Control Points in Glycolysis and the Committed Step · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Where glycolysis listens to control

Science · Biochemistry & Molecular Biology · ages 19-21
Name ______________________   Date ____________

  1. Most glycolytic steps sit near equilibrium and run in either direction.

    Circle one:   True   False

  2. Which enzyme catalyzes the committed step of glycolysis?

    • Phosphofructokinase
    • An enzyme of a near equilibrium step
    • An enzyme of the payoff phase

  3. What makes a step effectively irreversible in the cell?

    • A strongly negative free energy change under cell conditions
    • A step that runs in either direction
    • A step with no enzyme at all

  4. Citrate builds up in a cell. What does it signal and what follows?

    • Fuel shortage, so glycolysis speeds up
    • Full downstream capacity, so glycolysis slows down
    • Nothing, citrate never affects glycolysis

  5. A resting cell has plentiful ATP. What happens to glycolytic flux?

    • Flux rises because fuel is abundant
    • Flux falls because ATP slows phosphofructokinase
    • Flux stays fixed whatever the signals

  6. Hard exercise burns ATP and AMP climbs. What happens at the committed step?

    • The enzyme slows because AMP blocks it
    • The enzyme speeds up because AMP signals shortage
    • The enzyme ignores AMP entirely

  7. You must slow glycolysis with one targeted drug. Which target works best?

    • A near equilibrium step in the middle
    • A step of the payoff phase at random
    • Phosphofructokinase at the committed step

  8. A drug slightly weakens a near equilibrium step. A student predicts the flux will halve. Why is that wrong?

    • Near equilibrium steps self adjust, so other steps compensate
    • Near equilibrium steps are the main control valves
    • Drugs can only ever speed steps up
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Where glycolysis listens to control W1-mt_2vAoKtnmoz-s1

  1. True · Only three steps are effectively irreversible.
  2. Phosphofructokinase · Its product commits the sugar to breakdown.
  3. A strongly negative free energy change under cell conditions · The steep drop cannot be climbed back directly.
  4. Full downstream capacity, so glycolysis slows down · Citrate damps phosphofructokinase when the next cycle is full.
  5. Flux falls because ATP slows phosphofructokinase · Plentiful ATP feeds back to slow the committed step.
  6. The enzyme speeds up because AMP signals shortage · Rising AMP signals shortage and opens the valve wider.
  7. Phosphofructokinase at the committed step · Hit a valve, not a stretch that adjusts on its own.
  8. Near equilibrium steps self adjust, so other steps compensate · Control lives elsewhere, so flux barely changes.
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