The Citric Acid Cycle as a Hub, and Refilling It · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

The wheel that must stay full

Science · Biochemistry & Molecular Biology · ages 20-21
Name ______________________   Date ____________

  1. Which pair names intermediates withdrawn for biosynthesis?

    • Alpha-ketoglutarate for amino acids and oxaloacetate for glucose making
    • NADH for muscles and FADH2 for nerves
    • Pyruvate for lungs and lactate for skin

  2. What does it mean that the cycle is catalytic in oxaloacetate?

    • Oxaloacetate is destroyed permanently in every turn
    • Oxaloacetate is regenerated at the end of each turn
    • Oxaloacetate never participates in the cycle

  3. What enters the cycle per turn, and what leaves?

    • Two acetyl CoA enter and one carbon dioxide leaves
    • One glucose enters and one oxaloacetate leaves
    • One acetyl CoA enters, and two carbon dioxide plus carriers leave

  4. Which reaction refills oxaloacetate directly?

    • Lactate dehydrogenase making lactate from pyruvate
    • Pyruvate carboxylase making oxaloacetate from pyruvate
    • ATP synthase making ATP from the gradient

  5. A growing cell withdraws oxaloacetate for glucose making. Why does the whole cycle slow?

    • Withdrawing a catalyst leaves less capacity to process acetyl CoA
    • Glucose making poisons every cycle enzyme directly
    • Acetyl CoA stops entering whenever glucose is made

  6. When is refilling most needed?

    • In resting cells with no biosynthesis at all
    • In cells burning only glucose for energy
    • In growing cells with heavy biosynthetic demand

  7. Oxaloacetate is consumed permanently each turn and must be rebuilt from scratch every time.

    Circle one:   True   False

  8. A fast growing mutant lacks pyruvate carboxylase and drains oxaloacetate toward glucose making. What happens?

    • The cycle spins faster, since drains always help
    • Nothing changes, since intermediates never matter
    • The cycle slows unless another refilling reaction covers the drain
LightMySky · lightmysky.comW1-mt_BPPfI-vnab-s1

Answer key

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

The wheel that must stay full W1-mt_BPPfI-vnab-s1

  1. Alpha-ketoglutarate for amino acids and oxaloacetate for glucose making · Those two exits feed building pathways. Carriers and entry fuels are not withdrawals.
  2. Oxaloacetate is regenerated at the end of each turn · Regeneration is the point: the wheel remakes its own starter each turn. Only the acetyl carbons leave as waste.
  3. One acetyl CoA enters, and two carbon dioxide plus carriers leave · Two carbons in as acetyl, two out as carbon dioxide, with carriers loaded and oxaloacetate remade.
  4. Pyruvate carboxylase making oxaloacetate from pyruvate · Pyruvate carboxylase tops the intermediate back up. The other enzymes work in different pathways.
  5. Withdrawing a catalyst leaves less capacity to process acetyl CoA · The wheel needs its intermediates to turn. Draining one shrinks the whole operation.
  6. In growing cells with heavy biosynthetic demand · Heavy building drains intermediates fastest. Fast drains need fast refills or respiration grinds down.
  7. False · It is regenerated at the end of each turn. Only drains for biosynthesis need true refilling.
  8. The cycle slows unless another refilling reaction covers the drain · Apply the drain test: leaver named, refiller missing. Without cover, the wheel must slow.
Worksheet · LightMySky