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Clarification - Respiration (5 of 5) questions
1. What compound couples glycolysis to acetyl CoA formation?
ANS: pyruvate
Pyruvate couples glycolysis to acetyl CoA formation. It is an output of glycolysis and an input to acetyl CoA
formation. If acetyl CoA formation were to stop for some reason, glycolysis would also stop because of an
accumulation of pyruvate (unless another process utilized the pyruvate, as is the case with fermentation).
Wrong Answers for RCNEFF17934
%
Answer Response
Wrong
For a compound to couple two processes, it must be an output from one process and an
33.3%
NAD+
input to the other. NAD+ is an input to both glycolysis and acetyl CoA formation, but is it an
output from either stage?
For a compound to couple two processes, it must be an output from one process and an
33.3%
NADH input to the other. NADH is an output from both glycolysis and acetyl CoA formation, but is it
an input to either stage?
For a compound to couple two processes, it must be an output from one process and an
33.3%
glucose input to the other. Glucose is an input to glycolysis, but is it also an output from acetyl CoA
formation?
The following table summarizes the inputs and outputs of the stages of cellular respiration. Note that FAD and
FADH2 are not included in this table.
Glycolysis
Acetyl CoA Formation
and
the Citric Acid Cycle
Oxidative
Phosphorylation
Inputs
Outputs
Inputs
Outputs
Inputs
Outputs
glucose
pyruvate
pyruvate
CO2
O2
water
NAD+
NADH
NAD+
NADH
NADH
NAD+
ADP + Pi
ATP
ADP + Pi
ATP
ADP + Pi
ATP
2. Drag the labels on the left onto the diagram to identify the compounds that couple each stage. Labels
may be used once, more than once, or not at all.
See figure 9.11 (p. 170) in the textbook.
Locate the following:
NAD+/NADH,
ATP/ADP + Pi,
pyruvate,
CO2
The main coupling among the stages of cellular respiration is accomplished by NAD+ and NADH. In the first three stages,
NAD+ accepts electrons from the oxidation of glucose, pyruvate, and acetyl CoA. The NADH produced in these redox
reactions then gets oxidized during oxidative phosphorylation, regenerating the NAD+ needed for the earlier stages.
3. What compound couples oxidative phosphorylation to acetyl CoA formation?
ANS: NAD+
A lack of oxygen has a direct effect on oxidative phosphorylation—it stops electron transport and NADH oxidation. Thus,
there would be no production of NAD+ (an output from oxidative phosphorylation and an input to acetyl CoA formation).
Without NAD+ as an input, acetyl CoA production cannot continue.
Wrong Answers for RCNEFF17934
% Wrong Answer
Oxygen is required to convert glucose to
46.7%
pyruvate in glycolysis.
Without oxygen, no pyruvate can be made.
26.7%
Oxygen is an input to acetyl CoA formation.
26.7%
ATP is needed to convert pyruvate to acetyl
CoA. Without oxygen, no ATP can be made in
oxidative phosphorylation.
Response
Oxygen is not required for glycolysis
(otherwise it would be an input to that stage).
Oxygen is only an input to oxidative phosphorylation.
Oxygen is not an input to acetyl CoA formation.
Oxygen is only an input to oxidative phosphorylation.
ATP is not required for acetyl CoA formation
(otherwise it would be an input to that stage).
4. What happens to the amount of ATP in your muscle cells in that first second, as you leap out of the
room, slamming the door behind you?
ANS: The cellular ATP level decreases.
Which statement correctly describes how this increased demand would lead to an increased rate of ATP
production?
ATP levels would fall at first, decreasing the inhibition of PFK and increasing the rate of ATP production.
An increased demand for ATP by a cell will cause an initial decrease in the level of cellular ATP. Lower ATP decreases the
inhibition of the PFK enzyme, thus increasing the rate of glycolysis, cellular respiration, and ATP production. It is the
initial decrease in ATP levels that leads to an increase in ATP production.
Wrong Answers for RCNEFF17934
% Wrong Answer
40%
33.3%
26.7%
Response
If the demand for ATP exceeds supply, will
ATP levels would rise at first, increasing the inhibition the level of cellular ATP initially rise or fall?
of PFK and increasing the rate of ATP production.
How will this then affect the activity of PFK
and the rate of cellular respiration?
You are correct that ATP levels would initially
ATP levels would fall at first, increasing the inhibition of fall, but how would this affect the activity of
PFK and increasing the rate of ATP production.
PFK? If high levels of ATP inhibit PFK, what
would be true for low ATP levels?
If the demand for ATP exceeds supply, will
ATP levels would rise at first, decreasing the inhibition the level of cellular ATP initially rise or fall?
of PFK and increasing the rate of ATP production.
How will this then affect the activity of PFK
and the rate of cellular respiration?
5. How does the amount of ATP produced by fermentation compare to the amount produced by aerobic
respiration?
ANS: Fermentation produces less than 10% of the amount of ATP produced by aerobic respiration.
Fermentation produces a net of only 2 molecules of ATP per glucose, while the entire process of cellular respiration produces
about 36 to 38 ATP per glucose. This is only about 6% of the total amount of ATP produced in cellular respiration.
ATP made during fermentation comes from glycolysis, which produces a net of only 2 ATP per glucose molecule. In contrast,
aerobic cellular respiration produces about 36 ATP per glucose molecule. To meet the same ATP demand under anaerobic
conditions as under aerobic conditions, a cell’s rate of glycolysis and glucose utilization must increase nearly 20-fold.
Wrong Answers for RCNEFF17934
%
Answer
Wrong
33.3%
Glucose utilization would decrease a little.
33.3%
Glucose utilization would increase a little.
20%
Glucose utilization would remain the same.
13.3%
Glucose utilization would decrease a lot.
Response
Fermentation produces less than 10% of the amount
of ATP produced during aerobic cellular respiration
(2/36). Thus, if a cell’s demand for ATP remains the
same, its rate of glucose utilization must increase by
about 95%.
Fermentation produces less than 10% of the amount
of ATP produced during aerobic cellular respiration
(2/36). Thus, if a cell’s demand for ATP remains the
same, its rate of glucose utilization must increase by
about 95%.
Fermentation produces less than 10% of the amount
of ATP produced during aerobic cellular respiration
(2/36). Thus, if a cell’s demand for ATP remains the
same, its rate of glucose utilization must increase by
about 95%.
Fermentation produces less than 10% of the amount
of ATP produced during aerobic cellular respiration
(2/36). Thus, if a cell’s demand for ATP remains the
same, its rate of glucose utilization must increase by
about 95%.
FROM AP CURRICULUM:
Essential knowledge 2.A.1: All living systems require constant input of free energy.
a. Life requires a highly ordered system. Evidence of student learning is a demonstrated understanding of
each of the following:
1. Order is maintained by constant free energy input into the system.
2. Loss of order or free energy flow results in death.
3. Increased disorder and entropy are offset by biological processes that maintain or increase order.
b. Living systems do not violate the second law of thermodynamics, which states that entropy increases
over time. Evidence of student learning is a demonstrated understanding of each of the following:
1. Order is maintained by coupling cellular processes that increase entropy (and so have negative changes in
free energy) with those that decrease entropy (and so have positive changes in free energy).
2. Energy input must exceed free energy lost to entropy to maintain order and power cellular processes.
3. Energetically favorable exergonic reactions, such as ATP→ADP, that have a negative change in free energy
can be used to maintain or increase order in a system by being coupled with reactions that have a positive free
energy change.
c. Energy-related pathways in biological systems are sequential and may be entered at multiple points in
the pathway. [See also 2.A.2]
To foster student understanding of this concept, instructors can choose an illustrative example such as: Krebs
cycle, Glycolysis, Calvin cycle, Fermentation
Essential knowledge 2.A.2: Organisms capture and store free energy for use in biological processes.
b. Heterotrophs capture free energy present in carbon compounds produced by other organisms.
Evidence of student learning is a demonstrated understanding of each of the following:
1. Heterotrophs may metabolize carbohydrates, lipids and proteins by hydrolysis as sources of free energy.
2. Fermentation produces organic molecules, including alcohol and lactic acid, and it occurs in the absence of
oxygen.
✘✘ Specific steps, names of enzymes and intermediates of the pathways for these processes are
beyond the scope of the course and the AP Exam.
c. Different energy-capturing processes use different types of electron acceptors.
To foster student understanding of this concept, instructors can choose an illustrative example such as:
• NADP+ in photosynthesis
• Oxygen in cellular respiration
f. Cellular respiration in eukaryotes involves a series of coordinated enzyme-catalyzed reactions that
harvest free energy from simple carbohydrates.
Evidence of student learning is a demonstrated understanding of each of the following:
1. Glycolysis rearranges the bonds in glucose molecules, releasing free energy to form ATP from ADP and
inorganic phosphate, and resulting in the production of pyruvate.
2. Pyruvate is transported from the cytoplasm to the mitochondrion, where further oxidation occurs.
3. In the Krebs cycle, carbon dioxide is released from organic intermediates ATP is synthesized from ADP and
inorganic phosphate via substrate level phosphorylation and electrons are captured by coenzymes.
4. Electrons that are extracted in the series of Krebs cycle reactions are carried by NADH and FADH2 to the
electron transport chain.
✘✘Memorization of the steps in glycolysis and the Krebs cycle, or of the structures of the molecules and
the names of the enzymes involved, are beyond the scope of the course and the AP Exam.
g. The electron transport chain captures free energy from electrons in a series of coupled reactions that
establish an electrochemical gradient across membranes.
Evidence of student learning is a demonstrated understanding of each of the following:
1. Electron transport chain reactions occur in chloroplasts (photosynthesis), mitochondria (cellular respiration)
and prokaryotic plasma membranes.
2. In cellular respiration, electrons delivered by NADH and FADH2 are passed to a series of electron acceptors
as they move toward the terminal electron acceptor, oxygen. In photosynthesis, the terminal electron acceptor is
NADP+.
3. The passage of electrons is accompanied by the formation of a proton gradient across the inner mitochondrial
membrane or the thylakoid membrane of chloroplasts, with the membrane(s) separating a region of high proton
concentration from a region of low proton concentration. In prokaryotes, the passage of electrons is
accompanied by the outward movement of protons across the plasma membrane
4. The flow of protons back through membrane-bound ATP synthase by chemiosmosis generates ATP from ADP
and inorganic phosphate.
5. In cellular respiration, decoupling oxidative phosphorylation from electron transport is involved in
thermoregulation.
✘✘ The names of the specific electron carriers in the ETC are beyond the scope of the course and the AP Exam.
h. Free energy becomes available for metabolism by the conversion of ATP→ADP, which is coupled to
many steps in metabolic pathways.