Glycolysis MCAT Practice Questions with Worked Answers

Glycolysis questions on the MCAT rarely ask you to list all ten steps. They ask what the pathway nets, what regulates it, and what happens when oxygen is missing. The questions below practise exactly those three ideas. Try each before reading the answer.

Facts to have ready

• Net per glucose: 2 ATP, 2 NADH, 2 pyruvate. The pathway spends 2 ATP and makes 4.
• Committed step: phosphofructokinase-1 (PFK-1), fructose-6-phosphate to fructose-1,6-bisphosphate.
• PFK-1 is inhibited by ATP and citrate, and activated by AMP and fructose-2,6-bisphosphate.
• Location: cytosol. No oxygen is needed for glycolysis itself.

Question 1: net ATP

A cell metabolizes one glucose by glycolysis only. How much ATP does it gain overall?

Answer: 2 ATP. Two ATP are invested (hexokinase and PFK-1) and four are made by substrate-level phosphorylation (two per triose, in the phosphoglycerate kinase and pyruvate kinase steps). Four minus two is two. A common trap is answering 4, which is the gross yield.

Question 2: the rate-limiting step

Which enzyme is the main control point, and what does a high ATP level do to it?

Answer: PFK-1, and high ATP inhibits it. ATP binds an allosteric site and lowers the enzyme's affinity for fructose-6-phosphate. When energy is plentiful, the cell has no reason to break down more glucose.

Question 3: fermentation

Why do muscle cells convert pyruvate to lactate when oxygen is low?

Answer: to regenerate NAD⁺. Glyceraldehyde-3-phosphate dehydrogenase needs NAD⁺. Without oxygen the electron transport chain cannot reoxidize NADH, so lactate dehydrogenase uses NADH to reduce pyruvate to lactate, giving NAD⁺ back. Lactate fermentation makes no extra ATP; it only keeps glycolysis running.

Question 4: an inhibitor

A drug blocks pyruvate kinase. What happens to fructose-1,6-bisphosphate levels?

Answer: they rise. Blocking a late step makes intermediates upstream build up, and ATP production falls. In many cells pyruvate kinase is also activated by fructose-1,6-bisphosphate, which is a feed-forward signal.

Question 5: with oxygen

If oxygen is available, where does the pyruvate go?

Answer: into the mitochondrion, where pyruvate dehydrogenase converts it to acetyl-CoA, which enters the citric acid cycle. The NADH from glycolysis passes electrons to the electron transport chain through shuttles.

Common traps

• Gross vs net ATP. The gross yield is 4; the net is 2.
• Counting once, not twice. Everything after aldolase happens twice per glucose.
• Saying lactate makes ATP. It makes none; it regenerates NAD⁺.
• Wrong direction of regulation. ATP and citrate slow PFK-1; AMP speeds it up.

One-minute recap

• Glycolysis nets 2 ATP, 2 NADH and 2 pyruvate per glucose.
• PFK-1 is the committed, regulated step.
• Fermentation exists to regenerate NAD⁺.