Ch 9 Cellular Respiration: Harvesting Chemical Energy

Review Questions
1. Starting with glucose (C6H12O6), write the overall reactions for aerobic respiration.
C6H12O6 + 6O2 → 6H2O + 6 CO2 + 38 ATP
2. How is aerobic cellular respiration the opposite of photosynthesis?
C6H12O6 + 6O2 → 6H2O + 6 CO2 + 38 ATP CELLULAR RESPIRATION

6H2O + 6 CO2 + sunlight → C6H12O6 + 6O2 PHOTOSYNTHESIS

If we look at the two reactions closely we notice that the products for cellular
respiration are the reactants for photosynthesis and the products for
photosynthesis are the reactants for cellular respiration. Basically the two
reactions are the reverse of each other.
3. Name the five stages of cellular respiration.
Cells that completely break down glucose into carbon dioxide and water during
aerobic cellular respiration have the following five stages: glycolysis, transition
reaction, Krebb’s cycle, electron transport chain (system) and chemiosmosis.
4. Where in the cell does glycolysis take place?
All cells whether they have mitochondria or not have glycolysis occurring within
the cytosol of the cell. Only those cells that utilize oxygen continue to extract
energy from the pyruvate formed from glycolysis.
5. How much ATP is required to split a glucose molecule in half?
An initial input of two (2) ATP molecules is required to start the process of
glycolysis.
6. What is the net gain in ATP at the end of glycolysis?
At the end of glycolysis there are four ATP molecules formed, but there will only
be a net gain of two(2) ATP molecules since two ATP molecules were used in
the energy investment stage of glycolysis.
7. What is the difference between fermentation and aerobic cellular respiration?
“One catabolic process, fermentation, is a partial degradation of sugars that
occurs without the use of oxygen. However, the most prevalent and efficient
catabolic pathway is cellular respiration, in which oxygen is consumed as a
reactant along with the organic fuel (aerobic is from the Greek aer, air and bios,
life). The cells of most eukaryotic and many prokaryotic organisms cary out
aerobic cellular respiration.” (Text quoted from page 163 of the textbook)
8. Which produces more ATP per glucose molecule, fermentation or aerobic cellular
respiration?
During aerobic respiration a glucose molecule can yield 38 ATP molecules while
the process of glycolysis yields only 2 ATP molecules for each glucose molecule
consumed. Thus it is easy to see that Aerobic respiration yields 19 times more
ATP than fermentation.
9. Identify the products for each of the stages of cellular respiration.
Glycolysis forms the products of pyruvate which still have some potential energy
that can be extracted during the subsequent stages of cellular respiration. In
addition to the pyruvate, 2 NADH are formed and enter the electron transport
chain and a net gain of 2 ATP molecules also are produced in glycolysis.
During the transition reaction, each pyruvate is converted into Acetyl Coenzyme
A and a carbon dioxide molecule is formed. In addition a molecule of NADH is
also formed for each pyruvate molecule during the transition reaction. This
 NADH also enters the electron transport chain in the inner mitochondrial
membrane. Each acetyl coenzyme A that enters the Kreb’s cycle produces 1
ATP by substrate phosphorylation, 2 Carbon dioxide (CO2) molecules, NADH
and FADH2. Both the NADH and FADH2 enter the electron transport chain of the
inner mitochondrial membrane. In the electron transport chain, water is formed
and finally in chemiosmosis, 34 ATP are formed.
10. During which stage of cellular respiration is carbon dioxide first formed?
Carbon dioxide is first formed during the transition reaction where pyruvate is
converted to acetyl coenzyme A.
11. How does fermentation produce lactic acid?
“During lactic acid fermentation (Figure 9.18b), 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.” (Text quoted from page
178 of the textbook)
12. How do yeast cells produce alcohol?
“In alcohol fermentation (Figure 9.18a), 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. For thousands of years, humans have used yeast in
brewing, winemaking, and baking. The CO2 bubbles generated by baker’s yeast
allow bread to rise.” (Text quoted from page 178 of the textbook)
13. What events occur during the transition reaction?
During the transition reaction, carbon dioxide is formed as NAD removes
hydrogen atoms and their associated electrons forming NADH.
14. Where inside the cell does the transition reaction occur?
Pyruvate from glycolysis enters the mitochondrion by active transport. This
process takes place in the matrix of the mitochondrion.
15. What roles do NAD+ and FAD+ play in cellular respiration?
“At this point, molecules of NADH (and FADH2) account for most of the energy
extracted from the food. These electron escorts link glycolysis and the citric acid
cycle to the machinery of oxidative phosphorylation, which uses energy released
by the electron transport chain to power ATP synthesis.” (Text quoted from page
172 of the textbook)
16. How much ATP and carbon dioxide are produced during the Krebs cycle?
During each turn of the Kreb’s cycle, 2 CO2 molecules are formed and one ATP
molecule is formed by substrate-level phosphorylation.
17. Where inside the cell does the Krebs cycle occur?
The Kreb’s cycle takes place within the matrix of the mitochondrion.
18. What is the location of the electron transport chain?
The electron transport chain takes place within the inner mitochondrial
membrane.
19. What happens in the electron transport chain?
“During electron transport along the chain, electron carriers alternate between
reduced and oxidized states as they accept and donate electrons. Each
component of the chain becomes reduced when it accepts electrons from its
“uphill” neighbor, which has a lower affinity for electrons (is less electronegative).
 It then returns to its oxidized form as it passes electrons to its “downhill,” more
electronegative neighbor.” (Text quoted from page 172 of the textbook)

“The electron transport chain makes no ATP directly. Its function is to ease the
fall of electrons from food to oxygen, breaking a large free-energy drop into a
series of smaller steps that release energy in manageable amounts. How does
the mitochondrion (or prokaryotic plama membrane) couple this electron
transport and energy release to ATP synthesis? The answer is a mechanism
called chemiosmosis.” (text quoted from page 173 of the textbook)
20. Why is oxygen needed for cellular respiration?
“The last cytochrome of the chain, cyt a3, passes its electrons to oxygen, which is
very electronegative. Each oxygen atom also picks up a pair of hydrogen ions
from the aqueous solution, forming water.” (Text quoted from page 173 of the
textbook)
21. What happens in chemiosmosis?
“Populating the inner membrane of the mitochondrion are many copies of a
protein complex called ATP synthase, the enzyme that actually makes ATP from
ADP and inorganic phosphate.. ATP synthase works like an ion pump running in
reverse. Recall from Chapter 7 that ion pumps use ATP as an energy source to
transport ions against their gradients. In fact, the proton pump shown in Figure
7.19 is an ATP synthetase.” (text quoted from page 173 of the textbook)

Rather than hydrolyzing ATP to pump protons against their concentration

gradient, under the conditions fo cellular respiration, ATP synthase uses the
energy of an existing ion gradient to power ATP synthesis. The power source for
ATP synthetase is a difference in the concentration of H+ on opposite sides of
the inner mitochondrial membrane. (We can also think of this gradient as a
difference in pH, since pH is a measure of H+ concentration.) This process, in
which energy stored in the form of a hydrogen ion gradient across a membrane is
used to drive cellular work such as the synthesis of ATP, is called chemiosmosis
(from the Greek osmos, push).” (Text quoted from page 173 of the textbook)
22. How many ATP molecules are produced in chemiosmosis?
Chemiosmosis produces the majority of ATP formed during cellular respiration.
There are 34 ATP molecules formed through the oxidative phosphorylation
during chemiosmosis.
23. How many ATP molecules are produced from all five stages of cellular respiration?
A total of 38 ATP molecules are formed from the five stages of cellular
respiration. A net of 2ATP molecules result from glycolysis, 2 from the Kreb’s
cycle taking place twice and 34 ATP from chemiosmosis. Both glycolysis and the
Kreb’s cycle form ATP by substrate level phosphorylation while chemiosmosis
forms ATP by oxidative phosphorylation.
24. What happens to the excess of nitrogenous refuse from the digestion of proteins
and amino acids?
“Proteins can also be used for fuel, but they must first be converted into their
constituent amino acids. Many of the amino acids, of course are used by the
organism to build new proteins. Amino acids in excess are converted by
enzymes to intermediates of glycolysis and eh citric acid cycle. iBefore amino
acids can feed into glycolysis or the citric acid cycle, their amino groups must be
removed, a process called deamination. The nitrogenous refuse is excreted from
the animal in the form of ammonia, urea, or other waste products.” (Text quoted
from page 180 of the textbook)
25. For each glucose molecule processed in cells, what are the net molecular products
of glycolysis?
The net products from glycolysis are 2 pyruvate, 2 ATP and 2 NADH molecules.
26. A glucose fed yeast cell is removed from an aerobic environment to an anaerobic
one. If the cell continues to generate ATP at the same rate, how will the rate of
glucose consumption compare to the consumption in the aerobic environment?
If a yeast cell were fermenting glucose, it would have to consume glucose 19
times faster than the same yeast cell breaking down glucose in cellular
respiration. Remember that only a net of 2 ATP molecules result from glycolysis,
while 38 ATP molecules can be formed from complete oxidation of glucose by
cellular respiration.

KEY TERMS
Acetyl CoA FAD
Aerobic Facultative anaerobe
Alcohol fermentation Glycolysis
Anaerobic respitration Lactic acid fermentation
ATP synthetase NAD
Cellular respiration Obligate anaerobe
Chemiosmosis Oxidation
Citric acid cycle Oxidative phosphorylation
Cytochrome Pyruvate
Deamination Reduction
Electron transport chain Substrate level phosphorylation
Fermentation
Concept check 9.3 p 172
Concept check 9.4 p 177
Concept check 9.5 p 179
Concept check 9.6 p 182

Questions 2, 3, 4, 5, 6, 7, 8, 9, and 10 Pages 183 and 184