Energy for Cells
Chapter
7
Outline
	7.1	Cellular Respiration 
	•	The breakdown of glucose to CO2 and H2O during cellular respiration drives the synthesis 
of ATP.98
	•	The complete breakdown of glucose requires four phases: three metabolic pathways and one individual enzymatic reaction.99
	7.2	
Outside the Mitochondria: Glycolysis 
	•	Glycolysis is a metabolic pathway that partially breaks down glucose outside the mitochondria.101
	7.3	Inside the Mitochondria
	•	The preparatory reaction and the citric acid cycle, which occur inside the mitochondria, continue the breakdown of glucose products until carbon dioxide and water result.102
	•	The electron transport chain, which receives electrons from NADH and FADH2, produces  most of the ATP during cellular respiration.104
	•	Other nutrients in addition to glucose can be broken down to drive ATP synthesis.106
	7.4	Fermentation
	•	Fermentation is a metabolic pathway that partially breaks down glucose under anaerobic conditions.107
In American society, we think a lot about weight loss. Everywhere we look, advertisements claim that some product will rev up our slow metabolism and cure our weight problems for good. In the meantime, our society has more obese individuals than ever. A “miracle cure” for being heavy often comes in the form of a very odd diet or a nutritional supplement. Have you ever considered the science behind how these products claim to work? When you do, you will see that many of these claims simply can’t be verified. Yet millions of people spend a fortune hoping for a “miracle,” and many of them suffer health problems as a result of fad diets or the use of nutritional supplements. 
Numerous nutritional supplements are supposed to allow you to eat everything you want, with no need to exercise. It sounds nice, but the reality is that our cells don’t work that way! In order to acquire the energy you need to live, cells break down glucose in a process called cellular respiration. Both the glucose and the oxygen needed for cellular respiration are provided by the process of photosynthesis in plants. In this chapter, you will learn how cells use glucose to produce the ATP they need. Understanding this process will then help you make informed decisions about weight loss, diets, and nutritional supplements.
7.1		Cellular Respiration
Whether you go skiing, take an aerobics class, or just hang out, ATP molecules provide the energy needed for your muscles to contract. ATP molecules are produced during cellular respiration, a process that requires the participation of mitochondria. Cellular respiration is aptly named because just as you take in oxygen (O2) and give off carbon dioxide (CO2) during breathing, so do the mitochondria in your cells (Fig. 7.1). In fact, cellular respiration, which occurs in all cells of the body, is the reason you breathe. 
Oxidation of substrates is a fundamental part of cellular respiration. In living things, oxidation doesn’t occur by the addition of oxygen (O2). Instead, oxidation is the removal of hydrogen atoms from a molecule. As cellular respiration occurs, hydrogen atoms are removed from glucose (and glucose products) and transferred to oxygen atoms, forming carbon dioxide (CO2) and water (H2O):  
The breakdown of glucose releases a lot of energy. If you mistakenly burn sugar in a skillet, the energy escapes into the atmosphere as heat. A cell is more sophisticated than that. In a cell, glucose is broken down slowly—not all at once—and the energy given off isn’t all lost as heat. Hydrogen atoms are removed bit by bit, and this allows energy to be captured and used to make ATP molecules. 
Phases of Complete Glucose Breakdown
The enzymes that carry out oxidation during cellular respiration are assisted by nonprotein helpers called coenzymes. As glucose is oxidized, the coenzymes NAD and FAD1 receive hydrogen atoms (H  e2) and become NADH and FADH2, respectively (Fig. 7.2). 
During these phases, notice where CO2 and H2O are produced.
	•	Glycolysis, which occurs in the cytoplasm outside the mitochondria, is the breakdown of glucose to 2 molecules of pyruvate. Oxidation results in NADH, and there is enough energy left over for a net gain of 2 ATP molecules.
	•	The preparatory (prep) reaction takes place in the matrix of mitochondria. Pyruvate is broken down to a 2-carbon acetyl group carried by coenzyme A (CoA). Oxidation of pyruvate results in not only NADH, but also CO2. 
	•	The citric acid cycle also takes place in the matrix of mitochondria. As oxidation occurs, NADH and FADH2 result and more CO2 is released. The citric acid cycle is able to produce 2 ATP per glucose molecule.
	•	The electron transport chain is a series of electron carriers in the cristae of mitochondria. NADH and FADH2  give up electrons to the chain. Energy is released and captured as the electrons move from a higher energy to a lower energy state. Later, this energy will be used for the production of ATP. Oxygen (O2 ) finally shows up here as the last acceptor of electrons from the chain. Combination with hydrogen ions (H) produces water (H2O).
7.2		Outside the Mitochondria: Glycolysis
In eukaryotes, such as plants and animals, glycolysis takes place within the cytoplasm outside the mitochondria. During glycolysis, glucose, a C6 molecule, is broken down to 2 molecules of pyruvate, a C3 molecule. Glycolysis is divided into (1) the energy-investment steps when ATP is used, and (2) the energy-harvesting steps, when NADH and ATP are produced (Fig. 7.3). 
Energy-Investment Steps
During the energy-investment steps, 2 ATP transfer phosphate groups to substrates, and 2 ADP  sP  result. In other words, ATP has been broken down, not built up. However, the phosphate groups activate the substrates so that they can undergo reactions. 
Energy-Harvesting Steps
During the energy-harvesting steps, substrates are oxidized by the removal of hydrogen atoms, and 2 NADH result.
Oxidation produces substrates with energized phosphate groups, which are used to synthesize 4 ATP. As a phosphate group is transferred to ADP, ATP results. The process is called substrate-level ATP synthesis (Fig. 7.4).
What is the net gain of ATP from glycolysis? Confirm that 2 ATP are used to get started, and 4 ATP are produced by substrate-level ATP synthesis. Therefore, there is a net gain of 2 ATP from glycolysis.
If oxygen is available, pyruvate, the end product of glycolysis, enters  mitochondria, where it undergoes further breakdown. If oxygen is not available, pyruvate undergoes reduction. In humans, if oxygen is not available, pyruvate is reduced to lactate, as discussed on page 107.
7.3		Inside the Mitochondria
The other three phases of cellular respiration occur inside the mitochondria (Fig. 7.5).
Preparatory Reaction 
Occurring in the matrix, the preparatory (prep) reaction is so called because it produces a substrate that can enter the citric acid cycle. The preparatory reaction occurs twice per glucose molecule because glycolysis results in 2 pyruvate molecules. During the prep reaction:
	•	Pyruvate is oxidized, and a CO2 molecule is given off. This is part of the CO2 we breathe out!
	•	NAD1 accepts a hydrogen atom, and NADH results. 
	•	A C2 acetyl group is attached to coenzyme A (CoA), forming acetyl-CoA.
The Citric Acid Cycle
The citric acid cycle is a cyclical metabolic pathway located in the matrix of mitochondria (Fig. 7.6). It was originally called the Krebs cycle to honor the man who first studied it. At the start of the citric acid cycle, the C2 acetyl group -carried by CoA joins with a C4 molecule, and a C6 citrate molecule results. The CoA returns to the preparatory reaction to be used again. 
During the citric acid cycle: 
	•	The acetyl group is oxidized, and the rest of the CO2 we breathe out per glucose molecule is released.
	•	Both NAD1 and FAD accept hydrogen atoms, resulting in NADH and FADH2. 
	•	Substrate-level ATP synthesis occurs (see Fig. 7.4), and an ATP results.
Because the citric acid cycle turns twice for each original glucose molecule, the inputs and outputs of the citric acid cycle per glucose molecule are as follows: 
The Electron Transport Chain
The electron transport chain located in the cristae of mitochondria is a series of carriers that pass electrons from one to the other. NADH and FADH2 deliver electrons to the chain. Consider that the hydrogen atoms attached to NADH and FADH2 consist of an e2 and an H1. The members of the electron transport chain accept only electrons (e2) and not hydrogen ions (H1).
In Figure 7.7, high-energy electrons enter the chain, and low-energy electrons leave the chain. When NADH gives up its electrons, the next carrier gains the electrons and is reduced. This oxidation-reduction reaction starts the process, and each of the carriers in turn becomes reduced and then oxidized as the electrons move down the system. As the pair of electrons is passed from carrier to carrier, energy is released and captured for ATP production. The final acceptor of electrons is oxygen (O2), the very O2 we breathe in. It’s remarkable to think that the role of oxygen in cellular respiration is to keep the electrons moving from the first to the last carrier. Why can oxygen play this role? Because oxygen attracts electrons to a greater degree than the carriers of the chain. Once oxygen accepts electrons it combines with H1, and the other end product of cellular respiration (i.e., water) results (see the equation on page 98).
When each NADH delivers electrons to the first carrier of the electron transport chain, enough energy is captured by the time the electrons are received by O2 to permit the production of three ATP molecules. When each FADH2 delivers electrons to the electron transport chain, only 2 ATP are produced.
Once NADH has delivered electrons to the electron transport chain, NAD is regenerated and can be used again. In the same manner, FAD is regenerated and can be used again. The recycling of coenzymes, and for that matter ADP, increases cellular efficiency since it does away with the need to synthesize NAD1, FAD, and ADP anew.
The Cristae of a Mitochondrion
The carriers of the electron transport chain are located in molecular complexes within the inner mitochondrial membrane. ATP synthesis is carried out by ATP synthase complexes also located in this membrane (Fig. 7.8). 
The carriers of the electron transport chain accept electrons from NADH or FADH2 and then pass them from one to the other by way of two additional mobile carriers (orange arrow). What happens to the hydrogen ions (H1) carried by NADH and FADH2? The complexes use the energy released by oxidation-reduction to pump H1 from the mitochondrial matrix into the intermembrane space located between the outer and inner membrane of a mitochondrion. The pumping of H1 into the intermembrane space establishes an unequal distribution of H1 ions; in other words, there are many H1 in the intermembrane space and few in the matrix of a mitochondrion.
The energy stored in the H1 gradient is now used to drive forward ATP synthesis. The cristae of mitochondria (like the thylakoid membrane of chloroplasts) contain an ATP synthase complex that allows H1 to return to the matrix. The flow of H1 through the ATP synthase complex brings about a conformational change, which causes the enzyme ATP synthase to synthesize ATP from ADP 1 sP . ATP leaves the matrix by way of a channel protein. This ATP remains in the cell and is used for cellular work.
Energy Yield from Glucose Metabolism
Figure 7.9 calculates the ATP yield for the complete breakdown of glucose to CO2 and H2O. Per glucose molecule, there is a net gain of 2 ATP from glycolysis, which takes place in the cytoplasm. The citric acid cycle, which occurs in the matrix of mitochondria, accounts for 2 ATP per glucose molecule. This means that a total of 4 ATP form due to substrate-level ATP synthesis outside the electron transport chain.
Most of the ATP produced comes from the electron transport chain and the ATP synthase complex. Per glucose molecule, 10 NADH and 2 FADH2 take electrons to the electron transport chain. The maximum number of ATP produced by the chain is therefore 34 ATP, and the maximum number produced by both the chain and substrate-level ATP synthesis is 38. However, for reasons beyond the scope of this book, the maximum number of ATP produced per glucose molecule in some cells is only 36 ATP or lower. A yield of 36–38 ATP represents about 40% of the available energy in a glucose molecule. The rest of the energy is lost in the form of heat.
Alternative Metabolic Pathways
Let’s say you are on a low-carbohydrate diet. Will you then run out of ATP? No, because your cells can also utilize other energy sources—the components of fats and oils, namely glycerol and fatty acids, and amino acids, which are derived from proteins (Fig. 7.10).
Because glycerol is a carbohydrate, it enters the process of cellular respiration during glycolysis. Fatty acids can be metabolized to acetyl groups, which enter the citric acid cycle. A fatty acid with a chain of 18 carbons can make three times the number of acetyl groups as does glucose. For this reason, fats are an efficient form of stored energy—there are three long fatty acid chains per fat molecule. The complete breakdown of glycerol and fatty acids to carbon dioxide and water results in many more ATP molecules per molecule than does the breakdown of glucose. 
Only the hydrocarbon backbone of amino acids, not the amino group, can be used by the cellular respiration pathways. The amino group becomes ammonia (NH3), which becomes part of urea, the primary excretory product of humans. Just where the hydrocarbon backbone from an amino acid begins degradation to produce ATP molecules depends on its length. Figure 7.10 shows that the hydrocarbon backbone from an amino acid can enter cellular respiration pathways at pyruvate, at acetyl-CoA, or during the citric acid cycle.
The smaller molecules in Figure 7.10 can also be used to synthesize larger molecules. In such instances ATP is used instead of generated. You already know that amino acids can be employed to synthesize proteins. Also, some substrates of the citric acid cycle can become amino acids through the addition of an amino group. Of the 20 most common amino acids, humans have the ability to synthesize 11 amino acids in this way, but we cannot synthesize the other 9. These nine are called the essential amino acids, meaning that they must be present in the diet or else we suffer a protein deficiency. 
Similarly, substrates from glycolysis can become glycerol, and acetyl groups can be used to produce fatty acids. When glycerol and three fatty acids join, a fat results. This explains why you can gain weight from eating carbohydrate-rich foods. 
7.4		Fermentation
Fermentation is the anaerobic breakdown of glucose resulting in the buildup of 2 ATP and a toxic end product (Fig. 7.11). During fermentation in animal cells, the pyruvate formed by glycolysis accepts 2 hydrogen atoms and is reduced to lactate. Notice in Figure 7.11 that 2 NADH pass hydrogen atoms to pyruvate, reducing it. Why is it beneficial for pyruvate to be reduced to lactate when oxygen is not available? The answer is that this reaction regenerates NAD1, which can then pick up more electrons during the earlier reactions of glycolysis. This keeps glycolysis going, during which ATP is produced by substrate-level ATP synthesis.
The 2 ATP produced by fermentation represent only a small fraction of the potential energy stored in a glucose molecule. Following fermentation, most of this potential energy is still waiting to be released. Despite its low yield of only 2 ATP, fermentation is essential. It can provide a rapid burst of ATP, and muscle cells are more apt than other cells to carry on fermentation. When our muscles are working vigorously over a short period of time, as when we run, fermentation is a way to produce ATP even though oxygen is temporarily in limited supply.
However, one of its end products, lactate, is toxic to cells. At first, blood carries away all the lactate formed in muscles. But eventually, lactate begins to build up, changing the pH and causing the muscles to “burn” and then to fatigue so that they no longer contract. When we stop running, our bodies are in oxygen deficit, as signified by the fact that we continue to breathe very heavily for a time. Recovery is complete when all the lactate is transported to the liver, where it is reconverted to pyruvate. Some of the pyruvate is oxidized completely, and the rest is converted back to glucose.
Microorganisms and Fermentation
Bacteria utilize fermentation to produce an organic acid, such as lactate, or an alcohol and CO2, depending on the type of bacterium.
Yeasts are good examples of microorganisms that generate ethyl alcohol and CO2 when they carry out fermentation. When yeast is used to leaven bread, the CO2 makes the bread rise. When yeast is used to ferment grapes for wine production or to ferment wort—derived from barley—for beer production, ethyl alcohol is the desired product. However, the yeast are killed by the very product they produce.
The inputs and outputs of fermentation are as follows:
The Chapter in Review
Summary
7.1		Cellular Respiration
During cellular respiration, glucose from food is oxidized to CO2, which we exhale. Oxygen (O2), which we breathe in, is reduced to H2O. When glucose is oxidized, energy is released. Cellular respiration captures the energy of oxidation and uses it to produce ATP molecules. The following equation gives an overview of these events:
7.2		Outside the Mitochondria: Glycolysis
Glycolysis, the breakdown of glucose to 2 molecules of pyruvate, is a series of enzymatic reactions that occur in the cytoplasm. During glycolysis:
	•	Glucose is oxidized by removal of hydrogen atoms.
	•	When NAD accepts these electrons, NADH results. 
Breakdown releases enough energy to immediately give a net gain of 2 ATP by substrate-level ATP synthesis. The inputs and outputs of glycolysis are summarized here:
When oxygen is available, pyruvate from glycolysis enters a mitochondrion.
7.3		Inside the Mitochondria
Preparatory Reaction
During the preparatory reaction in the matrix:
	•	Oxidation occurs as CO2 is removed from pyruvate. 
	•	NAD1 accepts hydrogen atoms, and NADH results.
	•	An acetyl group, the end product, combines with CoA.
This reaction takes place twice per glucose molecule.
Citric Acid Cycle
Acetyl groups enter the citric acid cycle, a series of reactions occurring in the mitochondrial matrix. During one turn of the cycle, oxidation results in 2 CO2 molecules, 3 NADH molecules, and 1 FADH. One turn also produces 1 ATP molecule. The cycle must turn twice per glucose molecule.
Electron Transport Chain
The final stage of cellular respiration involves the electron transport chain located in the cristae of the mitochondria. The chain is a series of electron carriers that accept electrons (e2) from NADH and FADH2 and pass them along until they are finally received by oxygen, which combines with H to produce water.
The carriers of the electron transport chain are located in molecular complexes on the cristae of mitochondria. These carriers capture energy from the passage of electrons and use it to pump H into the intermembrane space of the mitochondrion. When H flows down its gradient into the matrix through ATP synthase complexes, energy is released and used to form ATP molecules from ADP and sP .
Energy Yield
Of the maximum 38 ATP formed by complete glucose breakdown, 4 are the result of substrate-level ATP synthesis, and the rest are produced as a result of the electron transport chain and ATP synthase: 
Alternative Metabolic Pathways
Besides carbohydrates, glycerol and fatty acids from fats, and amino acids from proteins can undergo cellular respiration by entering glycolysis and/or the citric acid cycle. These metabolic pathways also provide substrates for the synthesis of fats and proteins. 
7.4		Fermentation
Fermentation involves glycolysis followed by the reduction of pyruvate by NADH, either to lactate or to alcohol and CO2. The reduction of pyruvate regenerates NAD1, which can accept more hydrogen atoms during glycolysis. 
	•	Although fermentation results in only 2 ATP molecules, it still provides a quick burst of ATP energy for short-term, strenuous muscular activity.
	•	The accumulation of lactate puts the individual in oxygen deficit, which is the amount of oxygen needed when lactate is completely metabolized to CO2 and H2O.
Thinking Scientifically
	1.	Occasionally, you’ll hear a news story about a bin of grain that has undergone spontaneous combustion, resulting in a spectacular fire. It may seem odd that wet grain is more likely to burn than dry grain. However, the grain contains living plant seeds that are physiologically more active when moist than when dry. In addition, the surfaces of the kernels of grain are covered with microorganisms that increase their growth rates when moist. Explain how the consumption of oxygen by these organisms can contribute to a grain-bin fire.
	2.	One of the major risk factors for diabetes in the elderly is insulin resistance, which is decreased tissue sensitivity to the action of insulin. Tissues then compensate by increasing insulin secretion. Insulin resistance can result from the accumulation of fatty acids in muscle and liver tissue. Researchers have recently found a connection between fatty acid accumulation and mitochondrial function in elderly people. Logically, what might be this connection? Using this knowledge, how might elderly people reduce their risk of diabetes?
Testing Yourself
Choose the best answer for each question.
	1.	During cellular respiration, _________ is oxidized and _________ is reduced.
	a. glucose, oxygen
	b. glucose, water
	c. oxygen, water
	d. water, oxygen
	e. oxygen, carbon dioxide
	2.	The products of cellular respiration are energy and
	a. water.
	b. oxygen.
	c. water and carbon dioxide.
	d. oxygen and carbon dioxide.
	e. oxygen and water.
	3.	During the energy-harvesting steps of glycolysis, which are produced?
	a. ATP and NADH	c. ATP and NAD
	b. ADP and NADH	d. ADP and NAD
	4.	The end product of glycolysis is
	a. phosphoenol pyruvate.	c. phosphoglyceraldehyde.
	b. glucose.	d. pyruvate.
	5.	Acetyl-CoA is the end product of
	a. glycolysis.	c. the citric acid cycle.
	b. the preparatory reaction.	d. the electron transport chain.
	6.	The citric acid cycle results in the release of
	a. carbon dioxide.	c. oxygen.
	b. pyruvate.	d. water.
	7.	The following reactions occur in the matrix of the mitochondria:
	a. glycolysis and the preparatory reaction
	b. the preparatory reaction and the citric acid cycle
	c. the citric acid cycle and the electron transport chain
	d. the electron transport chain and glycolysis
	8.	Match the descriptions below to the lettered events in the preparatory reaction and citric acid cycle.
	Pyruvate is broken down to an acetyl group.
	Acetyl group is taken up and a C6 molecule results.
	Oxidation results in NADH and CO2.
	ATP is produced by substrate-level ATP synthesis.
	Oxidation produces more NADH and FADH2.
	9.	The strongest and final electron acceptor in the electron transport chain is
	a. NADH.		c. oxygen.
	b. FADH2.		d. water.
For questions 10–16, match the items to those in the key. Answers can be used more than once, and each question can have more than one answer.
Key:
	a. glycolysis
	b. preparatory reaction
	c. citric acid cycle
	d. electron transport chain
	10.	Produces ATP. 
	11.	Uses ATP.
	12.	Produces NADH.
	13.	Uses NADH. 
	14.	Produces carbon dioxide. 
	15.	Occurs in cytoplasm. 
	16.	Occurs in mitochondria. 
	17.	The carriers in the electron transport chain undergo
	a. oxidation only.
	b. reduction only.
	c. oxidation and reduction.
	d. the loss of hydrogen ions.
	e. the gain of hydrogen ions.
	18.	The final acceptor for hydrogen atoms during fermentation is
	a. O2.	c. FAD.
	b. acetyl CoA.	d. pyruvic acid.
	19.	Which of the following do not enter the cellular respiration pathways?
	a. fats	c. nucleic acids
	b. amino acids	d. carbohydrates
	20.	When animals carry out fermentation, they produce _________, while yeasts produce _________.
	a. lactate, malate
	b. lactate, ethyl alcohol
	c. malate, ethyl alcohol
	d. malate, lactate
	e. ethyl alcohol, lactate
	21.	Fermentation does not yield as much ATP as cellular respiration does because fermentation
	a.	generates mostly heat.
	b.	makes use of only a small amount of the potential energy in glucose.
	c.	creates by-products that require large amounts of ATP to break down.
	d.	creates ATP molecules that leak into the cytoplasm and are broken down.
	22.	Which type of human cells carries on the most fermentation?
	a.	fat
	b. muscle
	c. nerve
	d. bone
	23.	Cellular respiration cannot occur without 
	a. sodium.
	b. oxygen.
	c. lactate.
	d. All of these are correct.
	24.	The metabolic process that produces the most ATP molecules is
	a.	glycolysis.	
	b.	the citric acid cycle.
	c.	the electron transport chain.	
	d.	fermentation.
	25.	The greatest contributor of electrons to the electron transport chain is
	a. oxygen.
	b. glycolysis.
	c. the citric acid cycle.
	d. the preparatory reaction.
	e. fermentation.
	26.	Substrate-level ATP synthesis takes place in
	a. glycolysis and the citric acid cycle.
	b. the electron transport chain and the preparatory reaction.
	c. glycolysis and the electron transport chain.
	d. the citric acid cycle and the preparatory reaction.
	27.	Which of the following is not true of fermentation? Fermentation 
	a. has a net gain of only 2 ATP.
	b. occurs in the cytoplasm.
	c. donates electrons to the electron transport chain.
	d. begins with glucose.
	e. is carried on by yeast.
	28.	Match the terms to their definitions. Only four of these terms are needed:
anaerobic	oxygen deficit
citric acid cycle	pyruvate
fermentation	preparatory reaction
	a. Occurs in mitochondria and produces CO2, ATP, NADH, and FADH2.
	b. Growing or metabolizing in the absence of -oxygen.
	c. End product of glycolysis.
	d. Anaerobic breakdown of glucose that results in a gain of 
2 ATP and end products such as -alcohol and lactate.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
For millennia, humans have taken advantage of a product of fermentation in microbes—ethyl alcohol. However, alcohol is toxic to human cells, and alcohol abuse is a serious problem from many perspectives. For example, a woman who consumes large amounts of alcohol during pregnancy may cause her child to have fetal alcohol syndrome. Children with this syndrome suffer from mental retardation and physical problems. Some people believe the unborn child has a right to be protected from harm and argue that intervention is justified if a pregnant woman drinks heavily despite her doctor’s orders. In extreme cases, it may be necessary to incarcerate the woman throughout the pregnancy. The alternative point of view is that every person has a right to freedom of choice, and society has no authority to intervene in the life of a pregnant woman. In addition, forcing a pregnant woman to follow medical treatment against her will, for the sake of her fetus, is imposing an obligation that we do not impose on others—that is, other members of our society are not forced to change their lifestyles purely for the sake of others. 
Do you think society has an obligation to do whatever is necessary to prevent women from drinking excessively during pregnancy? Will this lead to attempts to control the lives of pregnant women in other ways as well, such as requiring them to exercise more or to abstain from caffeine?            
Understanding the Terms
acetyl-CoA102
citric acid cycle99, 102
coenzyme A (CoA)99
electron transport chain99
fermentation107
glycolysis99
intermembrane space105
oxygen deficit107
preparatory (prep) reaction99, 102
Match the terms to these definitions:
a. _______________ First step in cellular respiration. 
b. _______________ This metabolic pathway breaks down pyruvate in the mitochondrial matrix.
c. _______________ Reduction of pyruvate when oxygen is not available.
d. _______________ Hydrogen ions are pumped into this region during the electron transport chain. 
e. _______________ Acetyl-CoA is needed during this phase of cellular respiration.
Part IIntegration and Coordination in Humans
Part IIntegration and Coordination in Humans
Check Your Progress
What are the drawbacks and benefits of fermentation? 
Answer:Drawbacks: Most of the energy in a glucose molecule is unused and it results in a toxic end product. Benefits: The 2 ATP gained can be used as a burst of energy when oxygen is not available for complete glucose breakdown. 
Check Your Progress
Explain how the electron transport chain results in the synthesis of ATP.
Answer:As electrons move from one carrier to another in the cristae, energy is released, and this energy is used to pump hydrogen ions from the matrix to the intermembrane space. The flow of hydrogen ions back down the concentration gradient into the matrix drives the synthesis of ATP by ATP synthase.
1 NAD = Nicotinamide adenine dinucleotide; FAD = Flavin adenine dinucleotide
Check Your Progress
	1.	A C2 acetyl group enters the citric acid cycle. Where does it come from?
	2.	What are the products of the citric acid cycle as a result of further breakdown of glucose? 
Answers:1. The C2 acetyl group comes from the prep reaction.2. The citric acid cycle turns twice per glucose molecule, producing 2 CO2, 3 NADH, 
1 FADH2, 1 ATP per turn.
Check Your Progress
	1.	Contrast the energy-investment steps of glycolysis with 
the energy-harvesting steps.
	2.	What happens to pyruvate when oxygen is available in 
a cell?
Answers:1. During the energy-investment steps, ATP breakdown provides the phosphate groups to activate substrates. During the energy-harvesting steps, NADH and ATP are produced.2. Pyruvate enters the mitochondria for further breakdown.
Figure 7.11Fermentation.
Fermentation consists of glycolysis followed by a reduction of pyruvate by NADH. This regenerates NAD1, which returns to the glycolytic pathway to pick up more hydrogen atoms.
Figure 7.8The organization of cristae.
Molecular complexes that contain the electron transport carriers are located in the cristae as are ATP synthase complexes. a. As electrons move from one carrier to the other, hydrogen ions (H1) are pumped from the matrix into the intermembrane space. b. As hydrogen ions flow back down a concentration gradient through an ATP synthase complex, ATP is synthesized by the enzyme ATP synthase. 
Check Your Progress
	1.	Why is breathing necessary to cellular respiration?
	2.	
Explain why glucose is broken down slowly, rather than quickly, during cellular respiration.
	3.	List the four phases of complete glucose breakdown.
Answers:1. Breathing takes in oxygen needed for cellular respiration and rids the body of carbon dioxide, a waste product of cellular respiration.2. Slow breakdown allows much of the released energy to be captured and utilized by the cell.3. Glycolysis, the preparatory reaction, the citric acid cycle, and the electron transport chain.
Figure 7.6The citric acid cycle.
The acetyl-CoA from the preparatory reaction enters the citric acid cycle. The net result of one turn of this cycle of reactions is the oxidation of the acetyl group to 2 molecules of CO2 and the formation of 3 molecules of NADH and 1 molecule of FADH2. Substrate-level ATP synthesis occurs, and the result is 1 ATP molecule. The citric acid cycle turns twice per glucose molecule.
Figure 7.4Substrate-level ATP synthesis.
The net gain of 2 ATP from glycolysis is the result of substrate-level ATP synthesis. At an enzyme’s active site, ADP acquires an energized phosphate group from a substrate, and ATP results.
Figure 7.2
The four phases of complete glucose breakdown. 
a. The enzymatic reactions of glycolysis take place in the cytoplasm. b. The preparatory reaction, (c) the citric acid cycle, and (d) the electron transport chain occur in mitochondria.
Figure 7.1Cellular respiration.
Glucose from our food and the oxygen we breathe are requirements for cellular respiration, a process completed within the mitochondria.
Figure 7.3Glycolysis.
This metabolic pathway begins with glucose and ends with pyruvate. A net gain of 2 ATP molecules can be calculated by subtracting those expended during the energy-investment steps from those produced during the energy-harvesting steps. 
Figure 7.5Mitochondrion structure and function.
A mitochondrion is bounded by a double membrane. The inner membrane invaginates to form the shelflike cristae. Glycolysis takes place in the cytoplasm outside the mitochondria. The preparatory reaction and the citric acid cycle occur within the mitochondrial matrix. The electron transport chain is located on the cristae of a mitochondrion.
Figure 7.7The electron transport chain.
An electron transport chain operates like a bucket brigade. Each electron carrier (#1–5) is alternatively reduced (orange) and oxidized as if the electrons were a bucket being passed from person to person. As oxidation-reduction occurs, energy is released that will be used to make ATP.
Figure 7.9Calculating ATP energy yield per glucose molecule.
Substrate-level ATP synthesis during glycolysis and the citric acid cycle accounts for 4 ATP. The electron transport chain produces a maximum of 34 ATP, and the maximum total is 38 ATP. Some cells, however, produce only 36 ATP per glucose molecule or even less.
Figure 7.10Alternative metabolic pathways.
All the types of food in a pizza can be used to generate ATP.
Lack of oxygen for 
cellular respiration makes muscles ache.
Carbon monoxide and cyanide kill by stopping cellular respiration.
If a diet sounds too good to be true, it probably is.