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Campbell Biology: Concepts & Connections

Tenth Edition

Chapter 6

How Cells Harvest Chemical Energy

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Introduction

Oxygen is a reactant in cellular respiration, the process that breaks down sugar and other food molecules and generates A T P, the energy currency in cells, and heat.

Brown fat cells have a “short circuit” in their cellular respiration, which generates only heat, not A T P.

In this chapter, we explore the stages of cellular respiration and how cells produce A T P in the presence of oxygen.

Figure 6.0_1

Figure 6.0_1 Can brown fat keep a newborn warm and help keep an adult thin?

Figure 6.0_2

Figure 6.0_2 Chapter 6: Big Ideas

Long Description:

The details of the figure are as follows:

Cellular respiration: Aerobic harvesting of energy. An illustration shows the connection between breathing and cellular respiration.

Stages of cellular respiration. An illustration shows the three stages of cellular respiration, namely, (1) glycolysis which occurs in the cytosol, (2) pyruvate oxidation and citric acid cycle, and (3) oxidative phosphorylation which occur in the mitochondrion.

Fermentation: Anaerobic harvesting of energy. A photo shows wine barrels in a storage room.

Connections between metabolic pathways. A photo shows a few giraffes.

Cellular Respiration: Aerobic Harvesting of Energy

6.1 Photosynthesis and Cellular Respiration Provide Energy for Life (1 of 2)

Life requires energy.

In almost all ecosystems, energy ultimately comes from the sun.

In photosynthesis,

the energy of sunlight is used to rearrange the atoms of carbon dioxide (C O2) and water ( H 2 0),

producing organic molecules, and

releasing oxygen (O2).

Student Misconceptions and Concerns

Caution students against the assumption that energy is created when it is converted from one form to another. This might be a good time to review the principle of conservation of energy (the first law of thermodynamics, addressed in Module 5.10). (6.1–6.5)

Teaching Tips

You might wish to elaborate on the amount of solar energy striking Earth. Every day Earth is bombarded with solar radiation equal to the energy of 100 million atomic bombs. Of the tiny fraction of light that reaches photosynthetic organisms, only about 1% is converted to chemical energy by photosynthesis. (6.1)

Energy coupling 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 the employer. (We all might soon tire of a fast-food job that only paid employees in food!) Money permits the coupling of a generation of value (a paycheck, analogous to an energy-releasing reaction) to an energy-consuming reaction (money, which allows us to make purchases in distant locations). This idea of earning and spending is a common concept we all know well. (6.1–6.3)

Active Lecture Tips

See the Activity “Photosynthesis and Respiration: Are They Similar?” on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. (6.1)

Ask your students why they feel warm when it is 30ºC (86ºF) outside. If their core body temperature is about 37ºC (98.6ºF), shouldn’t they feel cold? Have students discuss ideas with others seated near them. Our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37ºC. Thus, we sweat and behave in ways that help us get rid of the extra heat from cellular respiration. (6.1–6.5)

6.1 Photosynthesis and Cellular Respiration Provide Energy for Life (2 of 2)

In cellular respiration,

O2 is consumed as organic molecules are broken down to C O2 and H 2 O and

the cell captures the energy released as A T P.

Checkpoint question What is misleading about the following statement? “Plant cells perform photosynthesis, and animal cells perform cellular respiration.”

Checkpoint Question Response

The statement implies that cellular respiration does not occur in plant cells. In fact, almost all eukaryotic cells use cellular respiration to obtain energy for their cellular work.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

Figure 6.1

Figure 6.1 The connection between photosynthesis and cellular respiration

Long Description:

Sunlight energy is converted to organic molecules and oxygen through photosynthesis in chloroplasts. The organic molecules and oxygen are converted to carbon dioxide and water through cellular respiration in mitochondria. Cellular respiration releases heat energy and also makes A T P that powers most cellular work. The carbon dioxide and water made by the mitochondria are then used in photosynthesis and the cycle continues.

6.2 Breathing Supplies O 2 for Use in Cellular Respiration and Removes C O 2

Respiration, often used as a synonym for “breathing,” refers to an exchange of gases.

An organism obtains O2 from its environment and

Releases C O2 as a waste product.

Breathing and cellular respiration are closely related.

Checkpoint question Are the oxygen atoms a runner exhales the same oxygen atoms she inhaled from the environment?

Checkpoint Question Response

No. The oxygen atoms inhaled become part of water, which may become part of the runner’s urine. The oxygen atoms in exhaled CO2 originate from glucose (or other food molecules).

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

Figure 6.2

Figure 6.2 The connection between breathing and cellular respiration

Long Description:

Breathing allows oxygen to enter the lungs when a person inhales. The oxygen in the lungs is transported in the bloodstream to the muscle cells. Muscle cells carry out cellular respiration. The equation for cellular respiration is, glucose plus oxygen yields carbon dioxide, plus water, plus A T P. The carbon dioxide then gets transported in the bloodstream to the lungs, where it is released from the body through breathing when a person exhales. The cycle starts over when the person inhales again.

6.3 Cellular Respiration Banks Energy in A T P Molecules

Cellular respiration

is an exergonic (energy-releasing) process that transfers energy from glucose to form A T P and

captures about 34% of the available energy originally stored in glucose with the rest of the energy lost as heat.

Checkpoint question Are the oxygen atoms a runner exhales the same oxygen atoms she inhaled from the environment?

Checkpoint Question Response

Exercise requires more ATP and thus a higher rate of cellular respiration. However, about 66% of the energy released from food produces heat instead of ATP.

Student Misconceptions and Concerns

Students often fail to realize that aerobic metabolism is a process generally similar to the burning of wood or the burning of gasoline in an automobile engine. Noting these general similarities can help students comprehend the overall reaction and heat generation associated with these processes. (6.3)

Teaching Tips

During cellular respiration, our cells convert about 34% of our food energy to useful work (Module 6.3). The other 66% of the energy is heat. We use this heat to maintain a relatively steady body temperature near 37ºC (98–99ºF). This is about the same amount of heat generated by a 75-watt incandescent lightbulb. If you choose to include a discussion of heat generation from aerobic metabolism, consider the following Teaching Tip. (6.3)

Share this calculation with your students. Depending on a person’s size and level of activity, a human might burn 2,000 dietary calories (kilocalories) a day. This is enough energy to raise the temperature of 20 liters of liquid water from 0º to 100ºC. This is something to think about the next time you heat water on the stove. (Note: Consider bringing a 2-liter bottle as a visual aid, or ten 2-liter bottles to make the point above. It takes 100 calories to raise 1 liter of water 100ºC; it takes much more energy to melt ice or evaporate water as steam.) (6.3)

Figure 6.3

Figure 6.3 Summary equation for cellular respiration

6.4 Connection: The Human Body Uses Energy from A T P for All Its Activities (1 of 2)

Your body requires a continuous supply of energy.

Cellular respiration provides energy for body maintenance and voluntary activities.

A balance of energy intake and expenditure is required to maintain a healthy weight.

Student Misconceptions and Concerns

Teaching Tips

You might share with your students that it takes about 10 million ATP molecules per second to power one active muscle cell. (6.4)

6.4 Connection: The Human Body Uses Energy from A T P for All Its Activities (2 of 2)

Checkpoint question While walking at 3 mph, how far would you have to travel to “burn off” the equivalent of an extra slice of pizza, which has about 475 kcal? How long would that take?

Checkpoint Question Response

You would have to walk about 6 miles, which would take you about 2 hours. (Now you understand why the most effective exercise for losing weight is pushing away from the table!)

Student Misconceptions and Concerns

Teaching Tips

You might share with your students that it takes about 10 million ATP molecules per second to power one active muscle cell. (6.4)

Long Description:

Activity is graphed on the horizontal axis, and kilocalories consumed per hour by a 67.5 kilograms, or a 105 pound person, is graphed on the vertical axis. The points on the graph are as follows,

Activity

kilocalories consumed per hour by a 105 pound person

sitting, or writing

driving a car

dancing slow

204

walking at 3 miles per hour

245

swimming at two miles per hour

408

bicycling at ten miles per hour)

490

dancing fast

510

running at 8 or 9 miles per hour

979

6.5 Cells Capture Energy from Electrons “Falling” from Organic Fuels to Oxygen (1 of 2)

How do your cells extract energy from fuel molecules? The answer involves the transfer of electrons in chemical reactions.

Electrons removed from fuel molecules (oxidation) are transferred to N A D+ (reduction).

N A D H passes electrons to an electron transport chain. As electrons “fall” from carrier to carrier and finally to O 2, energy is released.

Student Misconceptions and Concerns

The advantage of the gradual degradation of glucose may not be obvious to some students. Many analogies reveal the advantages of a gradual process. Fuel in an automobile burns slowly to best utilize the energy released from the fuel. A few fireplace logs release gradual heat to keep a room’s temperature steady. In both situations, excessive use of fuel becomes wasteful, reducing the efficiencies of the systems. (6.5)

Teaching Tips

The use of the word “falling” when discussing the movement of electrons in a redox reaction can be confusing. Consider explaining the use of the term “falling” in reference to potential energy of a falling object. (6.5)

Active Lecture Tips

See the Activity “Demonstration of Electron Transport and ATP Production in Aerobic Respiration Using Students and Balloons” on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. (6.5)

Figure 6.5a

Figure 6.5a Movement of hydrogen atoms (with their electrons) in the redox reactions of cellular respiration

Figure 6.5b

Figure 6.5b Oxidation of an organic fuel with accompanying reduction of NAD+ to NADH

Figure 6.5c

Figure 6.5c Electrons releasing energy for ATP synthesis as they fall down an energy staircase from NADH through an electron transport chain to O2

Long Description:

N A D H becomes N A D plus and a hydrogen cation. N A D H also goes through the electron transport chain and loses two electrons. As it passes through the electron transport chain, there is a controlled release of energy for synthesis of A T P. At the end of the electron transport chain, 2 hydrogen cations, 2 electrons, and half an O 2 molecule make water.

6.5 Cells Capture Energy from Electrons “Falling” from Organic Fuels to Oxygen (2 of 2)

Checkpoint question What chemical characteristic of the element oxygen accounts for its function in cellular respiration?

Checkpoint Question Response

Oxygen is extremely electronegative (see Module 2.6), making it very powerful in pulling electrons down the electron transport chain.

Student Misconceptions and Concerns

The advantage of the gradual degradation of glucose may not be obvious to some students. Many analogies exist that reveal the advantages of a gradual process. Fuel in an automobile burns slowly to best utilize the energy released from the fuel. A few fireplace logs release gradual heat to keep a room’s temperature steady. In both situations, excessive use of fuel becomes wasteful, reducing the efficiencies of the systems. (6.5)

Teaching Tips

Active Lecture Tips

Stages of Cellular Respiration

6.6 Overview: Cellular Respiration Occurs in Three Main Stages (1 of 3)

Stage 1: Glycolysis

occurs in the cytosol,

begins cellular respiration, and

breaks down glucose into two molecules of a three-carbon compound called pyruvate.

Stage 2: Pyruvate oxidation and the citric acid cycle

take place in mitochondria,

complete the breakdown of glucose to carbon dioxide, and

supply the third stage of respiration with electrons.

Student Misconceptions and Concerns

Perhaps more than anywhere else in general biology, students studying aerobic metabolism may fail to see the forest for the trees. Students may focus on the details of each stage of aerobic metabolism and devote little attention to the overall process and products. Consider emphasizing the products and energy yields associated with glycolysis, the citric acid cycle, and oxidative phosphorylation before detailing the specifics of each reaction. (6.6–6.13)

The location within a cell in which each reaction takes place is often forgotten in the details of the chemical processes, but it is important to emphasize. Consider using Figure 6.6 as a common reference to locate each stage as you discuss the details of cellular respiration. (6.6–6.13)

Students frequently think that plants have chloroplasts instead of mitochondria. Take care to point out the need for mitochondria in plants when photosynthesis is not efficient or possible (such as during the night). (6.6–6.13)

Teaching Tips

The production of NADH and FADH2 through glycolysis and the citric acid cycle, as compared to the direct production of ATP, can get confusing for students. Help students understand that these molecules have value to be cashed in by the electron transport chain. The NADH and FADH2 can therefore be thought of as casino chips, accumulated along the way to be cashed in at the electron transport cashier. (6.6–6.13)

Active Lecture Tips

See the Activity “Cell Respiration: Pair and Share” on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. (6.6–6.13)

Figure 6.6

Figure 6.6 An overview of the three stages of cellular respiration

Long Description:

Stage 1 of cellular respiration is glycolysis. In glycolysis, which occurs in the cytosol, glucose becomes pyruvate and makes A T P. Stages 2 and 3 of cellular respiration take place in the mitochondrion. Stage 2 of cellular respiration is pyruvate oxidation, and the citric acid cycle both of which makes C O 2. The citric acid cycle makes A T P. The electrons are carried by N A D H and F A D H 2. In stage 3 of cellular respiration, oxygen is converted to water and A T P is made through oxidative phosphorylation. Oxidative phosphorylation is electron transport and chemiosmosis.

Figure 6.6_1

Figure 6.6_1 An overview of the three stages of cellular respiration (part 1: enlarged)

Long Description:

The details of the illustration are as follows:

6.6 Overview: Cellular Respiration Occurs in Three Main Stages (2 of 3)

Stage 3: Oxidative phosphorylation involves electron transport and chemiosmosis.

N A D H and a related electron carrier, F A D H2, shuttle electrons to electron transport chains embedded in the inner mitochondrial membrane.

Most of the A T P produced by cellular respiration is generated by oxidative phosphorylation.

The electrons are finally passed to oxygen, which becomes reduced to H 2 O.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

6.6 Overview: Cellular Respiration Occurs in Three Main Stages (3 of 3)

Checkpoint question Of the three main stages of cellular respiration, which one does not take place in the mitochondria?

Checkpoint Question Response

Stage 1, glycolysis, occurs in the cytosol.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

6.7 Stage 1: Glycolysis Harvests Chemical Energy by Oxidizing Glucose to Pyruvate (1 of 2)

A T P is used to prime a glucose molecule, which is split in two.

These three-carbon intermediates are oxidized to two molecules of pyruvate, yielding a net of 2 A T P and 2 N A D P H.

A T P is formed by substrate-level phosphorylation, in which a phosphate group is transferred from an organic molecule to A D P.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

Figure 6.7a

Figure 6.7a An overview of glycolysis—stage 1 of cellular respiration

Figure 6.7_1

Figure 6.7a_1 An overview of glycolysis—stage 1 of cellular respiration (part 1: enlarged)

Figure 6.7b

Figure 6.7b Substrate-level phosphorylation: transfer of a phosphate group from a substrate to ADP, producing ATP

6.7 Stage 1: Glycolysis Harvests Chemical Energy by Oxidizing Glucose to Pyruvate (2 of 2)

Checkpoint question For each glucose molecule processed, what are the net molecular products of glycolysis?

Checkpoint Question Response

Two molecules of pyruvate, two molecules of ATP, and two molecules of NADH

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

6.8 Multiple Reactions in Glycolysis Split Glucose into Two Molecules (1 of 2)

Figure 6.8 shows simplified structures for the organic compounds in the nine chemical reactions of glycolysis.

The sequential steps of glycolysis illustrate how, in a metabolic pathway, each chemical step feeds into the next. In other words, the product of one reaction serves as the reactant for the next.

Compounds that form between an initial reactant and a final product are known as intermediates.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

6.8 Multiple Reactions in Glycolysis Split Glucose into Two Molecules (2 of 2)

The energy investment phase actually consumes energy.

In this phase, two molecules of A T P are used to add a phosphate group to each glucose molecule,

which is then split into two small sugars.

Steps 1–4 consume energy.

Steps 5–9 yield energy.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

Figure 6.8_1

Figure 6.8_1 Details of glycolysis (part 1: step 1)

Long Description:

The details of the figure are as follows:

Step

Description

Diagram

Steps 1 through 3

Glucose is energized, using A T P. A sequence of three chemical reactions converts glucose to an energized intermediate. the curved arrows indicate the transfer of a phosphate group from A T P to another molecule. The cell invests 2 A T P, one at step 1 and one at step 3, to produce a more reactive molecule.

At step 1 and A T P molecule gives up a phosphate group and phosphorylates glucose. At step 3, another A T P molecule gives up a phosphate group and adds another phosphate to glucose.

Figure 6.8_2

Figure 6.8_2 Details of glycolysis (part 1: step 2)

Long Description:

The details of the figure are as follows:

The energy investment phase is described across steps 1 to 4, as follows:

Step

Description

Diagram

Steps 1 through 3

At step 1 and A T P molecule gives up a phosphate group and phosphorylates glucose. At step 3, another A T P molecule gives up a phosphate group and adds another phosphate to glucose.

Step 4

A six carbon intermediate splits into two three carbon intermediates. An enzyme splits the reactive six carbon molecule into two three carbon molecules. Each of these molecules, called glyceraldehyde 3 phosphate, G 3 P, enters the next phase, so steps 5 through 9 occur twice per glucose molecule.

At step 4, the six carbon molecule becomes two three carbon molecules.

Figure 6.8_3

Figure 6.8_3 Details of glycolysis (part 2: step 1)

Long Description:

The details of the figure are as follows:

Step

Description

Diagram

Step 5

A redox reaction generates N A D H. The curved arrow indicates the transfer of hydrogen atoms as each G 3 P is oxidized and N A D plus is reduced to N A D H. This reaction also attached a phosphate group to the substrate.

Each three carbon molecule is oxidized and phosphorylated when N A D plus is reduced to N A D H.

Figure 6.8_4

Figure 6.8_4 Details of glycolysis (part 2: step 2)

Long Description:

The details of the figure are as follows:

The energy payoff phase is described across steps 5 to 9, as follows:

Step

Description

Diagram

Step 5

Each three carbon molecule is oxidized and phosphorylated when N A D plus is reduced to N A D H.

Step 6 through 9

A T P and pyruvate are produced. This series of four chemical reactions completes glycolysis, and produce two molecules of pyruvate for each initial molecule of glucose. In steps 6 and 9, A T P is produced by substrate level phosphorylation, which a total of 4 A T P produced in the energy payoff phase. Water is produced at step 8 as a byproduct.

In step 6, two A D P molecules gain a phosphate group, and make two A T P when the three carbon molecule loses a phosphate. In step 7, the phosphate is transferred from the third carbon to the middle carbon. In step 8, a water molecule is made. In step 9, two A D P molecules gain a phosphate group and make two A T P when the three carbon molecule loses a phosphate.

6.9 Stage 2: The Citric Acid Cycle Completes the Energy-Yielding Oxidation of Organic Molecules

The oxidation of pyruvate yields acetyl C o A, C O 2, and N A D H.

For each turn of the citric acid cycle,

two carbons from acetyl C o A are added,

2 C O 2 are released, and

3 N A D H and 1 F A D H 2 are produced.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

Figure 6.9

Figure 6.9 An overview of pyruvate oxidation and the citric acid cycle—stage 2 of cellular respiration

Long Description:

In pyruvate oxidation, the pyruvate molecule reacts with N A D plus to make N A D H, carbon dioxide and, with the help of coenzyme A, Acetyl C o A. Acetyl C o A goes through the citric acid cycle and loses the C o A, gives off 2 carbon dioxide molecules, reacts with N A D plus to make NADH, reacts with A D P to make A T P, and reacts with F A D to make F A D H 2.

Figure 6.9_1

Figure 6.9_1 An overview of pyruvate oxidation and the citric acid cycle—stage 2 of cellular respiration (part 1: pyruvate)

Figure 6.9_2

Figure 6.9_1 An overview of pyruvate oxidation and the citric acid cycle—stage 2 of cellular respiration (part 1: pyruvate)

Long Description:

The details of the figure are as follows:

Acetyl C o A obtained as a result of pyruvate oxidation goes through the citric acid cycle and loses the C o A, gives off 2 carbon dioxide molecules, reacts with N A D plus to make N A D H, reacts with A D P to make A T P, and reacts with F A D to make F A D H 2.

6.10 The Multiple Reactions of the Citric Acid Cycle Finish Off the Dismantling of Glucose

Figure 6.10 shows the six major steps of the citric acid cycle.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

Figure 6.10_1

Figure 6.10_1 A closer look at the citric acid cycle (remember that the cycle runs two times for each glucose molecule oxidized) (step 1)

Long Description:

The details are as follows:

Step 1

Acetyl C o A stokes the furnace.

A turn of the citric acid cycle begins as enzymes strip the C o A portion from acetyl C o A, and combine the two carbon group that remains with the four carbon molecule oxaloacetate already present in the mitochondrion. The product of this reaction is the six carbon molecule citrate. All the acid compounds in this cycle exist in the cell in their ionized form, hence the suffix, A T E.

Figure 6.10_2

Figure 6.10_2 A closer look at the citric acid cycle (remember that the cycle runs two times for each glucose molecule oxidized) (step 2)

Long Description:

The figure shows a closer look at the first three steps of the citric acid cycle, as follows:

Step 1

Acetyl C o A stokes the furnace.

Steps 2 through 3

N A D H, A T P, and C O 2 are generated during redox reactions.

Successive redox reactions harvest energy when they strip the hydrogen atoms from citrate and alpha ketoglutarate and producing energy laden N A D H molecules. In two places, an intermediate compound loses a C O 2 molecule. Energy is harvested by substrate level phosphorylation of A D P to produce A T P. A four carbon compound called succinate emerges at the end of step 3.

Figure 6.10

Figure 6.10 A closer look at the citric acid cycle (Remember that the cycle runs two times for each glucose molecule oxidized) (step 3)

Long Description:

A closer look at the six steps of the citric acid cycle.

Step 1

Acetyl C o A stokes the furnace.

Steps 2 through 3

N A D H, A T P, and C O 2 are generated during redox reactions.

Steps 4 through 6

Further redox reactions generate F A D H 2 and more N A D H.

Succinate is oxidized as the electron carrier F A D is reduced to F A D H 2. Fumarate is converted to malate, which is then oxidized as one last N A D plus is reduced to N A D H. One turn of the citric acid cycle is completed with the regeneration of oxaloacetate, which is then ready to start the next cycle by accepting an acetyl group from acetyl C o A.

6.11 Stage 3: Visualizing the Concept: Most A T P Production Occurs by Oxidative Phosphorylation (1 of 2)

In mitochondria, electrons from N A D H and F A D H 2 are passed down the electron transport chain to O 2, which picks up H+ to form water.

Energy released by these redox reactions is used to pump H+ into the intermembrane space.

In chemiosmosis, the H+ gradient drives H+ back through the enzyme complex A T P synthase in the inner membrane, synthesizing A T P.

Student Misconceptions and Concerns

Teaching Tips

The function of the inner mitochondrial membrane is like a dam. A “reservoir” of hydrogen ions is built up between the inner and outer mitochondrial membranes, like a dam holding water. As the hydrogen ions move down their concentration gradient, they “spin” the ATP synthase, which helps generate ATP. In a dam, water rushing downhill turns giant turbines, which generate electricity. (6.11)

Active Lecture Tips

As you relate the structure of the inner mitochondrial membrane to its functions, challenge students to explain the adaptive advantage of the many folds of this inner membrane (see Figures 6.6 and 6.11). (These folds greatly increase the surface area available for the associated reactions.) (6.11)

Figure 6.11

Figure 6.11 Most ATP production occurs by oxidative phosphorylation

Long Description:

Oxidative phosphorylation happens at the membrane proteins on the inner mitochondrial membrane. The folds, or cristae, of the inner membrane enlarge its surface area, which provides space for thousands of electron transport chains and A T P synthases. Oxidative phosphorylation also has two parts, the electron transport chain and chemiosmosis. In the electron transport chain, electrons are shuttled from glycolysis, pyruvate oxidation, and the citric acid cycle are delivered to the electron transport chain. Hydrogen atoms from N A D H are carried through complex electron carrier I and become N A D plus. The N A D H travels to mobile electron carrier Q and Cyt c. Electron carrier two converts F A D H 2 to F A D. F A D H 2 also travels to mobile electron carrier Q and Cyt c. Some electron carriers pump H plus across the membrane as they transfer electrons. H plus cannot diffuse back through the membrane and its concentration gradient across the membrane stores potential energy. The electrons from Cyt c travel through electron carrier four and react with oxygen. Oxygen accepts 2 electrons and picks up 2 H plus, which form water. Here, oxygen finally steps in to play its critical role in cellular respiration. H plus cannot diffuse back through the membrane, and its concentration gradient across the membrane stores potential energy. In chemiosmosis, the flow of H plus through A T P synthase acts somewhat like a stream that rushes to a water wheel. H plus ions move one by one into binding sites, which cause the rotor to spin. The rotor turns an internal rod, which activates sites that phosphorylate A D P to A T P.

Figure 6.11_1

Figure 6.11_1 Most ATP production occurs by oxidative phosphorylation (part 1: enlarged)

Long Description:

The details of the figure are as follows:

Oxidative phosphorylation happens at the membrane proteins on the inner mitochondrial membrane. The folds, or cristae, of the inner membrane enlarge its surface area, which provides space for thousands of electron transport chains and A T P synthases. Oxidative phosphorylation also has two parts, the electron transport chain and chemiosmosis. In the electron transport chain, electrons that are shuttled from glycolysis, pyruvate oxidation, and the citric acid cycle are delivered to the electron transport chain. Hydrogen atoms from N A D H are carried through complex electron carrier I and become N A D plus. The N A D H travels to mobile electron carrier Q and Cyt c. Electron carrier two converts F A D H 2 to F A D. F A D H 2 also travels to mobile electron carrier Q and Cyt c. Some electron carriers pump H plus across the membrane as they transfer electrons. H plus cannot diffuse back through the membrane and its concentration gradient across the membrane stores potential energy. The electrons from Cyt c travel through electron carrier four and react with oxygen. Oxygen accepts 2 electrons and picks up 2 H plus, which form water. Here, oxygen finally steps in to play its critical role in cellular respiration. H plus cannot diffuse back through the membrane, and its concentration gradient across the membrane stores potential energy. In chemiosmosis, the flow of H plus through A T P synthase acts somewhat like a stream that rushes to a water wheel. H plus ions move one by one into binding sites, which cause the rotor to spin. The rotor turns an internal rod, which activates sites that phosphorylate A D P to A T P.

6.11 Stage 3: Visualizing the Concept: Most A T P Production Occurs by Oxidative Phosphorylation (2 of 2)

Checkpoint question What effect would an absence of oxygen (O 2) have on the process of oxidative phosphorylation?

Checkpoint Question Response

Without oxygen to “pull” electrons down the electron transport chain, the energy stored in NADH and FADH2 could not be harnessed for ATP synthesis.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

6.12 Scientific Thinking: Scientists Have Discovered Heat-Producing, Calorie-burning Brown Fat in Adults (1 of 3)

Mitochondria in brown fat can burn fuel and produce heat without making A T P.

Ion channels spanning the inner mitochondrial membrane

allow H+ to flow freely across the membrane and

dissipate the H+ gradient that the electron transport chain produced, which does not allow A T P synthase to make A T P.

All the energy from the burning of fuel molecules would be released as heat.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

6.12 Scientific Thinking: Scientists Have Discovered Heat-Producing, Calorie-burning Brown Fat in Adults (2 of 3)

Until recently, brown fat in humans was thought to disappear after infancy.

Recent research indicates that brown fat may be present in most people and when activated by cold, the brown fat of lean individuals is more active (burns more calories).

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

Figure 6.12

Figure 6.12 Activity level of brown fat of lean and overweight/obese participants after cold exposure

Long Description:

Average activity of brown fat is graphed on the vertical axis. One bar represents the brown fat activity of the lean group. 10 lean subjects with B M I’s less than 25 participated. The other bar represents the brown fat activity of the overweight or obese group. 14 overweight subjects with B M I’s equal to or greater than 25 participated. The brown fat activity of the lean group was at least three times greater than the brown fat activity of the overweight or obese group.

6.12 Scientific Thinking: Scientists Have Discovered Heat-Producing, Calorie-burning Brown Fat in Adults (3 of 3)

Checkpoint question The initial study discussed identified brown fat in less than 10% of the patients whose scans were analyzed. The second study identified brown fat in 96% of participants. What accounts for this difference in research results?

Checkpoint Question Response

Brown fat was activated and thus identified in response to the cold temperature treatment of the second study.

Student Misconceptions and Concerns

Teaching Tips

Active Lecture Tips

6.13 Review: Each Molecule of Glucose Yields Many Molecules of A T P

Substrate-level phosphorylation and oxidative phosphorylation produce up to 32 A T P molecules for every glucose molecule oxidized in cellular respiration.

Checkpoint question Explain where O 2 is used and C O 2 is produced in cellular respiration.

Checkpoint Question Response

O2 accepts electrons at the end of the electron transport chain. CO2 is released during the oxidation of intermediate compounds in pyruvate oxidation and the citric acid cycle.

Student Misconceptions and Concerns

Teaching Tips

As the authors note in Module 6.13, the ATP yield of up to 32 ATP per glucose molecule is only a potential. The complex chemistry of aerobic metabolism can yield this amount only under ideal conditions, when every substrate and enzyme is immediately available. Such circumstances may occur only rarely in a working cell. (6.13)

Active Lecture Tips

Figure 6.13

Figure 6.13 An estimated tally of the ATP produced per molecule of glucose by substrate-level and oxidative phosphorylation in cellular respiration

Long Description:

The steps of the cellular respiration process, and the amount of A T P produced for the corresponding step, is explained.

Step of Cellular Respiration

Amount of A T P produced

Glycolysis

2 A T P by substrate level phosphorylation

Pyruvic Oxidation and Citric Acid Cycle

2 A T P by substrate level phosphorylation

Oxidative Phosphorylation

About 28 A T P by oxidative phosphorylation

Total or maximum amount per glucose molecule

About 32 A T P

Fermentation: Anaerobic Harvesting of Energy

6.14 Fermentation Enables Cells to Produce A T P Without Oxygen (1 of 2)

Fermentation is a way of harvesting energy that does not require oxygen.

Under anaerobic conditions, muscle cells, yeasts, and certain bacteria produce A T P by glycolysis.

N A D+ is recycled from N A D H as pyruvate is reduced to

lactate (lactic acid fermentation) or

alcohol and C O 2 (alcohol fermentation).

Student Misconceptions and Concerns

Students may expect that fermentation will produce alcohol and maybe even carbon dioxide. Take the time to clarify the different possible products of fermentation and correct this general misconception. (6.14)

Teaching Tips

The text notes that some microbes are useful in the dairy industry because they produce lactic acid. However, the impact of acids on milk may not be obvious to many students. Consider a simple demonstration mixing about equal portions of milk (skim or 2%) with some acid (vinegar will work). Notice the accumulation of strands of milk curd (protein) on the side of the container and stirring device. (6.14)

Dry wines are produced when the yeast cells use up all or most of the sugar available. Sweet wines result when the alcohol accumulates enough to inhibit fermentation before the sugar is depleted. (6.14)

Exposing fermenting yeast to oxygen will slow or stop the process, because the yeast will switch back to aerobic respiration. When fermentation is rapid, the carbon dioxide produced drives away the oxygen immediately above the wine. However, as fermentation slows down, the wine must be sealed to prevent oxygen exposure and permit the fermentation process to finish. (6.14)

Figure 6.14a

Figure 6.14a Lactic acid fermentation. NAD+ is regenerated as pyruvate is reduced to lactate

Long Description:

Glucose is converted to 2 pyruvates through glycolysis. 2 A D P are converted to 2 A T P and N A D plus becomes N A D H. N A D plus is then regenerated when the pyruvates are reduced to 2 lactate. The 2 N A D plus molecules reenter the glycolysis process.

Figure 6.14b

Figure 6.14b Alcohol fermentation. NAD+ is regenerated as pyruvate is broken down to CO2 and ethanol

Figure 6.14c_1

Figure 6.14c_1 Wine barrels

Figure 6.14c_2

Figure 6.14c_2 Beer fermentation vats

6.14 Fermentation Enables Cells to Produce A T P Without Oxygen (2 of 2)

Checkpoint question A glucose-fed yeast cell is moved from an aerobic environment to an anaerobic one. For the cell to continue generating A T P at the same rate, how would its rate of glucose consumption need to change?

Checkpoint Question Response

The cell would have to consume glucose at a rate about 16 times the consumption rate in the aerobic environment (2 ATP per glucose molecule is made by fermentation versus 32 ATP by cellular respiration.)

Student Misconceptions and Concerns

Teaching Tips

6.15 Evolution Connection: Glycolysis Evolved Early in the History of Life on Earth

Glycolysis occurs in the cytosol of the cells of nearly all organisms and is thought to have evolved in ancient prokaryotes.

Checkpoint question List some of the characteristics of glycolysis that indicate that it is an ancient metabolic pathway.

Checkpoint Question Response

Glycolysis occurs universally (functioning in both fermentation and respiration), does not require oxygen, and does not occur in a membrane-enclosed organelle.

Teaching Tips

The widespread occurrence of glycolysis, which takes place in the cytosol and independent of organelles, suggests that this process had an early evolutionary origin. Since atmospheric oxygen was not available in significant amounts during the early stages of Earth’s history, and glycolysis does not require oxygen, it is likely that this chemical pathway was used by the prokaryotes in existence at that time. Students focused on the evolution of large, readily apparent structures such as wings and teeth may have never considered the evolution of cellular chemistry. (6.15)

Connections Between Metabolic Pathways

6.16 Cells Use Many Kinds of Organic Molecules as Fuel for Cellular Respiration (1 of 2)

You obtain most of your calories as

carbohydrates (such as sucrose and other disaccharide sugars and starch, a polysaccharide),

fats, and

proteins.

A cell can use these three types of molecules to make A T P.

Teaching Tips

Figure 6.16 is an important visual synthesis of the diverse fuels that can enter into cellular respiration and the various stages of this process. Figures such as this can serve as a visual anchor to integrate the many aspects of this chapter. (6.16)

Active Lecture Tips

Challenge your students to explain why most extra energy in the human body is stored as fat and not sugars or proteins. Have students exchange ideas with others seated nearby. The general answer is this. The same mass of fat stores nearly twice as many calories (about 9 kcal per gram) as an equivalent mass of protein or carbohydrates (about 4.5–5 kcal per gram). Fat is therefore an efficient way to store energy in animals and many plants. Storing 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 more than 40 pounds overweight if the same energy were stored as carbohydrates or proteins.) (6.16–6.17)

Figure 6.16

Figure 16.16 Pathways that break down various food molecules

Long Description:

Food, such as peanuts, break down into carbohydrates, fats, and proteins. Carbohydrates break down into sugars, which turn into G 3 P and then pyruvate in the glycolysis process. Glycolysis yields Acetyl C o A, which enters the citric acid cycle, and finally results in oxidative phosphorylation. Fats, become glycerol or fatty acids. Glycerol enters the glycolysis process, which again ends at oxidative phosphorylation, just like sugar. Fatty acids skip glycolysis and become Acetyl C o A, which enters citric acid cycle and results in oxidative phosphorylation. Proteins break down into amino acids, which can either become amino groups, can undergo glycolysis, or can skip glycolysis and become Acetyl C o A and result in oxidative phosphorylation.

Figure 6.16_1

Figure 16.16_1 Pathways that break down various food molecules (part 1: enlarged)

Long Description:

The details of the figure are as follows:

Food breaks down into carbohydrates, fats, and proteins. Carbohydrates break down into sugars, which turn into G 3 P and then pyruvate in the glycolysis process. Glycolysis yields Acetyl C o A, which enters the citric acid cycle, and finally results in oxidative phosphorylation. Fats become glycerol or fatty acids. Glycerol enters the glycolysis process, which again ends at oxidative phosphorylation, just like sugar. Fatty acids skip glycolysis and become Acetyl C o A, which enters citric acid cycle and results in oxidative phosphorylation. Proteins break down into amino acids, which can either become amino groups, can undergo glycolysis, or can skip glycolysis and become Acetyl C o A and result in oxidative phosphorylation.

6.16 Cells Use Many Kinds of Organic Molecules as Fuel for Cellular Respiration (2 of 2)

Checkpoint question Can a human survive on a diet consisting primarily of fats and proteins and almost no sugar?

Checkpoint Question Response

Yes. Breakdown products from fats and proteins enter the cellular respiration pathway through glycolysis and the citric acid cycle to make ATP.

Teaching Tips

Active Lecture Tips

6.17 Organic Molecules from Food Provide Raw Materials for Biosynthesis (1 of 2)

Cells use intermediates from cellular respiration and A T P for biosynthesis of other organic molecules.

Metabolic pathways are often regulated by feedback inhibition.

Student Misconceptions and Concerns

Some students may only view nutrients as sources of calories. As noted in Module 6.17, the building blocks in many nutrients are recycled into biosynthetic pathways of organic molecules. (6.17)

Teaching Tips

The final modules in this chapter may raise questions about obesity and proper diet. The Centers for Disease Control and Prevention website www.cdc.gov/nccdphp/dnpao/index.html discusses many aspects of nutrition, obesity, and general physical fitness and is a useful reference for teachers and students. (6.16–6.17)

Active Lecture Tips

Figure 6.17

Figure 6.17 Biosynthesis of organic molecules from intermediates of cellular respiration

Long Description:

A T P is needed to drive biosynthesis can go one of three ways. It can move to the citric acid cycle, Acetyl C o A, or Glucose synthesis, which consists of three parts, pyruvate, G P 3, and glucose. The citric acid cycle moves on to amino acid, where amino groups also enter. Amino acids move on to proteins, which ends at cells, tissues, and organisms. Acetyl C o A can move on to amino acids, or go towards fatty acids, which move to fats, and end at cells, tissues, and organisms. The pyruvate portion of glucose synthesis moves towards amino acids. The G 3 P of glucose synthesis transitions to Glycerol, which moves to fats, and ends at cells, tissues, and organisms. Lastly, glucose turns into sugars, which turns into carbohydrates, which ends at cells, tissues, and organisms.

Figure 6.17_1

Long Description:

The details of the figure are as follows:

A T P is needed to drive biosynthesis can go one of three ways. It can move to the citric acid cycle, Acetyl C o A, or Glucose synthesis, which consists of three parts, pyruvate, G P 3, and glucose. The citric acid cycle moves on to amino acid, where amino groups also enter. Amino acids move on to proteins which ends at cells, tissues, and organisms. Acetyl C o A can move on to amino acids, or go towards fatty acids, which move to fats, and end at cells, tissues, and organisms. The pyruvate portion of glucose synthesis moves towards amino acids. The G 3 P of glucose synthesis transitions to Glycerol, which moves to fats, and ends at cells, tissues, and organisms. Lastly, glucose turns into sugars, which turns into carbohydrates, which ends at cells, tissues, and organisms.

Figure 6.17_2

6.17 Organic Molecules from Food Provide Raw Materials for Biosynthesis (2 of 2)

Checkpoint question Explain how someone can gain weight and store fat even when on a low-fat diet.

Checkpoint Question Response

If caloric intake is excessive, body cells use metabolic pathways to convert the excess to fat. The glycerol and fatty acids of fats are made from G3P and acetyl CoA, respectively, both produced from the oxidation of carbohydrates.

Student Misconceptions and Concerns

Some students may only view nutrients as sources of calories. As noted in Module 6.17, the building blocks in many nutrients are recycled into biosynthetic pathways of organic molecules. (6.17)

Teaching Tips

Active Lecture Tips

You Should Now Be Able to (1 of 3)

Compare the processes and locations of cellular respiration and photosynthesis.

Explain how breathing and cellular respiration are related.

Provide the overall chemical equation for cellular respiration.

Explain how the human body uses its daily supply of A T P.

You Should Now Be Able to (2 of 3)

Explain how the energy in a glucose molecule is released during cellular respiration.

Explain how redox reactions are used in cellular respiration.

Describe the general roles of dehydrogenase, N A D H, and the electron transport chain in cellular respiration.

Compare the reactants, products, and energy yield of the three stages of cellular respiration.

You Should Now Be Able to (3 of 3)

Describe the special function of brown fat.

Compare the reactants, products, and energy yield of alcohol and lactic acid fermentation.

Distinguish between obligate anaerobes and facultative anaerobes.

Explain how carbohydrates, fats, and proteins are used as fuel for cellular respiration.

Figure 6.U N01

Figure 6.UN01 Reviewing the concepts, 6.3

Figure 6.U N02

Figure 6.UN02 Reviewing the concepts, 6.6

Long Description:

1. Glycolysis turn glucose into pyruvate. 2. The pyruvate oxidizes, and the citric acid cycle begins and the energy yielding oxidation Organic molecules occurs. 3. The electron transport and chemiosmosis, Oxidative Phosphorylation, occurs.

Figure 6.U N03

Figure 6.UN03 Connecting the concepts, question 1

Long Description:

Cellular respiration generates A T P energy for cellular work and has three stages as follows.

Blank, produces some A T P energy for cellular work.

Blank, produces many A T P energy for cellular work, by a process called chemiosmosis which uses H plus gradient

Cellular respiration uses the following.

Blank, and pumps H plus to create an H plus gradient.

Blank, to pull electrons down to E, blank.

Cellular respiration oxidizes glucose and organic fuels to F, blank, and G, blank.