AP Biology: Cellular Respiration — Glycolysis Practice Questions
These original AP Biology practice questions focus on how glycolysis converts glucose to pyruvate while transferring energy to ATP and NADH. They are independent study practice, not official AP exam questions.
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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Answer the questions below — you get instant feedback and a full explanation for each.
1. Where does glycolysis occur in both eukaryotic and prokaryotic cells?
Explanation. Glycolysis occurs in the cytosol and does not require a mitochondrion. This shared location helps explain why both prokaryotes, which lack mitochondria, and eukaryotes can use the pathway to begin glucose breakdown.
2. Which set lists the net products of glycolysis from one glucose molecule?
Explanation. One six-carbon glucose is converted to two three-carbon pyruvate molecules. Glycolysis produces four ATP but uses two, giving a net gain of two ATP, and it also reduces two NAD+ to two NADH.
3. Why are two ATP molecules used during the energy-investment phase of glycolysis?
Explanation. The investment reactions transfer phosphate groups from ATP to sugar intermediates. This traps and energizes the carbon compound, preparing the six-carbon intermediate to split into two three-carbon molecules.
4. Glycolysis forms four ATP molecules but consumes two ATP molecules. What is its net ATP yield per glucose?
Explanation. Net yield equals ATP produced minus ATP invested. The payoff phase forms four ATP by substrate-level phosphorylation, while the investment phase uses two, so the pathway adds two ATP overall.
5. What happens to NAD+ during the energy-payoff phase of glycolysis?
Explanation. As a three-carbon intermediate is oxidized, NAD+ accepts high-energy electrons and hydrogen and becomes NADH. The carrier can later transfer those electrons to other pathways rather than supplying phosphate or carbon.
6. Which description best defines substrate-level phosphorylation during glycolysis?
Explanation. In substrate-level phosphorylation, an enzyme directly moves a phosphate group from a phosphorylated organic molecule to ADP. This differs from oxidative phosphorylation, which depends on an electron transport chain and proton gradient.
7. Why can glycolysis continue for a short time when oxygen is absent?
Explanation. No glycolytic reaction directly consumes oxygen, so the pathway itself can run without it. Continued glycolysis still requires NAD+ regeneration, which fermentation can provide when aerobic electron transport is unavailable.
8. After the six-carbon intermediate in glycolysis splits into two three-carbon molecules, what is the main consequence for later reactions?
Explanation. The two three-carbon molecules proceed through the same payoff reactions. Therefore each downstream ATP-forming and NADH-forming step occurs twice for every original six-carbon glucose molecule.
9. A drug blocks a glycolytic enzyme that normally transfers phosphate from a high-energy organic intermediate to ADP. Which immediate change is most likely?
Explanation. Direct phosphate transfer from a glycolytic intermediate to ADP is substrate-level phosphorylation. Blocking the responsible enzyme reduces ATP formation in glycolysis; the other choices describe photosynthetic processes rather than glycolysis.
10. A cell metabolizes one glucose molecule whose six carbon atoms are all labeled. Before any later respiration step releases carbon dioxide, where will those labels be after glycolysis?
Explanation. Glycolysis rearranges and divides the six-carbon glucose but does not release carbon dioxide. Its two pyruvate products contain three carbons each, so together they retain all six labeled carbon atoms.
11. An experimental cell cannot regenerate NAD+ from NADH, but its glycolytic enzymes are otherwise functional. Why will glycolysis eventually slow or stop?
Explanation. A payoff-phase oxidation requires NAD+ as an electron acceptor. If NAD+ is not regenerated, the available pool becomes depleted, electron transfer at that step stops, and glycolytic flux cannot be maintained.
12. What is the key role of fermentation in supporting glycolysis when an electron transport chain cannot oxidize NADH?
Explanation. Fermentation transfers electrons from NADH to an organic molecule, regenerating NAD+. The restored NAD+ can again accept electrons during glycolysis, allowing the pathway's small ATP yield to continue without an active electron transport chain.
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FAQ
Does glycolysis require oxygen?
No glycolytic reaction uses oxygen directly. However, glycolysis needs a continuing supply of NAD+, so cells without aerobic electron transport must use fermentation or another process to regenerate NAD+ from NADH.
Why is the net ATP yield two if glycolysis makes four ATP?
The energy-investment phase consumes two ATP before the sugar splits. The payoff phase then produces four ATP, so four produced minus two consumed gives a net gain of two ATP per glucose.
Does glycolysis release carbon dioxide?
No. Glycolysis divides one six-carbon glucose into two three-carbon pyruvate molecules, retaining all six carbons. Carbon dioxide can be released later during pyruvate oxidation and the citric acid cycle.