram) stores nearly twice as many Calories as an equivalent mass of protein or carbohydrates (about 4.5–5 kcal per gram). Thus, when comparing equal masses of fat, protein, and lipid, the fat has nearly twice the potential energy. Fat is therefore an efficient way to store energy in animals and many plants. To store an equivalent amount of energy in the form of carbohydrates or proteins would require about twice the mass, adding a significant burden to the organism’s structure. (For example, if you were 20 pounds overweight, you would be nearly 40 pounds overweight if the same energy were stored as carbohydrates or proteins instead of fat.) 7

Conservation of Energy
Energy is defined as the capacity to cause change. Some forms of energy are used to perform work – such as moving an object against an opposing force (e.g. gravity) Energy is the ability to rearrange a collection of matter. Imagine a diver climbing to the top of a platform and diving off. To get to the top of the platform, the diver must perform work to overcome the opposing force of gravity. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students with limited exposure to physics may have never understood the concepts of “energy” and “the conservation of energy” or distinguished between potential and kinetic energy. Understanding such broad and new abstract concepts requires time and concrete examples. 2. Although typically familiar with the concept of dietary Calories, students often struggle to think of Calories as a source of potential energy. For many students, it is not immediately clear how energy is stored as Calories in food. Teaching Tips 1. In our daily lives, we rely upon many energy transformations. On our classroom walls, a clock converts electrical energy to mechanical energy to sweep the hands around the clock’s face (unless it is digital!). Our physical (mechanical) activities walking to and from the classroom rely upon the chemical energy from our diet. This chemical energy in our diet also helps us maintain a steady body temperature (heat). Consider challenging your students to come up with additional examples of such common energy conversions in their lives. 2. Some students can relate well to the concept of entropy as it relates to the room where they live. Despite cleaning up and organizing the room on a regular (or irregular) basis, the room increasingly becomes disorganized, a victim of entropy, until another energy input (or effort) is exerted to make the room more orderly again. Students might even get to know “entropy: as the “dorm room effect”. 3. All too often we hear or read that some thing or reaction creates energy. We might hear or read that a power plant “produces” energy or that mitochondria “make” energy. Even in our classroom conversations, we may occasionally slip into this error. When discussing the first law of thermodynamics, consider emphasizing the inaccuracy of such statements. 4. The heat produced by the engine of a car is typically used to heat the car during cold weather. But is this same heat available in warmer weather? Students are often unaware that their car “heater” works very well in the summer too. Just as exercise can warm us when it is cold, the same extra heat is released when we exercise in warm conditions. A car engine in the summer struggles to dissipate heat in the same way that a human struggles to cool off after exercising when weather is warm. 5. Here is a question that might make cellular respiration a little more meaningful to your students. Ask your students why they feel warm when it is 30°C (86°F) outside if their core body temperature is about 37°C (98.6°F)? Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that help release our extra heat generated in cellular respiration. 6. Here is a calculation that has impressed some students. Depending on the size and activity of a person, a human might burn 2,000 dietary Calories (kilocalories) a day. This is enough energy to raise the temperature of 20 L of liquid water from 0° to 100°C. This is something to think about the next time you heat water on the stove! If you can bring in ten 2-L bottles you can help students visualize how much liquid water can be raised from 0° to 100°C. (Note: 100 Calories raises about 1 L of water 100°C; but it takes much more energy to melt ice or to convert boiling water into steam.) 7. The same mass of fat (about 9 kcal per gram) stores nearly twice as many Calories as an equivalent mass of protein or carbohydrates (about 4.5–5 kcal per gram). Thus, when comparing equal masses of fat, protein, and lipid, the fat has nearly twice the potential energy. Fat is therefore an efficient way to store energy in animals and many plants. To store an equivalent amount of energy in the form of carbohydrates or proteins would require about twice the mass, adding a significant burden to the organism’s structure. (For example, if you were 20 pounds overweight, you would be nearly 40 pounds overweight if the same energy were stored as carbohydrates or proteins instead of fat.) 8

Specifically, chemical energy from food in the diver is converted to kinetic energy, the energy of motion. In this case, the kinetic energy takes the form of muscle movement. What happens to the kinetic energy when the diver reaches the top of the platform? Has it disappeared? The answer is NO. The principal known at the conservation of energy states that energy cannot be created out of nothing nor can it be destroyed. © 2013 Pearson Education, Inc.

Energy can only be converted from one form to another. A power plant, for example, does not make energy; it only converts it from one form (e.g. coal) to another form (e.g. electricity) That is what happens when a diver climbs the steps. The kinetic energy of muscle movement (climbing) is now stored energy. It is now called potential energy because of the location and/or structure of the object. This is like energy stored in water behind a dam or by a compresses spring. © 2013 Pearson Education, Inc.

In our example, the diver on top of the platform has potential energy because of this elevated location. The act of diving off the platform into the water converts potential energy back into kinetic energy. Life depends upon countless similar conversions of energy from one for into another. © 2013 Pearson Education, Inc.

Kinetic energy is the energy of motion. Potential energy is stored energy. It is energy that an object has because of its location or structure. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students with limited exposure to physics may have never understood the concepts of “energy” and “the conservation of energy” or distinguished between potential and kinetic energy. Understanding such broad and new abstract concepts requires time and concrete examples. 2. Although typically familiar with the concept of dietary Calories, students often struggle to think of Calories as a source of potential energy. For many students, it is not immediately clear how energy is stored as Calories in food. Teaching Tips 1. In our daily lives, we rely upon many energy transformations. On our classroom walls, a clock converts electrical energy to mechanical energy to sweep the hands around the clock’s face (unless it is digital!). Our physical (mechanical) activities walking to and from the classroom rely upon the chemical energy from our diet. This chemical energy in our diet also helps us maintain a steady body temperature (heat). Consider challenging your students to come up with additional examples of such common energy conversions in their lives. 2. Some students can relate well to the concept of entropy as it relates to the room where they live. Despite cleaning up and organizing the room on a regular (or irregular) basis, the room increasingly becomes disorganized, a victim of entropy, until another energy input (or effort) is exerted to make the room more orderly again. Students might even get to know “entropy: as the “dorm room effect”. 3. All too often we hear or read that some thing or reaction creates energy. We might hear or read that a power plant “produces” energy or that mitochondria “make” energy. Even in our classroom conversations, we may occasionally slip into this error. When discussing the first law of thermodynamics, consider emphasizing the inaccuracy of such statements. 4. The heat produced by the engine of a car is typically used to heat the car during cold weather. But is this same heat available in warmer weather? Students are often unaware that their car “heater” works very well in the summer too. Just as exercise can warm us when it is cold, the same extra heat is released when we exercise in warm conditions. A car engine in the summer struggles to dissipate heat in the same way that a human struggles to cool off after exercising when weather is warm. 5. Here is a question that might make cellular respiration a little more meaningful to your students. Ask your students why they feel warm when it is 30°C (86°F) outside if their core body temperature is about 37°C (98.6°F)? Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that help release our extra heat generated in cellular respiration. 6. Here is a calculation that has impressed some students. Depending on the size and activity of a person, a human might burn 2,000 dietary Calories (kilocalories) a day. This is enough energy to raise the temperature of 20 L of liquid water from 0° to 100°C. This is something to think about the next time you heat water on the stove! If you can bring in ten 2-L bottles you can help students visualize how much liquid water can be raised from 0° to 100°C. (Note: 100 Calories raises about 1 L of water 100°C; but it takes much more energy to melt ice or to convert boiling water into steam.) 7. The same mass of fat (about 9 kcal per gram) stores nearly twice as many Calories as an equivalent mass of protein or carbohydrates (about 4.5–5 kcal per gram). Thus, when comparing equal masses of fat, protein, and lipid, the fat has nearly twice the potential energy. Fat is therefore an efficient way to store energy in animals and many plants. To store an equivalent amount of energy in the form of carbohydrates or proteins would require about twice the mass, adding a significant burden to the organism’s structure. (For example, if you were 20 pounds overweight, you would be nearly 40 pounds overweight if the same energy were stored as carbohydrates or proteins instead of fat.) 12

Greatest potential energy Climbing converts kinetic energy to
Figure 5.1 Greatest potential energy Climbing converts kinetic energy to potential energy. Diving converts potential energy to kinetic energy. Least potential energy Figure 5.1 Energy conversions during a dive

Machines and organisms can transform kinetic energy to potential energy and vice versa. In all such energy transformations, total energy is conserved. Energy cannot be created or destroyed. Energy can be converted from one form to another. This is the principle of conservation of energy. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students with limited exposure to physics may have never understood the concepts of “energy” and “the conservation of energy” or distinguished between potential and kinetic energy. Understanding such broad and new abstract concepts requires time and concrete examples. 2. Although typically familiar with the concept of dietary Calories, students often struggle to think of Calories as a source of potential energy. For many students, it is not immediately clear how energy is stored as Calories in food. Teaching Tips 1. In our daily lives, we rely upon many energy transformations. On our classroom walls, a clock converts electrical energy to mechanical energy to sweep the hands around the clock’s face (unless it is digital!). Our physical (mechanical) activities walking to and from the classroom rely upon the chemical energy from our diet. This chemical energy in our diet also helps us maintain a steady body temperature (heat). Consider challenging your students to come up with additional examples of such common energy conversions in their lives. 2. Some students can relate well to the concept of entropy as it relates to the room where they live. Despite cleaning up and organizing the room on a regular (or irregular) basis, the room increasingly becomes disorganized, a victim of entropy, until another energy input (or effort) is exerted to make the room more orderly again. Students might even get to know “entropy: as the “dorm room effect”. 3. All too often we hear or read that some thing or reaction creates energy. We might hear or read that a power plant “produces” energy or that mitochondria “make” energy. Even in our classroom conversations, we may occasionally slip into this error. When discussing the first law of thermodynamics, consider emphasizing the inaccuracy of such statements. 4. The heat produced by the engine of a car is typically used to heat the car during cold weather. But is this same heat available in warmer weather? Students are often unaware that their car “heater” works very well in the summer too. Just as exercise can warm us when it is cold, the same extra heat is released when we exercise in warm conditions. A car engine in the summer struggles to dissipate heat in the same way that a human struggles to cool off after exercising when weather is warm. 5. Here is a question that might make cellular respiration a little more meaningful to your students. Ask your students why they feel warm when it is 30°C (86°F) outside if their core body temperature is about 37°C (98.6°F)? Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that help release our extra heat generated in cellular respiration. 6. Here is a calculation that has impressed some students. Depending on the size and activity of a person, a human might burn 2,000 dietary Calories (kilocalories) a day. This is enough energy to raise the temperature of 20 L of liquid water from 0° to 100°C. This is something to think about the next time you heat water on the stove! If you can bring in ten 2-L bottles you can help students visualize how much liquid water can be raised from 0° to 100°C. (Note: 100 Calories raises about 1 L of water 100°C; but it takes much more energy to melt ice or to convert boiling water into steam.) 7. The same mass of fat (about 9 kcal per gram) stores nearly twice as many Calories as an equivalent mass of protein or carbohydrates (about 4.5–5 kcal per gram). Thus, when comparing equal masses of fat, protein, and lipid, the fat has nearly twice the potential energy. Fat is therefore an efficient way to store energy in animals and many plants. To store an equivalent amount of energy in the form of carbohydrates or proteins would require about twice the mass, adding a significant burden to the organism’s structure. (For example, if you were 20 pounds overweight, you would be nearly 40 pounds overweight if the same energy were stored as carbohydrates or proteins instead of fat.) 14

Entropy If energy cannot be created or destroyed, where has it gone when the diver hits the water? It has been converted to heat, a form of kinetic energy contained in the random motion o f atoms and molecules. All energy conversions generate some heat. Although heat production does not destroy energy, it does make it less useful. “Heat” of all forms of energy is the most difficult to “tame” or harness for useful work. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students with limited exposure to physics may have never understood the concepts of “energy” and “the conservation of energy” or distinguished between potential and kinetic energy. Understanding such broad and new abstract concepts requires time and concrete examples. 2. Although typically familiar with the concept of dietary Calories, students often struggle to think of Calories as a source of potential energy. For many students, it is not immediately clear how energy is stored as Calories in food. Teaching Tips 1. In our daily lives, we rely upon many energy transformations. On our classroom walls, a clock converts electrical energy to mechanical energy to sweep the hands around the clock’s face (unless it is digital!). Our physical (mechanical) activities walking to and from the classroom rely upon the chemical energy from our diet. This chemical energy in our diet also helps us maintain a steady body temperature (heat). Consider challenging your students to come up with additional examples of such common energy conversions in their lives. 2. Some students can relate well to the concept of entropy as it relates to the room where they live. Despite cleaning up and organizing the room on a regular (or irregular) basis, the room increasingly becomes disorganized, a victim of entropy, until another energy input (or effort) is exerted to make the room more orderly again. Students might even get to know “entropy: as the “dorm room effect”. 3. All too often we hear or read that some thing or reaction creates energy. We might hear or read that a power plant “produces” energy or that mitochondria “make” energy. Even in our classroom conversations, we may occasionally slip into this error. When discussing the first law of thermodynamics, consider emphasizing the inaccuracy of such statements. 4. The heat produced by the engine of a car is typically used to heat the car during cold weather. But is this same heat available in warmer weather? Students are often unaware that their car “heater” works very well in the summer too. Just as exercise can warm us when it is cold, the same extra heat is released when we exercise in warm conditions. A car engine in the summer struggles to dissipate heat in the same way that a human struggles to cool off after exercising when weather is warm. 5. Here is a question that might make cellular respiration a little more meaningful to your students. Ask your students why they feel warm when it is 30°C (86°F) outside if their core body temperature is about 37°C (98.6°F)? Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that help release our extra heat generated in cellular respiration. 6. Here is a calculation that has impressed some students. Depending on the size and activity of a person, a human might burn 2,000 dietary Calories (kilocalories) a day. This is enough energy to raise the temperature of 20 L of liquid water from 0° to 100°C. This is something to think about the next time you heat water on the stove! If you can bring in ten 2-L bottles you can help students visualize how much liquid water can be raised from 0° to 100°C. (Note: 100 Calories raises about 1 L of water 100°C; but it takes much more energy to melt ice or to convert boiling water into steam.) 7. The same mass of fat (about 9 kcal per gram) stores nearly twice as many Calories as an equivalent mass of protein or carbohydrates (about 4.5–5 kcal per gram). Thus, when comparing equal masses of fat, protein, and lipid, the fat has nearly twice the potential energy. Fat is therefore an efficient way to store energy in animals and many plants. To store an equivalent amount of energy in the form of carbohydrates or proteins would require about twice the mass, adding a significant burden to the organism’s structure. (For example, if you were 20 pounds overweight, you would be nearly 40 pounds overweight if the same energy were stored as carbohydrates or proteins instead of fat.) 15

Entropy Heat is energy in its most chaotic form, the energy of aimless molecular movement. Entropy is a measure of the amount of disorder, chaos or randomness in a system. Everytime energy is converted from one form to another, the entropy increases. © 2013 Pearson Education, Inc.

Chemical Energy How can molecules derived from the food we eat provide energy for our working cells? The molecules from food, gasoline and other fuels have a special form of energy called chemical energy which arises form the arrangement of atoms and can be released by a chemical reaction. Carbohydrates, fats, and gasoline have structures that make them especially rich in chemical energy. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students with limited exposure to physics may have never understood the concepts of “energy” and “the conservation of energy” or distinguished between potential and kinetic energy. Understanding such broad and new abstract concepts requires time and concrete examples. 2. Although typically familiar with the concept of dietary Calories, students often struggle to think of Calories as a source of potential energy. For many students, it is not immediately clear how energy is stored as Calories in food. Teaching Tips 1. In our daily lives, we rely upon many energy transformations. On our classroom walls, a clock converts electrical energy to mechanical energy to sweep the hands around the clock’s face (unless it is digital!). Our physical (mechanical) activities walking to and from the classroom rely upon the chemical energy from our diet. This chemical energy in our diet also helps us maintain a steady body temperature (heat). Consider challenging your students to come up with additional examples of such common energy conversions in their lives. 2. Some students can relate well to the concept of entropy as it relates to the room where they live. Despite cleaning up and organizing the room on a regular (or irregular) basis, the room increasingly becomes disorganized, a victim of entropy, until another energy input (or effort) is exerted to make the room more orderly again. Students might even get to know “entropy: as the “dorm room effect”. 3. All too often we hear or read that some thing or reaction creates energy. We might hear or read that a power plant “produces” energy or that mitochondria “make” energy. Even in our classroom conversations, we may occasionally slip into this error. When discussing the first law of thermodynamics, consider emphasizing the inaccuracy of such statements. 4. The heat produced by the engine of a car is typically used to heat the car during cold weather. But is this same heat available in warmer weather? Students are often unaware that their car “heater” works very well in the summer too. Just as exercise can warm us when it is cold, the same extra heat is released when we exercise in warm conditions. A car engine in the summer struggles to dissipate heat in the same way that a human struggles to cool off after exercising when weather is warm. 5. Here is a question that might make cellular respiration a little more meaningful to your students. Ask your students why they feel warm when it is 30°C (86°F) outside if their core body temperature is about 37°C (98.6°F)? Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that help release our extra heat generated in cellular respiration. 6. Here is a calculation that has impressed some students. Depending on the size and activity of a person, a human might burn 2,000 dietary Calories (kilocalories) a day. This is enough energy to raise the temperature of 20 L of liquid water from 0° to 100°C. This is something to think about the next time you heat water on the stove! If you can bring in ten 2-L bottles you can help students visualize how much liquid water can be raised from 0° to 100°C. (Note: 100 Calories raises about 1 L of water 100°C; but it takes much more energy to melt ice or to convert boiling water into steam.) 7. The same mass of fat (about 9 kcal per gram) stores nearly twice as many Calories as an equivalent mass of protein or carbohydrates (about 4.5–5 kcal per gram). Thus, when comparing equal masses of fat, protein, and lipid, the fat has nearly twice the potential energy. Fat is therefore an efficient way to store energy in animals and many plants. To store an equivalent amount of energy in the form of carbohydrates or proteins would require about twice the mass, adding a significant burden to the organism’s structure. (For example, if you were 20 pounds overweight, you would be nearly 40 pounds overweight if the same energy were stored as carbohydrates or proteins instead of fat.) 17

Chemical Energy Living cells and car engines use the same basic processes to make chemical energy stored in their fuels available for work. In both cases, this process breaks organic fuel into smaller waste molecules (like carbon dioxide) that have much less chemical energy. For example, the engine of a car mixes oxygen with gasoline in an explosive chemical reaction that breaks down food molecules and pushes the pistons that eventually move the wheels. The wast products are carbon dioxide and water. © 2013 Pearson Education, Inc.

Chemical Energy Only about 25% of the energy that a car engine extracts from the fuel is converted to the energy of motion of the car. Most of the rest is converted to heat. Cells also use oxygen in reactions that release energy from fuel molecules. Just as with a car, the “exhaust” is also carbon dioxide and water. The combustion of food in cells is called cellular respiration. © 2013 Pearson Education, Inc.

Chemical Energy Cellular respiration is the energy-releasing chemical breakdown of fuel molecules and the storage of that energy in a form the cell can use to perform work (a molecule called ATP). You convert about 34% of the energy released to walk across a room, about 66% goes to form heat that keeps your body warm. Humans and other animals use the unused heat energy to keep their bodies warm. It also shows why you get so hot when you exercise. © 2013 Pearson Education, Inc.

Energy conversion in a car
Figure 5.2 Fuel rich in chemical energy Waste products poor in chemical energy Energy conversion Heat energy Octane (from gasoline) Carbon dioxide Combustion Kinetic energy of movement Oxygen Water Energy conversion in a car Heat energy Cellular respiration Glucose (from food) Carbon dioxide ATP Water Oxygen Energy for cellular work Energy conversion in a cell Figure 5.2 Energy conversions in a car and a cell

Food Calories Read any packaged food and you’ll find the number calories in each serving of that food. Calories are units of energy. A calorie is the amount of energy that can raise one gram of water one degree Celsius. You could actually measure the calorie content of peanut butter by burning it under a beaker of water and then measuring the increase in water temperature. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students with limited exposure to physics may have never understood the concepts of “energy” and “the conservation of energy” or distinguished between potential and kinetic energy. Understanding such broad and new abstract concepts requires time and concrete examples. 2. Although typically familiar with the concept of dietary Calories, students often struggle to think of Calories as a source of potential energy. For many students, it is not immediately clear how energy is stored as Calories in food. Teaching Tips 1. In our daily lives, we rely upon many energy transformations. On our classroom walls, a clock converts electrical energy to mechanical energy to sweep the hands around the clock’s face (unless it is digital!). Our physical (mechanical) activities walking to and from the classroom rely upon the chemical energy from our diet. This chemical energy in our diet also helps us maintain a steady body temperature (heat). Consider challenging your students to come up with additional examples of such common energy conversions in their lives. 2. Some students can relate well to the concept of entropy as it relates to the room where they live. Despite cleaning up and organizing the room on a regular (or irregular) basis, the room increasingly becomes disorganized, a victim of entropy, until another energy input (or effort) is exerted to make the room more orderly again. Students might even get to know “entropy: as the “dorm room effect”. 3. All too often we hear or read that some thing or reaction creates energy. We might hear or read that a power plant “produces” energy or that mitochondria “make” energy. Even in our classroom conversations, we may occasionally slip into this error. When discussing the first law of thermodynamics, consider emphasizing the inaccuracy of such statements. 4. The heat produced by the engine of a car is typically used to heat the car during cold weather. But is this same heat available in warmer weather? Students are often unaware that their car “heater” works very well in the summer too. Just as exercise can warm us when it is cold, the same extra heat is released when we exercise in warm conditions. A car engine in the summer struggles to dissipate heat in the same way that a human struggles to cool off after exercising when weather is warm. 5. Here is a question that might make cellular respiration a little more meaningful to your students. Ask your students why they feel warm when it is 30°C (86°F) outside if their core body temperature is about 37°C (98.6°F)? Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that help release our extra heat generated in cellular respiration. 6. Here is a calculation that has impressed some students. Depending on the size and activity of a person, a human might burn 2,000 dietary Calories (kilocalories) a day. This is enough energy to raise the temperature of 20 L of liquid water from 0° to 100°C. This is something to think about the next time you heat water on the stove! If you can bring in ten 2-L bottles you can help students visualize how much liquid water can be raised from 0° to 100°C. (Note: 100 Calories raises about 1 L of water 100°C; but it takes much more energy to melt ice or to convert boiling water into steam.) 7. The same mass of fat (about 9 kcal per gram) stores nearly twice as many Calories as an equivalent mass of protein or carbohydrates (about 4.5–5 kcal per gram). Thus, when comparing equal masses of fat, protein, and lipid, the fat has nearly twice the potential energy. Fat is therefore an efficient way to store energy in animals and many plants. To store an equivalent amount of energy in the form of carbohydrates or proteins would require about twice the mass, adding a significant burden to the organism’s structure. (For example, if you were 20 pounds overweight, you would be nearly 40 pounds overweight if the same energy were stored as carbohydrates or proteins instead of fat.) 22

Food Calories Calories are tiny units of energy, so using them to described the fuel content of foods is not practical. Instead, it’s conventional to use kilocalories (kcal) or 1,000 calories. 1 kcal = 1 Calorie (Cal) 1 scientific kilocalorie = 1 dietary Calorie © 2013 Pearson Education, Inc.

Figure 5.3 Some caloric accounting
Food Food Calories Activity Food Calories consumed per hour by a 150-pound person* Cheeseburger 295 Running (7min/mi) 979 Spaghetti with sauce (1 cup) 241 Dancing (fast) 510 Baked potato (plain, with skin) 220 Bicycling (10 mph) 490 Fried chicken (drumstick) 193 Swimming (2 mph) 408 Bean burrito 189 Walking (3 mph) 245 Pizza with pepperoni (1 slice) 181 Dancing (slow) 204 Peanuts (1 ounce) 166 Playing the piano 73 Apple 81 Driving a car 61 Garden salad (2 cups) 56 Sitting (writing) 28 Popcorn (plain, 1 cup) 31 *Not including energy necessary for basic functions, such as breathing and heartbeat Broccoli (1 cup) 25 (a) Food Calories (kilocalories) in various foods (b) Food Calories (kilocalories) we burn in various activities Figure 5.3 Some caloric accounting

Food Food Calories (a) Food Calories (kilocalories) in various foods
Figure 5.3a Food Food Calories Cheeseburger 295 Spaghetti with sauce (1 cup) 241 Baked potato (plain, with skin) 220 Fried chicken (drumstick) 193 Bean burrito 189 Pizza with pepperoni (1 slice) 181 Peanuts (1 ounce) 166 Apple 81 Garden salad (2 cups) 56 Popcorn (plain, 1 cup) 31 Broccoli (1 cup) 25 (a) Food Calories (kilocalories) in various foods Figure 5.3 Some caloric accounting (part 1)

Food Calories consumed per hour by a 150-pound person*
Figure 5.3b Activity Food Calories consumed per hour by a 150-pound person* Running (7min/mi) 979 Dancing (fast) 510 Bicycling (10 mph) 490 Swimming (2 mph) 408 Walking (3 mph) 245 Dancing (slow) 204 Playing the piano 73 Driving a car 61 Sitting (writing) 28 *Not including energy necessary for basic functions, such as breathing and heartbeat (b) Food Calories (kilocalories) we burn in various activities Figure 5.3 Some caloric accounting (part 2)

ATP AND CELLULAR WORK The carbohydrates, fats and other fuel molecules we obtain from food do not drive work in our cells directly. Instead, chemical energy released by the breakdown of organic molecules during cellular respiration is used to generate molecules of ATP. These molecules of ATP then power cellular work. ATP acts like an energy shuttle, storing energy obtained from food and then releasing it as needed at a later time. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Although students might already understand mechanical and chemical work in a cell, the idea of transport work might be new to them. Chapter 6, in a section on the electron transport system, develops an analogy between transport work and water behind a dam. Therefore, at this point you might consider making an analogy between transport work (against a gradient) and pumping water up into a reservoir (against gravity)—perhaps into a water tower. 2. Energy shuttling at the cellular level may be new to many students, but it is a familiar concept when related to the use of money in our society. Students might be discouraged if the only benefit of work was the ability to make purchases from your employer. (Most of us would soon tire of a fast-food job that pays its employees only in free food!) Money permits the generation of value (a paycheck) analogous to an energy-releasing reaction to be coupled to an energy (money)-consuming reaction, making purchases in distant locations. This idea of “earn and spend” is a common concept most students know well. Teaching Tips 1. The authors suggest an analogy between ATP and a spring. When a phosphate group is attached to ADP, it is like compressing the spring. When a phosphate group is lost, the spring is relaxed, and energy is released. 2. When introducing ATP and ADP, consider having students think of the terms as A-3-P and A-2-P, noting that the word roots “tri’ means 3 and “di” means 2. It might help students to keep track of the number of phosphates more easily. 3. Recycling is essential in cell biology. Damaged organelles are broken down intracellularly and the chemical components recycled, monomers of the cytoskeleton are routinely recycled, and ADP is recycled. Several advantages are common to both human recycling of components of garbage and cellular recycling. For example, both save energy by avoiding the need to remanufacture the basic units, and both avoid an accumulation of waste products that could interfere with other “environmental” chemistry (the environment of the cell or the environment of the human population). 27

The Structure of ATP ATP (adenosine triphosphate)
consists of an organic molecule called adenosine plus a tail of three phosphate groups and is broken down to ADP and a phosphate group, releasing energy. The triphosphate tail is the “business end” of ATP, the part that provided energy for cellular work. Each phosphate group is negatively charged and these charges repel each other. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Although students might already understand mechanical and chemical work in a cell, the idea of transport work might be new to them. Chapter 6, in a section on the electron transport system, develops an analogy between transport work and water behind a dam. Therefore, at this point you might consider making an analogy between transport work (against a gradient) and pumping water up into a reservoir (against gravity)—perhaps into a water tower. 2. Energy shuttling at the cellular level may be new to many students, but it is a familiar concept when related to the use of money in our society. Students might be discouraged if the only benefit of work was the ability to make purchases from your employer. (Most of us would soon tire of a fast-food job that pays its employees only in free food!) Money permits the generation of value (a paycheck) analogous to an energy-releasing reaction to be coupled to an energy (money)-consuming reaction, making purchases in distant locations. This idea of “earn and spend” is a common concept most students know well. Teaching Tips 1. The authors suggest an analogy between ATP and a spring. When a phosphate group is attached to ADP, it is like compressing the spring. When a phosphate group is lost, the spring is relaxed, and energy is released. 2. When introducing ATP and ADP, consider having students think of the terms as A-3-P and A-2-P, noting that the word roots “tri’ means 3 and “di” means 2. It might help students to keep track of the number of phosphates more easily. 3. Recycling is essential in cell biology. Damaged organelles are broken down intracellularly and the chemical components recycled, monomers of the cytoskeleton are routinely recycled, and ADP is recycled. Several advantages are common to both human recycling of components of garbage and cellular recycling. For example, both save energy by avoiding the need to remanufacture the basic units, and both avoid an accumulation of waste products that could interfere with other “environmental” chemistry (the environment of the cell or the environment of the human population). 28

The Structure of ATP The crowding of the negative charges in the triphosphate tail contributes to the potential energy of ATP. For ATP power, it is the release of the phosphate at the tip of the triphosphate tail that makes energy available to the working cell. © 2013 Pearson Education, Inc.

(transferred to another
Figure 5.4 Energy Triphosphate Diphosphate Adenosine P P P Adenosine P P P Phosphate (transferred to another molecule) ATP ADP Figure 5.4 ATP power

Phosphate Transfer When ATP drives work in cells, phosphate groups do not just fly off into space. ATP energizes other molecules in cells by transferring phosphate groups to those molecules. This transfer of phosphate groups helps cells perform three kinds of work: mechanical, transport and chemical. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. Although students might already understand mechanical and chemical work in a cell, the idea of transport work might be new to them. Chapter 6, in a section on the electron transport system, develops an analogy between transport work and water behind a dam. Therefore, at this point you might consider making an analogy between transport work (against a gradient) and pumping water up into a reservoir (against gravity)—perhaps into a water tower. 2. Energy shuttling at the cellular level may be new to many students, but it is a familiar concept when related to the use of money in our society. Students might be discouraged if the only benefit of work was the ability to make purchases from your employer. (Most of us would soon tire of a fast-food job that pays its employees only in free food!) Money permits the generation of value (a paycheck) analogous to an energy-releasing reaction to be coupled to an energy (money)-consuming reaction, making purchases in distant locations. This idea of “earn and spend” is a common concept most students know well. Teaching Tips 1. The authors suggest an analogy between ATP and a spring. When a phosphate group is attached to ADP, it is like compressing the spring. When a phosphate group is lost, the spring is relaxed, and energy is released. 2. When introducing ATP and ADP, consider having students think of the terms as A-3-P and A-2-P, noting that the word roots “tri’ means 3 and “di” means 2. It might help students to keep track of the number of phosphates more easily. 3. Recycling is essential in cell biology. Damaged organelles are broken down intracellularly and the chemical components recycled, monomers of the cytoskeleton are routinely recycled, and ADP is recycled. Several advantages are common to both human recycling of components of garbage and cellular recycling. For example, both save energy by avoiding the need to remanufacture the basic units, and both avoid an accumulation of waste products that could interfere with other “environmental” chemistry (the environment of the cell or the environment of the human population). 31

Phosphate Transfer Imagine a bicyclist riding up a hill. In his muscle cells ATP transfers phosphate groups to motor proteins of the muscles. ATP also allows transport of ions across the cell membranes of the rider’s nerve cells. Lastly, ATP and phosphate groups allow for the attachment of one large molecule to another large molecule. © 2013 Pearson Education, Inc.

(a) Motor protein performing mechanical work (moving a muscle fiber)
Figure 5.5a Motor protein ATP ADP P ADP P Protein moved (a) Motor protein performing mechanical work (moving a muscle fiber) Figure 5.5 How ATP drives cellular work (part 1)

(b) Transport protein performing transport work (importing a solute)
Figure 5.5b Solute Transport protein P P ATP ADP P Solute transported (b) Transport protein performing transport work (importing a solute) Figure 5.5 How ATP drives cellular work (part 2)

Figure 5.5c P ATP X P X Y ADP P Y Reactants Product made (c) Chemical reactants performing chemical work (promoting a chemical reaction) Figure 5.5 How ATP drives cellular work (part 3)

The ATP Cycle Your cells spend ATP continuously. Fortunately, it is a renewable resource. ATP can be restored by adding a phosphate group back on to the ADP molecule. That requires energy. The chemical energy that cellular respiration harvests from sugars and other organic fuels is put to work regenerating a supply of ATP. © 2013 Pearson Education, Inc.

Cellular respiration: chemical energy harvested from fuel molecules
Figure 5.6 ATP Cellular respiration: chemical energy harvested from fuel molecules Energy for cellular work ADP P Figure 5.6 The ATP cycle

https://www.youtube.com/watch?v=5GMLIMIVUvo ATP (9.45)

ENZYMES As you’ve seen, a living organism contains a vast collection of chemicals, with countless chemical reactions. In a sense, a living organism is a complex “chemical square dance” with molecular dancers continually changing partners via chemical reactions. Metabolism is the word used to describe ALL of the chemical reactions in a cell or ogranism © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students not previously familiar with activation energy, analogies can make all the difference. Activation energy can be thought of as a small input that is needed to trigger a large output. This is like (a) an irritated person who needs only a bit more frustration to explode in anger, (b) small waves that lift debris over a dam, or (c) lighting a match around lighter fluid. In each situation, the output was much greater than the input. 2. The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. New concepts, like pitching a tent, are best constructed with many lines of support. Teaching Tips 1. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments. 2. Enzyme inhibitors that block the active site are like (a) a person sitting in your assigned theater seat or (b) a car parked in your parking space. Analogies for inhibitors that change the shape of the active site are more difficult to imagine. Consider challenging your students to think of such analogies. (Perhaps someone adjusting the driver seat of the car differently from your preferences and then leaving it that way when you try to use the car.) 3. Feedback regulation relies on the negative feedback of the accumulation of a product. Ask students in class to suggest other products of reactions that inhibit the process that made them, when the product reaches high enough levels. (Gas station pumps routinely shut off when a high level of gasoline is detected. Furnaces typically turn off when enough heat has been produced, and toilets stop running when the tank is refilled.) 4. Consider challenging your class to suggest advantages to using specific enzyme inhibitors as insect poisons. Many advantages exist and include (a) the ability to target chemical reactions of only certain types of pest organisms and (b) the ability to target chemical reactions that are found in insects but not in humans. 5. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. Yet in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out, that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, which are proteins with the ability to control the production of carbohydrates and lipids. 39

ENZYMES Almost NO metabolic reactions in a cell or organism can occur without assistance. The protein molecules that assist chemical reactions are called enzymes. Enzymes bind to reactant molecules helping to speed up a chemical reaction. © 2013 Pearson Education, Inc.

Activation Energy For a chemical reaction to begin, chemical bonds in the reactant molecule must be broken. This process requires that reactant molecules absorb energy from their surroundings. This energy is called activation energy because it activates the reactants and triggers the chemical reaction. Enzymes enable chemical reactions to proceed because they reduce the amount of activation energy required to break the bonds of the reactant molecules. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students not previously familiar with activation energy, analogies can make all the difference. Activation energy can be thought of as a small input that is needed to trigger a large output. This is like (a) an irritated person who needs only a bit more frustration to explode in anger, (b) small waves that lift debris over a dam, or (c) lighting a match around lighter fluid. In each situation, the output was much greater than the input. 2. The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. New concepts, like pitching a tent, are best constructed with many lines of support. Teaching Tips 1. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments. 2. Enzyme inhibitors that block the active site are like (a) a person sitting in your assigned theater seat or (b) a car parked in your parking space. Analogies for inhibitors that change the shape of the active site are more difficult to imagine. Consider challenging your students to think of such analogies. (Perhaps someone adjusting the driver seat of the car differently from your preferences and then leaving it that way when you try to use the car.) 3. Feedback regulation relies on the negative feedback of the accumulation of a product. Ask students in class to suggest other products of reactions that inhibit the process that made them, when the product reaches high enough levels. (Gas station pumps routinely shut off when a high level of gasoline is detected. Furnaces typically turn off when enough heat has been produced, and toilets stop running when the tank is refilled.) 4. Consider challenging your class to suggest advantages to using specific enzyme inhibitors as insect poisons. Many advantages exist and include (a) the ability to target chemical reactions of only certain types of pest organisms and (b) the ability to target chemical reactions that are found in insects but not in humans. 5. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. Yet in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out, that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, which are proteins with the ability to control the production of carbohydrates and lipids. 41

(a) Without enzyme (b) With enzyme Activation energy barrier
Figure 5.7 Activation energy barrier Activation energy barrier reduced by enzyme Enzyme Reactant Reactant Energy Energy Products Products (a) Without enzyme (b) With enzyme Figure 5.7 Enzymes and activation energy

(a) Without enzyme Activation energy barrier Reactant Energy Products
Figure 5.7a Activation energy barrier Reactant Energy Products (a) Without enzyme Figure 5.7 Enzymes and activation energy (part 1)

(b) With enzyme Activation energy barrier reduced by Enzyme enzyme
Figure 5.7b Activation energy barrier reduced by enzyme Enzyme Reactant Energy Products (b) With enzyme Figure 5.7 Enzymes and activation energy (part 2)

The Process of Science: Can Enzymes Be Engineered?
Observation: Genetic sequences suggest that many of our genes were formed through a type of molecular evolution. Question: Can laboratory methods mimic this process through artificial selection? Hypothesis: An artificial process could be used to modify the gene that codes for lactase into a new gene coding for an enzyme with a new function. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students not previously familiar with activation energy, analogies can make all the difference. Activation energy can be thought of as a small input that is needed to trigger a large output. This is like (a) an irritated person who needs only a bit more frustration to explode in anger, (b) small waves that lift debris over a dam, or (c) lighting a match around lighter fluid. In each situation, the output was much greater than the input. 2. The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. New concepts, like pitching a tent, are best constructed with many lines of support. Teaching Tips 1. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments. 2. Enzyme inhibitors that block the active site are like (a) a person sitting in your assigned theater seat or (b) a car parked in your parking space. Analogies for inhibitors that change the shape of the active site are more difficult to imagine. Consider challenging your students to think of such analogies. (Perhaps someone adjusting the driver seat of the car differently from your preferences and then leaving it that way when you try to use the car.) 3. Feedback regulation relies on the negative feedback of the accumulation of a product. Ask students in class to suggest other products of reactions that inhibit the process that made them, when the product reaches high enough levels. (Gas station pumps routinely shut off when a high level of gasoline is detected. Furnaces typically turn off when enough heat has been produced, and toilets stop running when the tank is refilled.) 4. Consider challenging your class to suggest advantages to using specific enzyme inhibitors as insect poisons. Many advantages exist and include (a) the ability to target chemical reactions of only certain types of pest organisms and (b) the ability to target chemical reactions that are found in insects but not in humans. 5. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. Yet in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out, that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, which are proteins with the ability to control the production of carbohydrates and lipids. 46

Ribbon model showing the polypeptide chains of the enzyme lactase
Figure 5.8c Ribbon model showing the polypeptide chains of the enzyme lactase Figure 5.8 Directed evolution of an enzyme (part 3)

Induced Fit An enzyme is very selective in the reaction it catalyzes. This is based one the enzyme’s ability to recognize a certain reactant molecule, called the substrate. Each enzyme recognizes a substrate, a specific reactant molecule. The active site fits to the substrate, and the enzyme changes shape slightly. This interaction is called induced fit because the entry of the substrate induces the enzyme to change shape slightly. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students not previously familiar with activation energy, analogies can make all the difference. Activation energy can be thought of as a small input that is needed to trigger a large output. This is like (a) an irritated person who needs only a bit more frustration to explode in anger, (b) small waves that lift debris over a dam, or (c) lighting a match around lighter fluid. In each situation, the output was much greater than the input. 2. The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. New concepts, like pitching a tent, are best constructed with many lines of support. Teaching Tips 1. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments. 2. Enzyme inhibitors that block the active site are like (a) a person sitting in your assigned theater seat or (b) a car parked in your parking space. Analogies for inhibitors that change the shape of the active site are more difficult to imagine. Consider challenging your students to think of such analogies. (Perhaps someone adjusting the driver seat of the car differently from your preferences and then leaving it that way when you try to use the car.) 3. Feedback regulation relies on the negative feedback of the accumulation of a product. Ask students in class to suggest other products of reactions that inhibit the process that made them, when the product reaches high enough levels. (Gas station pumps routinely shut off when a high level of gasoline is detected. Furnaces typically turn off when enough heat has been produced, and toilets stop running when the tank is refilled.) 4. Consider challenging your class to suggest advantages to using specific enzyme inhibitors as insect poisons. Many advantages exist and include (a) the ability to target chemical reactions of only certain types of pest organisms and (b) the ability to target chemical reactions that are found in insects but not in humans. 5. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. Yet in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out, that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, which are proteins with the ability to control the production of carbohydrates and lipids. 48

Induced Fit This is just like a handshake. As your hand makes contact with another hand, they both change shape slightly. After the products of the reaction are released by the enzyme, another substrate molecule can bind. Enzymes can function over and over again, a key characteristic of enzymes. Many enzymes are named for their substrates, but with an –ase ending. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students not previously familiar with activation energy, analogies can make all the difference. Activation energy can be thought of as a small input that is needed to trigger a large output. This is like (a) an irritated person who needs only a bit more frustration to explode in anger, (b) small waves that lift debris over a dam, or (c) lighting a match around lighter fluid. In each situation, the output was much greater than the input. 2. The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. New concepts, like pitching a tent, are best constructed with many lines of support. Teaching Tips 1. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments. 2. Enzyme inhibitors that block the active site are like (a) a person sitting in your assigned theater seat or (b) a car parked in your parking space. Analogies for inhibitors that change the shape of the active site are more difficult to imagine. Consider challenging your students to think of such analogies. (Perhaps someone adjusting the driver seat of the car differently from your preferences and then leaving it that way when you try to use the car.) 3. Feedback regulation relies on the negative feedback of the accumulation of a product. Ask students in class to suggest other products of reactions that inhibit the process that made them, when the product reaches high enough levels. (Gas station pumps routinely shut off when a high level of gasoline is detected. Furnaces typically turn off when enough heat has been produced, and toilets stop running when the tank is refilled.) 4. Consider challenging your class to suggest advantages to using specific enzyme inhibitors as insect poisons. Many advantages exist and include (a) the ability to target chemical reactions of only certain types of pest organisms and (b) the ability to target chemical reactions that are found in insects but not in humans. 5. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. Yet in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out, that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, which are proteins with the ability to control the production of carbohydrates and lipids. 49

1 Ready for substrate Active site Enzyme (sucrase) Figure 5.9-1
Figure 5.9 How an enzyme works (step 1)

Substrate (sucrose) 1 Ready for substrate Active site 2 Substrate
Figure 5.9-2 Substrate (sucrose) 1 Ready for substrate Active site 2 Substrate binding Enzyme (sucrase) Figure 5.9 How an enzyme works (step 2)

Figure 5.9-3 Substrate (sucrose) 1 Ready for substrate Active site 2 Substrate binding Enzyme (sucrase) H2O 3 Catalysis Figure 5.9 How an enzyme works (step 3)

Figure 5.9-4 Substrate (sucrose) 1 Ready for substrate Active site 2 Substrate binding Enzyme (sucrase) Fructose H2O Glucose 4 Product released 3 Catalysis Figure 5.9 How an enzyme works (step 4)

Enzyme Inhibitors Certain molecules can inhibit a metabolic reaction by binding to an enzyme and disrupting its function. Come of these inhibitors are actually substrate imposters that block the active site of the enzyme. Other inhibitors bind to the enzyme at a site remote from the active site, but binding changes the enzyme’s shape so the active site no longer accepts the substrate. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students not previously familiar with activation energy, analogies can make all the difference. Activation energy can be thought of as a small input that is needed to trigger a large output. This is like (a) an irritated person who needs only a bit more frustration to explode in anger, (b) small waves that lift debris over a dam, or (c) lighting a match around lighter fluid. In each situation, the output was much greater than the input. 2. The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. New concepts, like pitching a tent, are best constructed with many lines of support. Teaching Tips 1. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments. 2. Enzyme inhibitors that block the active site are like (a) a person sitting in your assigned theater seat or (b) a car parked in your parking space. Analogies for inhibitors that change the shape of the active site are more difficult to imagine. Consider challenging your students to think of such analogies. (Perhaps someone adjusting the driver seat of the car differently from your preferences and then leaving it that way when you try to use the car.) 3. Feedback regulation relies on the negative feedback of the accumulation of a product. Ask students in class to suggest other products of reactions that inhibit the process that made them, when the product reaches high enough levels. (Gas station pumps routinely shut off when a high level of gasoline is detected. Furnaces typically turn off when enough heat has been produced, and toilets stop running when the tank is refilled.) 4. Consider challenging your class to suggest advantages to using specific enzyme inhibitors as insect poisons. Many advantages exist and include (a) the ability to target chemical reactions of only certain types of pest organisms and (b) the ability to target chemical reactions that are found in insects but not in humans. 5. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. Yet in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out, that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, which are proteins with the ability to control the production of carbohydrates and lipids. 54

(a) Enzyme and substrate binding normally
Figure 5.10 (a) Enzyme and substrate binding normally Substrate Active site Enzyme (b) Enzyme inhibition by a substrate imposter Inhibitor Substrate Active site Enzyme (c) Inhibition of an enzyme by a molecule that causes the active site to change shape Substrate Active site Inhibitor Enzyme Figure 5.10 Enzyme inhibitors

(a) Enzyme and substrate binding normally
Figure 5.10a Substrate Active site Enzyme (a) Enzyme and substrate binding normally Figure 5.10 Enzyme inhibitors (part 1)

(b) Enzyme inhibition by a substrate imposter
Figure 5.10b Inhibitor Substrate Active site Enzyme (b) Enzyme inhibition by a substrate imposter Figure 5.10 Enzyme inhibitors (part 2)

(c) Inhibition of an enzyme by a molecule
Figure 5.10c Substrate Active site Inhibitor Enzyme (c) Inhibition of an enzyme by a molecule that causes the active site to change shape Figure 5.10 Enzyme inhibitors (part 3)

https://www. youtube. com/watch. v=ok9esggzN18 Enzymes (11
Enzymes (11.51) What are Enymes and How do they Work (6.29)

Enzyme Inhibitors Some products of a reaction may inhibit the enzyme required for its production. This is called feedback regulation. It prevents the cell from wasting resources. Many beneficial drugs work by inhibiting enzymes. Penicillin blocks the active site of an enzyme that bacteria use in making cell walls. Ibuprofen inhibits an enzyme involved in sending pain signals. Many cancer drugs inhibit enzymes that promote cell division. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students not previously familiar with activation energy, analogies can make all the difference. Activation energy can be thought of as a small input that is needed to trigger a large output. This is like (a) an irritated person who needs only a bit more frustration to explode in anger, (b) small waves that lift debris over a dam, or (c) lighting a match around lighter fluid. In each situation, the output was much greater than the input. 2. The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. New concepts, like pitching a tent, are best constructed with many lines of support. Teaching Tips 1. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments. 2. Enzyme inhibitors that block the active site are like (a) a person sitting in your assigned theater seat or (b) a car parked in your parking space. Analogies for inhibitors that change the shape of the active site are more difficult to imagine. Consider challenging your students to think of such analogies. (Perhaps someone adjusting the driver seat of the car differently from your preferences and then leaving it that way when you try to use the car.) 3. Feedback regulation relies on the negative feedback of the accumulation of a product. Ask students in class to suggest other products of reactions that inhibit the process that made them, when the product reaches high enough levels. (Gas station pumps routinely shut off when a high level of gasoline is detected. Furnaces typically turn off when enough heat has been produced, and toilets stop running when the tank is refilled.) 4. Consider challenging your class to suggest advantages to using specific enzyme inhibitors as insect poisons. Many advantages exist and include (a) the ability to target chemical reactions of only certain types of pest organisms and (b) the ability to target chemical reactions that are found in insects but not in humans. 5. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. Yet in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out, that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, which are proteins with the ability to control the production of carbohydrates and lipids. 60

MEMBRANE FUNCTION Cells must also regulate the flow of materials in and out of itself. Transport proteins are membrane proteins that help move substances across a cell membrane. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 61

Intercellular joining Cell-cell recognition
Figure 5.11 Cell signaling Enzymatic activity Cytoplasm Fibers of extracellular matrix Cytoskeleton Cytoplasm Attachment to the cytoskeleton and extracellular matrix Transport Intercellular joining Cell-cell recognition Figure 5.11 Primary functions of membrane proteins

Passive Transport: Diffusion across Membranes
Molecules are restless. Molecules contain heat energy that causes them to vibrate and wander randomly. One result of this motion is diffusion. Diffusion is the movement of molecules from an area where they are in higher concentration to an area of lower concentration until they are equal throughout the entire space. Imagine the molecules of a perfume bottle. If you remove the bottle top, the molecules will move about but the overall movement will be out of the bottle. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 63

Diffusion across a membrane is an example of passive transport – because the cell does not use any energy for this transport to occur. However, membranes are selectively permeable – they decide what will come in and go out. In passive transport, a substance will move down its concentration gradient from area of higher solute concentration to area of lower concentration. © 2013 Pearson Education, Inc.

Some molecules have to be assisted by membrane proteins in order to get across the cell membrane. This type of assisted transport is called facilitated diffusion. This process does not require the cell to use any of its energy. © 2013 Pearson Education, Inc.

(a) Passive transport of one type of molecule
Figure 5.12 Molecules of dye Membrane Net diffusion Net diffusion Equilibrium (a) Passive transport of one type of molecule Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium (b) Passive transport of two types of molecules Figure 5.12 Passive transport: diffusion across a membrane

(a) Passive transport of one type of molecule
Figure 5.12a Molecules of dye Membrane Net diffusion Net diffusion Equilibrium (a) Passive transport of one type of molecule Figure 5.12 Passive transport: diffusion across a membrane (part 1)

(b) Passive transport of two types of molecules
Figure 5.12b Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium (b) Passive transport of two types of molecules Figure 5.12 Passive transport: diffusion across a membrane (part 2)

Osmosis and Water Balance
The diffusion of water across a selectively permeable membrane is called osmosis. Imagine a membrane separating two different solutions with different concentrations of solute. A solute is a substance that is dissolved in a solvent such as salt dissolved in water. The membrane allows the water to pass through it but not the salt. The solution with the higher concentration of solute is called the hypertonic solution. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 69

Osmosis and Water Balance
The solution with the lower concentration of solute is called the hypotonic solution. In this case, if the solute cannot cross the membrane, the water will cross the membrane going from the hypotonic side to the hypertonic side. Two solutions that are of equal concentration are said to be isotonic to each other. © 2013 Pearson Education, Inc.

Hypotonic solution Hypertonic solution Osmosis
Figure Hypotonic solution Hypertonic solution Sugar molecule Selectively permeable membrane Osmosis Figure 5.13 Osmosis (step 1)

Hypotonic solution Hypertonic solution Isotonic solutions Osmosis
Figure Hypotonic solution Hypertonic solution Isotonic solutions Osmosis Sugar molecule Selectively permeable membrane Osmosis Figure 5.13 Osmosis (step 2)

https://www. youtube. com/watch
Diffusion and Osmosis (18.04)

Water Balance in Animal Cells
The survival of a cell depends on its ability to balance water uptake and loss. When an animal cell, like a blood cell, is placed in an isotonic solution, the cell’s volume will remain constant because it loses and gains water equally. But what happens if a blood cell is placed in a hypotonic solution? It will gain water and possibly explode due to too much water gain. To survive in a certain environment, the animal must have a way to balance uptake and loss of water. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 74

Water Balance in Animal Cells
To survive in a certain environment, the animal must have a way to balance uptake and loss of water. This control of water balance is called osmoregulation. © 2013 Pearson Education, Inc.

Water Balance in Plant Cells
Problems of water balance in plant cells are somewhat different because they have rigid cell walls around their cell membranes. A plant cell placed in an isotonic solution is flaccid. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 76

Figure 5.15 Figure 5.15 Plant turgor

In a hypertonic solution, a plant cell will shrivel due to the loss of water from it. As a plant cell loses water, it shrivels and its plasma membrane may pull away from the cell wall in the process of plasmolysis, which usually kills the cell. © 2013 Pearson Education, Inc.

(a) Isotonic solution (b) Hypotonic solution (c) Hypertonic solution
Figure 5.14 Animal cell H2O H2O H2O H2O Normal Lysing Shriveled Plant cell Plasma membrane H2O H2O H2O H2O Flaccid (wilts) Turgid (normal) Shriveled (a) Isotonic solution (b) Hypotonic solution (c) Hypertonic solution Figure 5.14 The behavior of animal and plant cells in different osmotic environments

(a) Isotonic solution Animal cell H2O H2O Normal Plant cell H2O H2O
Figure 5.14a Animal cell H2O H2O Normal Plant cell H2O H2O Flaccid (wilts) (a) Isotonic solution Figure 5.14 The behavior of animal and plant cells in different osmotic environments (part 1)

(b) Hypotonic solution H2O Lysing H2O Turgid (normal) Figure 5.14b
Figure 5.14 The behavior of animal and plant cells in different osmotic environments (part 2)

(c) Hypertonic solution H2O Shriveled Plasma membrane H2O Shriveled
Figure 5.14c H2O Shriveled Plasma membrane H2O Shriveled (c) Hypertonic solution Figure 5.14 The behavior of animal and plant cells in different osmotic environments (part 3)

As a plant cell loses water, it shrivels and its plasma membrane may pull away from the cell wall in the process of plasmolysis, which usually kills the cell. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 84

Active Transport: The Pumping of Molecules across Membranes
In contrast to passive transport, active transport requires that a cell expend energy to move molecules across a membrane. Cellular energy (usually ATP)is used to drive a transport protein that pumps a solute against the solute’s concentration gradient. Active transport allows cells to maintain internal concentrations of small solutes that differ from environmental conditions. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 85

Active Transport: The Pumping of Molecules across Membranes
One example is the concentration gradient of K+ and Na+ on either side of nerve cells. Animal nerve cells has a Na+/K+ ATPase pump that pumps these ions on either side of the cell membrane. © 2013 Pearson Education, Inc.

Active transport Lower solute concentration Solute ATP
Figure Lower solute concentration Solute ATP Higher solute concentration Active transport Figure 5.16 Active transport (step 1)

Active transport Lower solute concentration Solute ATP
Figure Lower solute concentration Solute ATP Higher solute concentration Active transport Figure 5.16 Active transport (step 2)

MEMBRANE TRANSPORT Passive Transport (requires no energy)
Figure 5.UN03 MEMBRANE TRANSPORT Passive Transport (requires no energy) Active Transport (requires energy) Diffusion Facilitated diffusion Osmosis Higher solute concentration Higher solute concentration Higher water concentration (lower solute concentration) Solute Solute Solute Water Solute ATP Lower solute concentration Lower water concentration (higher solute concentration) Lower solute concentration Figure 5.UN03 Summary of Key Concepts: Passive Transport, Osmosis, and Active Transport

Exocytosis and Endocytosis: Traffic of Large Molecules
So far, we have focused on how water and small solutes enter and leave cells by moving through the plasma membrane. The story is much different for large molecules such as proteins. The transport of larger molecules like proteins depends upon the formation and movement of vesicles. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 90

Proteins packaged into transport vesicles move toward the cell membrane then merge with the cell membrane where they undergo exocytosis. Exocytosis is the merging of a transport vesicle with the plasma membrane allowing the contents to move to the outside of the cell. The reverse of this process, the engulfment of molecules from the outside of the cell to the inside of the cells is called endocytosis. © 2013 Pearson Education, Inc.

Exocytosis Outside of cell Plasma membrane Cytoplasm Figure 5.17
Figure 5.17 Exocytosis

Endocytosis takes material in via vesicles that bud inward from the plasma membrane. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 93

When a cell brings a food vacuole into itself, kind of endocytosis is called phagocytosis (“eating” “cell” “process”) In liver cells, this is the manner in which cholesterol is taken out of the blood. © 2013 Pearson Education, Inc.

Figure 5.18 Endocytosis Figure 5.18 Endocytosis

The Role of Membranes in Cell Signaling
In addition to transport, the plasma membrane and its imbedded proteins play key roles in conveying signals from the external environment into the cell and also between cells. Communication begins when a receptor protein in the plasma membrane receives a stimulus such as a hormone. The hormone triggers a chain reaction in one or more molecules that function in transduction (moving the signal along). © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 96

The Role of Membranes in Cell Signaling
The proteins and other molecules of this signal transduction pathway relay the signal and convert it to chemical forms that can function in the cell. The ultimate signal may lead to various responses. © 2013 Pearson Education, Inc.

Proteins of signal transduction
Figure 5.19 Outside of cell Cytoplasm Reception Transduction Response Receptor protein Hydrolysis of glycogen releases glucose for energy Proteins of signal transduction pathway Epinephrine (adrenaline) from adrenal glands Plasma membrane Figure 5.19 An example of cell signaling

Evolution Connection: The Origin of Membranes
Phospholipids are key ingredients of membranes, were probably among the first organic compounds that formed from chemical reactions on early Earth, and self-assemble into simple membranes. © 2013 Pearson Education, Inc. Student Misconceptions and Concerns 1. For students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives. 2. Students easily confuse the terms hypertonic and hypotonic. One challenge is to understand that these are relative terms, such as heavier, darker, or fewer. No single solution is “heavier,” no single cup of coffee is “darker,” and no single bag of M & M’s has “fewer” candies. Such terms only apply when comparing two or more items. A solution with a higher concentration is “hypertonic.” But the same solution might also be “hypotonic” to a third solution. Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sources—the smell of dinner on the stove, the scent of a perfume or cologne to its bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away. 2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Or release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration. 3. The word root “hypo” means “below.” Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations below that of the other solution(s). 4. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A marine salmon moves from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____environment. (Answers: hypertonic to a hypotonic) 5. Your students may have noticed that their fingers wrinkle after taking a long shower or bath, or washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water since the soap removes the natural layer of oil from our skin. Our skin is hypertonic to the solutions that produce the swelling that appears as large wrinkles. 6. The effects of hypertonic and hypotonic solutions are easily demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations. 7. The three stages of cell signaling can be compared to a human reaction. Consider someone calling your name. (1) Reception—The sound waves of someone’s voice hitting and changing the shape of your eardrum. (2) Transduction—Your ear converts sound waves to nerve impulses and sends a signal to your brain, which registers that your name has been called. (3) Response—You look around to see who is calling. 8. The hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids. 99

Exocytosis Endocytosis Figure 5.UN04
Figure 5.UN04 Summary of Key Concepts: Exocytosis and Endocytosis

Chapter 5 The Working Cell.

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