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Glycolysis Quiz
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Which statement best explains the purpose of the early phosphorylation steps in glycolysis during its initial phase?
- A They produce ATP by substrate-level phosphorylation You picked ✕
- B They directly generate NADH for use in oxidative phosphorylation You picked ✕
- C They trap glucose inside the cell and increase its chemical reactivity You picked ✓
- D They cleave glucose into two three-carbon molecules You picked ✕
- E They convert glucose into lactate under anaerobic conditions You picked ✕
Correct!
Incorrect
The answer is C: They trap glucose inside the cell and increase its chemical reactivity
Explanation
Why C is correct
See Phase 1. Reactions 1 to 3. Option C is correct. The first phase of glycolysis, reactions 1 to 3, is the energy investment phase. In it the glucose is phosphorylated, meaning that phosphate groups are added to it, using up 2 ATP. This has two purposes. First, the phosphate group carries a negative charge, so the glucose can no longer pass back out across the cell membrane: it is trapped inside the cell. Second, phosphorylation makes the glucose more reactive, which primes it for the later reactions. Reactions 1 and 3 have large negative ΔG values (they release a lot of free energy), which makes them irreversible, and so they are the main places where glycolysis is controlled. The first reaction is catalysed by hexokinase (or glucokinase in the liver) and the third by phosphofructokinase. The energy spent here is repaid in the second phase, where more ATP is made than was used.
- Phase: Reactions 1 to 3 are the energy investment phase.
- Cost: Phosphorylating glucose uses up 2 ATP.
- Purpose: Phosphorylated glucose is trapped inside the cell and is more reactive.
A helpful aid is to think of the first phase as paying a deposit: ATP is spent to lock glucose in the cell and make it ready to be split.
Why the other options are incorrect
- A. They produce ATP by substrate-level phosphorylation: The early phosphorylation steps use up ATP rather than make it. Substrate-level phosphorylation makes ATP later, in the second phase. Your answer
- B. They directly generate NADH for use in oxidative phosphorylation: NADH is made later, at reaction 6 in the second phase. The early steps use ATP and do not make NADH. Your answer
- D. They cleave glucose into two three-carbon molecules: The splitting of the six-carbon sugar into two three-carbon molecules is reaction 4, which marks the start of the second phase. It is not one of the phosphorylation steps. Your answer
- E. They convert glucose into lactate under anaerobic conditions: Lactate is made from pyruvate by lactate dehydrogenase after glycolysis, when oxygen is limited. It is not what the early phosphorylation steps do. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
Which metabolite acts as an allosteric inhibitor of phosphofructokinase-1 (PFK-1) in glycolysis, signalling that the cell has sufficient energy?
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2
Which statement best describes the energetic outcome of the latter half of glycolysis following cleavage of glucose into triose phosphates?
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