in oxidative phosphorylation. Arrow 9 could also be ADP, but ADP is not among the answers.
7. D. Pathway B represents the Krebs cycle. The Krebs cycle uses the energy in pyruvate (arrow 2) to generate
FADH2 and NADH (arrows 6 and 7).
8. B. These molecules each have the potential to produce the following amounts of ATP: glucose, 36 ATP; pyruvate,
15 ATP; acetyl CoA, 12 ATP; NADH, 3 ATP (or 2 ATP if they originate in glycolysis); FADH2, 2 ATP. The
metabolic pathway that breaks down ethanol to H2O and CO2 in the human liver is variable. However, answer A
can be eliminated without knowing how many ATP molecules ethanol can yield because glucose produces more
ATP than does pyruvate.
9. C. When O2 is absent (or very low), anaerobic respiration (alcohol fermentation) is initiated. Alcohol fermentation
releases CO2. Photosynthesis, which would consume CO2 to produce glucose, is obviously not occurring. This
indicates that the plant activity illustrated by the graph is occurring at night (or during a heavily clouded day).
10. C. CO2 is produced in the Krebs cycle. As in the previous question, the production of CO2, rather than its
consumption, indicates that photosynthesis is not occurring, and that the plant activity is taking place at night.
11. C. Lactic acid fermentation, the conversion of pyruvate to lactate, removes electrons from NADH to make
NAD+. No ATP is generated by this step.
12. E. Protons, not electrons, pass through ATP synthase as they move down the proton gradient. A proton gradient
is the same as a pH gradient, and an electrical gradient or voltage is produced by the greater number of positive
charges (from the protons) in the intermembrane space relative to the number of positive charges inside the crista
13. D. During strenuous exercise, glucose is broken down to pyruvate. Aerobic respiration produces CO2. Anaerobic
respiration, which would occur during strenuous exercise, would increase lactate formation. Exercise would also
consume ATP, producing ADP and Pi.
14. D. The purpose of O2 is to accept the electrons at the end of the electron transport chain in oxidative phosphorylation.
The electrons, O2, and H+ form water. Products from the break down of lipids and proteins are converted to pyruvate,
acetyl CoA, or intermediate carbon compounds used in the Krebs cycle.
15. C. Oxidative phosphorylation describes the transfer of electrons from NADH and FADH2 to electron acceptors
that pump H+ across the inner mitochondrial membrane. Oxygen is required as the final electron acceptor of these
electrons. However, no CO2 is involved. In contrast, all the remaining answer choices describe processes that
release CO2. Note that answer choices B and D describe the same process.
Part I: Subject Area Reviews
Free-response questions on the AP exam might require you to provide information from a narrow area of biology, or
they might consist of parts that require you to assemble information from diverse areas of biology. The questions that
follow are typical of either an entire AP exam question or merely that part of a question that is related to this section.
Directions: Answer the questions below as completely and as thoroughly as possible. Answer the question in essay
form (NOT outline form), using complete sentences. You may use diagrams to supplement your answers, but a diagram
alone without appropriate discussion is inadequate.
1. Discuss the Krebs cycle and oxidative phosphorylation. Specifically address ATP and coenzyme production, the
location where these biosynthetic pathways occur, and chemiosmotic theory.
2. Describe, at the molecular level, how cells extract energy from starches, proteins, and lipids by the process of
3. A. Explain, at the molecular level, why many organisms need oxygen to maintain life.
B. Explain, at the molecular level, how some organisms can sustain life in the absence of oxygen.
Some Typical Answers to Free-Response Questions
A. The Krebs cycle and oxidative phosphorylation are the oxygen-requiring processes involved in obtaining ATP
from pyruvate. Pyruvate is derived from glucose through glycolysis, a process that does not require oxygen.
Before pyruvate enters the Krebs cycle, it combines with coenzyme A. During this initial reaction with coenzyme A,