AP Biology: Unit 3, Cellular Respiration Overview (Drill 11)

Question 1. Which feature of ATP best explains why it serves as the immediate energy donor for cellular work, rather than cells drawing energy directly from glucose?

• A) ATP hydrolysis releases no usable free energy at all, so cells rely on ATP mainly as a molecule for storing genetic information rather than for powering cellular work.
• B) ATP is synthesized in the nucleus, where most energy-requiring reactions occur.
• C) ATP contains more total chemical energy per molecule than glucose, making it a more concentrated fuel source.
• D) ATP hydrolysis releases a small, manageable packet of free energy (~7.3 kcal/mol) that can be directly coupled to endergonic cellular reactions such as active transport, biosynthesis, and motor protein movement. ✓

Explanation: Correct answer: D. Cells require energy in controlled, precisely sized increments. ATP hydrolysis releases ~7.3 kcal/mol -- a packet of energy well-matched to the requirements of individual molecular events: pumping an ion against its concentration gradient, forming a peptide bond, or driving a conformational change in a motor protein. This allows energy to be transacted one reaction at a time. Glucose, by contrast, contains ~686 kcal/mol of total chemical energy; releasing it all at once would be uncontrollable and wasteful. Glucose is broken down stepwise through metabolic pathways precisely to capture that energy in the manageable currency of ATP. (A) is incorrect; glucose is the primary fuel source available to virtually all cells and is universally present where metabolism is occurring. (B) is incorrect; ATP is synthesized in the cytoplasm (glycolysis) and in mitochondria, not the nucleus, and energy-requiring reactions occur throughout the cell. (C) is incorrect and reverses the correct relationship; a single glucose molecule contains far more total chemical energy than a single ATP molecule -- ATP is not a more concentrated fuel but a more manageable transactional unit.

Question 2. Based on Table 1, why can skeletal muscle sustain maximum-effort contractions for only a few seconds before force production drops?

• A) Oxidative phosphorylation cannot produce ATP fast enough to sustain maximum effort, and stored ATP and PCr are depleted within seconds.
• B) Stored ATP (5-8 mmol/kg) and phosphocreatine (15-17 mmol/kg) are the only sources with resynthesis rates fast enough for maximum contraction, and both are exhausted within a few seconds at maximum effort. ✓
• C) Anaerobic glycolysis is the primary ATP source during maximum effort but cannot sustain contraction because lactic acid inhibits glycolytic enzymes within seconds.
• D) The mitochondria are physically destroyed by the mechanical force generated during maximum muscle contractions, halting aerobic ATP production within seconds.

Explanation: Correct answer: B. The data in Table 1 show that the fastest ATP resynthesis rates belong to phosphocreatine (PCr) at 9.0 mmol/kg/s and anaerobic glycolysis at 4.5 mmol/kg/s. Maximum-effort contractions require ATP faster than oxidative phosphorylation (1.5 mmol/kg/s) can supply. The cell therefore draws on stored ATP (5-8 mmol/kg) and PCr (15-17 mmol/kg) -- totaling ~20-25 mmol/kg. At a very high ATP demand rate, these stores are exhausted within approximately 2-5 seconds, after which force must drop. Recovery takes longer because PCr must be replenished by oxidative phosphorylation during rest. (A) is correct in principle but incomplete -- it mentions only oxidative phosphorylation's slowness without fully explaining that both stored ATP and PCr are also depleted; B is more complete. (C) is partially true -- lactate accumulation does eventually impair glycolysis -- but the primary reason for the initial rapid fatigue within a few seconds is PCr and stored ATP depletion, not immediate lactate inhibition. (D) is biologically implausible; mitochondria are not physically destroyed by normal muscle contraction forces.

Question 3. The partial recovery after a brief rest, and more complete recovery after 3-5 minutes, is best explained by which sequence of events?

• A) Brief rest: aerobic respiration partially restores PCr; full rest: complete PCr resynthesis via oxidative phosphorylation and clearance of metabolic byproducts such as lactate and Pi. ✓
• B) Brief rest: the muscle cell increases glucose uptake to fuel glycolysis; full rest: mitochondria multiply in number to increase the cell's oxidative capacity.
• C) Brief rest: new ATP molecules are synthesized de novo from adenine and ribose; full rest: the entire glycolytic pathway is upregulated.
• D) Brief rest: stored ATP is released from glycogen granules; full rest: the neuromuscular junction resets to allow higher action potential frequency.

Explanation: Correct answer: A. During brief rest, oxidative phosphorylation (which has a large total ATP capacity but slow rate) begins restoring PCr levels: ADP + PCr -> ATP + creatine, driven by the creatine kinase reaction in reverse when ATP is being replenished. This partially restores the fast-acting PCr pool, explaining partial recovery. Full recovery over 3-5 minutes allows complete PCr resynthesis, clearance of Pi and lactate (which inhibit contractile proteins and glycolytic enzymes), and restoration of cellular redox balance. (B) is incorrect because mitochondria do not multiply within a few minutes; mitochondrial biogenesis is a process taking days to weeks. (C) is incorrect because cells do not synthesize ATP de novo from precursors rapidly; ATP is regenerated from ADP via phosphorylation, not synthesized from scratch during rest. (D) is incorrect; ATP is not stored in glycogen granules -- glycogen stores glucose, not ATP. Neuromuscular junction resetting is not the rate-limiting step for force recovery in this context.

Question 4. Which of the following accurately describes the structural basis for ATP's ability to release energy upon hydrolysis?

• A) The adenine base of ATP contains double bonds that store potential energy released when the base is cleaved.
• B) The ribose sugar of ATP is held under significant ring strain that is relieved when the molecule is hydrolyzed, releasing the stored free energy.
• C) The three phosphate groups of ATP carry negative charges that repel each other; hydrolysis of the terminal phosphate relieves this electrostatic strain and produces more stable products (ADP + Pi), releasing free energy. ✓
• D) ATP contains a high-energy covalent bond that requires more energy to form than it releases when broken.

Explanation: Correct answer: C. The free energy released by ATP hydrolysis arises from several structural factors, the most important being: (1) the three adjacent negatively charged phosphate groups experience significant electrostatic repulsion, creating an unstable, higher-energy state; and (2) the products of hydrolysis -- ADP and inorganic phosphate -- are more stable than ATP because the charges are better distributed through resonance in the separated phosphate groups. The system moves to a lower free-energy state, releasing the difference as usable energy. (A) is incorrect; the adenine base is not the energy-storing component; it is a structural recognition element that allows ATP to bind to enzymes and motor proteins. (B) is incorrect; ribose ring strain is not the basis for ATP's energy release. (D) contains a fundamental error: it conflates bond breaking with energy release. High-energy phosphate bonds actually release energy when broken because the products are more stable -- not because the bond requires more energy to form (which would mean forming ATP is energetically favored, the opposite of what occurs).

Question 5. Cyanide is a toxin that binds to and inhibits cytochrome c oxidase (Complex IV of the electron transport chain). Which of the following best predicts the effect of cyanide exposure on ATP production, and which energy pathway would be least directly affected?

• A) ATP production from oxidative phosphorylation would increase; glycolysis would be most affected.
• B) ATP production from oxidative phosphorylation and the citric acid cycle would be severely reduced; glycolysis (substrate-level phosphorylation) would be least affected, though limited by NADH accumulation. ✓
• C) All ATP production would immediately and permanently cease throughout the cell because cyanide destroys the mitochondrial DNA needed to encode the electron transport chain.
• D) ATP production from oxidative phosphorylation would cease; glycolysis would be completely unaffected and would compensate fully for the loss.

Explanation: Correct answer: B. Blocking Complex IV (cytochrome c oxidase) halts the entire ETC because electrons cannot be transferred to O2, the terminal electron acceptor. Without ETC activity, NADH and FADH2 cannot be re-oxidized, causing NADH to accumulate. This accumulation inhibits the citric acid cycle, which requires NAD+ to proceed. Both the ETC and the citric acid cycle are therefore effectively blocked. Glycolysis occurs in the cytoplasm and does not directly require the ETC, making it the least directly affected pathway. However, glycolysis also generates NADH; without the ETC to re-oxidize it, cells must shift to fermentation (producing lactate or ethanol) to regenerate NAD+ and allow glycolysis to continue. Glycolysis is therefore constrained but not eliminated. (A) is the opposite of correct; cyanide inhibits oxidative phosphorylation. (C) is incorrect; cyanide inhibits a protein enzyme -- cytochrome c oxidase -- not DNA; mitochondrial DNA is unaffected directly. (D) is incorrect because it overstates glycolysis as completely unaffected; the forced shift to fermentation and its byproduct accumulation represent a real limitation.