AP Biology: Fermentation Practice Questions
Follow carbon atoms and electron carriers through original fermentation problems. Use the stated assumptions to distinguish direct observations from explanations and to calculate net ATP production. This is independent study practice, not an official AP resource.
Free original practice questions for study purposes. Open Exam Prep is an independent study resource and is not affiliated with, endorsed by, or sponsored by the makers of AP Exams.
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1. A cell obtains ATP from glycolysis while converting the resulting pyruvate to lactate. Which role of the lactate-producing reaction allows glycolysis to keep operating?
Explanation. Glycolysis requires NAD+ as an electron acceptor. Reduction of pyruvate to lactate consumes NADH and restores NAD+, preventing the available carrier pool from remaining entirely reduced.
2. A model separates glucose breakdown into glycolysis and the subsequent conversion of pyruvate to lactate. In this model, where is the net ATP gain produced?
Explanation. Glycolytic reactions transfer phosphate from organic intermediates to ADP, yielding a net ATP gain. The subsequent lactate-producing reaction maintains carrier availability but adds no ATP in this pathway.
3. In a cell extract, pyruvate is converted to lactate as NADH becomes NAD+. Which molecule is reduced in that reaction?
Explanation. Pyruvate accepts electrons and is reduced to lactate. NADH donates electrons and is oxidized, so the two changes form a coupled oxidation-reduction reaction rather than two unrelated events.
4. A yeast pathway removes one carbon from each three-carbon pyruvate before reducing the remaining two-carbon compound. Which pair correctly identifies the released product and the two-carbon intermediate?
Explanation. Pyruvate decarboxylation releases carbon dioxide and leaves acetaldehyde. In the next step, acetaldehyde accepts electrons from NADH and becomes ethanol; acetyl-CoA belongs to a different route of pyruvate processing.
5. A simplified yeast culture processes five glucose molecules entirely through glycolysis and alcoholic fermentation. No carbon enters biomass or other products. How many ethanol molecules and carbon dioxide molecules result?
Explanation. Each glucose gives two pyruvates, which produce two ethanol molecules and two carbon dioxide molecules. Five glucose molecules therefore give ten of each; 10 times 2 plus 10 times 1 accounts for all 30 carbon atoms.
6. In a teaching model, each glucose processed by glycolysis requires an investment of 2 ATP and yields 4 ATP before pyruvate is converted to lactate. For nine glucose molecules, what is the net ATP gain from the modeled pathway?
Explanation. Subtract the investment from the gross output: 4 minus 2 gives a net gain of 2 ATP per glucose. Nine glucose molecules yield 18 net ATP. The specified conversion to lactate does not add another ATP-producing step.
7. An oxygen-free cell extract relies solely on the pyruvate-to-lactate reaction to recycle NADH. A selective block stops that reaction while glycolytic enzymes remain intact. After the initial NAD+ supply is used, why must sustained glycolysis slow?
Explanation. The extract cannot indefinitely reduce NAD+ without regenerating it. Blocking its only NADH oxidation route makes NAD+ limiting even if glucose and the glycolytic enzymes are still present; the effect need not be instantaneous.
8. Two oxygen-free bacterial cultures make ATP. Culture P reduces pyruvate to lactate without a respiratory chain. Culture Q uses a respiratory electron transport chain ending in nitrate reduction. Which classification fits these observations?
Explanation. The absence of oxygen alone does not distinguish these routes. P recycles its carrier through an organic fermentation product, whereas Q uses a respiratory chain with a terminal acceptor other than oxygen.
9. An idealized lactate pathway converts one six-carbon glucose to two three-carbon lactate molecules without diverting any carbon. What carbon dioxide release is required by this pathway's carbon balance?
Explanation. Two lactate molecules contain 2 times 3, or six, carbon atoms. No decarboxylation is needed in this specified route. This carbon balance concerns lactate fermentation and does not describe every possible microbial fermentation pathway.
10. Two yeast cultures have identical cell numbers, glucose concentrations, temperatures, and oxygen-free conditions. During ten minutes, culture R produces 8 units of carbon dioxide and culture S produces 3. Assume all measured gas comes from alcoholic fermentation. Which conclusion is directly supported?
Explanation. Equal cell numbers and observation times allow comparison of average gas production per cell. The larger output in R supports that rate comparison, but does not by itself identify an enzyme change or establish a different ATP yield per glucose.
11. A simplified system converts each glucose into two pyruvates and two NADH, then converts both pyruvates to lactate. Each lactate-forming reaction consumes one NADH. What is the net change in the number of NADH molecules after processing six glucose molecules through both stages?
Explanation. The first stage forms 6 times 2 = 12 NADH, and the second consumes the same 12. Thus the net change is zero even though carriers cycle continuously. Zero net accumulation does not mean that no electron transfer occurred.
12. Researchers test whether an intact cell extract converts pyruvate to lactate using NADH. Both tubes begin with identical pyruvate and NADH supplies; one receives active lactate dehydrogenase and the other heat-inactivated enzyme. Which result most strongly supports the proposed enzyme-dependent conversion?
Explanation. Measuring both disappearance of an electron donor and formation of the proposed product links the observations to the reaction. The matched inactive-enzyme tube helps distinguish enzyme-dependent conversion from background changes in the extract.
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FAQ
How should I track fermentation in a pathway diagram?
Label the carbon-containing reactants and products, then follow NADH and NAD+. Keep ATP accounting in a separate column so that carrier recycling is not counted as an extra ATP-producing step.
Does an oxygen-free culture necessarily use fermentation?
No. Look for evidence about the mechanism. A respiratory electron transport chain using a different terminal acceptor indicates anaerobic respiration; oxygen availability alone is not enough to distinguish the pathways.
Can a gas measurement identify the exact cause of a change in fermentation?
A controlled measurement can compare gas production rates, but a change may have several causes. Match cell numbers, time, temperature, and starting substrates, and collect additional evidence before attributing the change to one enzyme.