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Chapter 9: Cellular Respiration: Harvesting Chemical Energy
Overview: Before getting involved with the details of cellular respiration and photosynthesis, take a second to
look at the big picture. Photosynthesis and cellular respiration are key ecological concepts involved with energy
flow. Use Figure 9.2 to label the missing parts below.
See page 163 of your text for labeled figure.
Concept 9.1 Catabolic pathways yield energy by oxidizing organic fuels
Explain the difference between fermentation and cellular respiration.
Fermentation is a partial degradation of sugars or other organic fuel that occurs without the use of oxygen.
Cellular respiration consumes oxygen as a reactant along with the organic fuel.
Give the formula (with names) for the catabolic degradation of glucose by cellular respiration.
Organic Compounds + Oxygen  Carbon dioxide + Water + Energy
Both cellular respiration and photosynthesis are redox reactions. In redox, reactions pay attention to the flow
of electrons. What is the difference between oxidation and reduction?
In a redox reaction, the loss of electrons from one substance is called oxidation, and the addition of electrons
to another substance is known as reduction.
The following is a generalized formula for a redox reaction:
Xe– + Y  X + Ye–
Draw an arrow showing which component (X or Y) is oxidized and which is reduced.
Xe- is the reducing agent in this reaction, and Y is the oxidizing agent.
When compounds lose electrons, they lose energy; when compounds gain electrons, they gain energy.
In cellular respiration, electrons are not transferred directly from glucose to oxygen. Following the
movement of hydrogens allows you to follow the flow of electrons. The hydrogens are held in the cell
temporarily by what electron carrier? What electron carrier is hydrogen transferred to first?
Copyright © 2011 Pearson Education, Inc.
7. The correct answer to question 6 is NAD+. It is a coenzyme. What is a coenzyme? What are coenzymes? (If
you have forgotten, look back to a few pages in Chapter 8.)
An organic molecule serving as a cofactor. Most vitamins function as coenzymes in metabolic reactions.
Describe what happens when NAD+ is reduced. What enzyme is involved?
The enzymatic transfer of 2 electrons and 1 proton (H+) from an organic molecule in food to NAD+ reduces
the NAD+ to NADH; the second proton (H+) is released. The enzyme involved is dehydrogenase.
It is essential for you to understand the concept of oxidation/reduction and energy transfer. For the following
pair, which molecule is the oxidized form, and which is reduced? Which molecule holds higher potential
energy? Which is lower in potential energy?
Oxidized or Reduced?
Higher Energy/Lower Energy
Lower energy
Higher energy
10. What is the function of the electron transport chain in cellular respiration?
The electron transport chain shuttles electrons down a series of redox reactions that release energy used to
make ATP.
11. Electron transport involves a series of electron carriers.
Where are these found in eukaryotic cells? Membrane of the mitochondria
Where are these found in prokaryotic cells? Plasma membrane
12. What strongly electronegative atom, pulling electrons down the electron transport chain, is the final electron
13. Understanding the overall map of how cellular respiration works will make the details easier to learn. Use
Figure 9.6 to label the missing information in the figure that follows.
See page 167 in your text for the labeled figure.
14. Three types of phosphorylation (adding a phosphate) are covered in the text, and two of these occur in
cellular respiration. Explain how the electron transport chain is utilized in oxidative phosphorylation.
This mode of ATP synthesis is powered by the redox reactions of the electron transport chain.
Copyright © 2011 Pearson Education, Inc.
15. The second form of phosphorylation is substrate level. Label the figure below to show the direct transfer of a
phosphate from an organic substrate to ADP to form ATP.
See page 168 in your text for the labeled figure.
Concept 9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
16. What is the meaning of glycolysis? What occurs in this step of cellular respiration?
Glycosis is a series of reactions that ultimately splits glucose into pyruvate. Glycolysis occurs in almost all
living cells, serving as the starting point for fermentation or cellular respiration.
17. The starting product of glycolysis is the six-carbon sugar glucose, and the ending products are two threecarbon molecules of pyruvate.
The ten individual steps of glycolysis can be divided into two stages: energy investment and energy payoff.
These steps are shown in Figure 9.9, which details the enzymes and reactions at each of the ten steps. While
you are not expected to memorize these steps and enzymes, you should study the figure carefully. The next
few questions will help you focus your study.
The figures below show a summary of glycolysis. Label the energy investment phase below and complete
the figure. Then turn to Figure 9.9 on page 169 to find the two specific steps where ATP is used.
See page 168 in your text for the labeled figure.
19. The second step in glycolysis is the energy payoff phase. Label this phase and complete the figure. Note that
it provides both ATP and NADH. Look at Figure 9.9 to locate the two steps where ATP is formed and the
one step where NADH is formed.
See page 168 in your text for the labeled figure.
20. This final figure shows the net gain of energy for the cell after glycolysis. Most of the energy is still present
in the two molecules of pyruvate. Complete the figure below to show the net energy gains.
See page 168 in your text for the labeled figure.
21. Notice that glycolysis occurs in the cystol of the cell. Is oxygen required? No
Concept 9.3 After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic
22. To enter the citric acid cycle, pyruvate must enter the mitochondria by active transport. Three things are
necessary to convert pyruvate to acetyl CoA. Complete the missing parts of the chart below and then explain
the three steps in the conversion process.
See page 170 in your text for the labeled figure.
Copyright © 2011 Pearson Education, Inc.
a. Pyruvate’s carboxyl group, which is already fully oxidized and thus has little chemical energy, is removed
and given off as a molecule of CO2 .
b. The remaining two-carbon fragment is oxidized, forming acetate (the ionized form of acetic acid). The
extracted electrons are transferred to NAD+, storing energy in the form of NADH.
c. Finally, coenzyme A, a sulfur compound derived from a B vitamin, is attached via its sulfur atom to the
acetate, for acetyl CoA, which has a high potential energy. This molecule will now feed its acetyl group into
the citric acid cycle for further oxidation.
23. Use Figure 9.11 to help you answer the following summary questions about the citric acid cycle:
a. How many NADHs are formed? 1
b. How many total carbons are lost as pyruvate is oxidized? 3
c. The carbons have been lost in the molecule carbon dioxide.
d. How many FADH2 have been formed? 1
e. How many ATPs are formed? 1
f. How many times does the citric acid cycle occur for each molecule of glucose? 2
24. The step that converts pyruvate to acetyl CoA at the top of the diagram occurs twice per glucose. This
oxidation of pyruvate accounts for two additional reduced NADH molecules and two molecules of CO2.
25. Explain what has happened to each of the six carbons found in the original glucose molecule.
For each pyruvate molecule formed from the original glucose molecule, the pyruvate is broken down to three
CO2 molecules, including the molecule ofCO2 released during the conversion of pyruvate to acetyl CoA.
Concept 9.4 During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
26. Oxidative phosphorylation involves two components: the electron transport chain and ATP synthesis.
Referring to Figure 9.13, notice that each member of the electron transport chain is lower in free energy than
the preceding member of the chain, but higher in electronegativity. The molecule at zero free energy, which
is oxygen, is lowest of all the molecules in free energy and highest in electronegativity.
27. Oxygen is the ultimate electron acceptor. Why is this?
Oxygen is extremely electronegative.
28. Oxygen stabilizes the electrons by combining with two hydrogen ions to form what compound? H2O
29. The two electron carrier molecules that feed electrons into the electron transport system are NADH and
Copyright © 2011 Pearson Education, Inc.
30. Using Figure 9.14, explain the overall concept of how ATP synthase uses the flow of hydrogen ions to
produce ATP.
H+ ions flow down their gradient and enter binding sites within a rotor, changing the shape of each subunit
so that the rotor spins within the membrane, before leaving the rotor and passing through a second half
channel into the mitochondrial matrix. The spinning of the rotor causes an internal rod to spin, activating
catalytic sites in the knob that produce ATP.
31. What is the role of the electron transport chain in forming the H+ gradient across the inner mitochondrial
Certain members of the electron transport chain accept and release protons (H+) along with electrons. (The
aqueous solutions inside and surrounding the cell are a ready source of H+.) At certain steps along the chain,
electron transfers cause H+ to be taken up and released into the surrounding solution. In eukaryotic cells, the
electron carriers are spatially arranged in the inner mitochondrial membrane in such a way that H+ is
accepted from the mitochondrial matrix and deposited in the intermembrane space. The H+ gradient that
results is referred to as a proton-motive force, emphasizing the capacity of the gradient to perform work. The
force drives H+ back across the membrane through the H+ channels provided by ATP synthases.
32. Two key terms are chemiosmosis and proton-motive force. Relate both of these terms to the process of
oxidative phosphorylation.
Chemiosmosis refers to the energy-coupling mechanism that uses energy stored in the form of a hydrogen
ion gradient across a membrane to drive cellular work, such as the synthesis of ATP.
Proton-motive force refers to the potential energy stored in the form of a proton electrochemical gradient,
generated by the pumping of hydrogen ions (H+) across a biological membrane during chemiosmosis.
33. Figure 9.15 will help you understand the production of ATP in the mitochondria. Label the figure below to
study this process. Then, use one color to trace the flow of electrons and another color to show the flow of
See page 175 in your text for the labeled figure.
34. At this point, you should be able to account for the total number of ATPs that could be formed from a
glucose molecule. To accomplish this, we have to add the ATPs formed by substrate-level phosphorylation
in glycolysis and the citric acid cycle to the ATPs formed by chemiosmosis. Each NADH can form a
maximum of 3 ATP molecules. Each FADH2, which donates electrons that activate only two proton pumps,
makes 2 ATP molecules.
35. Label this figure to show the processes of cellular respiration. Then, show the production of NADH and
FADH2. Finally, show where ATP is formed, and indicate whether it is by substrate-level or oxidative
phosphorylation. Use the text to be sure you understand how each subtotal on the bar below the figure is
See page 176 in your text for labeled figure.
36. Why is the total count about 36 or 38 ATP molecules rather than a specific number?
There are three reasons we cannot state an exact number of ATP molecules generated by the breakdown of
Copyright © 2011 Pearson Education, Inc.
one molecule of glucose. First, phosphorylation and the redox reactions are not directly coupled to each
other, so the ratio of the number of NADH molecules to the number of ATP molecules is not a whole
number. Second, the ATP yield varied slightly, depending on the type of shuttle used to transport electrons
from the cytosol into the mitochondrion. A third variable that reduces the yield of ATP is the use of the
proton-motive force generated by the redox reactions of respiration to drive other kinds of work.
*Inconsistency: Question 36 counts about 36 or 38 ATP molecules, while Figure 9.16 and the text suggest 30
or 32.
Concept 9.5 Fermentation enables some cells to produce ATP without the use of oxygen
37. Fermentation allows for the production of ATP without using either oxygen or any electron transport chain.
38. For aerobic respiration to continue, the cell must be supplied with oxygen—the ultimate electron acceptor.
What is the electron acceptor in fermentation?
39. Alcohol fermentation starts with glucose and yields ethanol. Explain this process, and be sure to describe
how NAD+ is recycled.
In alcohol fermentation pyruvate is converted to ethanol (ethyl alcohol) in two steps. The first step releases
carbon dioxide from the pyruvate, which is converted to the two-carbon compound acetaldehyde. In the
second step, acetaldehyde is reduced by NADH to ethanol. This regenerates the supply of NAD needed for
the continuation of glycolysis. Many bacteria carry out alcohol fermentation under anaerobic conditions.
Yeast (a fungus) also carries out alcohol fermentation.
39. Lactic acid fermentation starts with glucose and yields lactate. Explain this process, and be sure to describe
how NAD+ is recycled.
During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate as an end product,
with no release of CO2. (Lactate is the ionized form of lactic acid.) Lactic acid fermentation by certain fungi
and bacteria is used in the dairy industry to make cheese and yogurt. Pyruvate, the end product of glycolysis,
serves as an electron acceptor for oxidizing NADH back to NAD+, which can then be reused in glycolysis.
40. Label the figure below and then explain why pyruvate is a key juncture in metabolism.
See page 179 in your text for the labeled figure.
Glycolysis is common to fermentation and cellular respiration. The end product of glycolysis, pyruvate,
represents a fork in the catabolic pathways of glucose oxidation. In a facultative anaerobe or a muscle cell,
which are capable of both aerobic cellular respiration and fermentation, pyruvate is committed to one of
those two pathways, usually depending on whether or not oxygen is present.
Copyright © 2011 Pearson Education, Inc.
Concept 9.6 Glycolysis and the citric acid cycle connect to many other metabolic pathways
41. What three organic macromolecules are often utilized to make ATP by cellular respiration?
Possible examples include fats, proteins, sucrose, and starch.
42. Explain the difference in energy usage between the catabolic reactions of cellular respiration and anabolic
pathways of biosynthesis.
Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration. Many
carbohydrates can enter glycolysis, most often after conversion to glucose. Amino acids of proteins must be
deaminated before being oxidized. The fatty acids of fats undergo beta oxidation to two-carbon fragments
and then enter the citric acid cycle as acetyl CoA. Anabolic pathways can use small molecules from food
directly or build other substances using intermediates of glycolysis or the citric acid cycle.
43. Study Figure 9.20 in your text. Explain how AMP stimulates cellular respiration while citrate and ATP
inhibit it.
If the cell is working hard and its ATP concentration begins to drop, respiration speeds up. When there is
plenty of ATP to meet demand, respiration slows down, sparing valuable organic molecules for other
44. Phosphofructokinase is an allosteric enzyme that catalyzes an important step in glycolysis. Explain how this
step is a control point in cellular respiration.
As ATP accumulates, inhibition of the enzyme slows down glycolysis. The enzyme becomes active again as
cellular work converts ATP to ADP (and AMP) faster than ATP is being regenerated. This mechanism helps
synchronize the rates of glycolysis and the citric acid cycle.
Test Your Understanding Answers
Now you should be ready to test your knowledge. Place your answers here:
1. d
2. c
3. c
4. a
5. e
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6. a
7. b