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.
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Keeping the cycle supplied with intermediates

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

  1. Which pair names cycle intermediates that can supply biosynthesis?

    • Alpha-ketoglutarate for amino acids and oxaloacetate for glucose production
    • NADH as the carbon skeleton for amino acids and bound FAD as the carbon skeleton for glucose
    • Acetyl CoA as the regenerated cycle starter and lactate as a circulating cycle intermediate

  2. What does it mean that oxaloacetate has a catalytic role in the cycle?

    • Oxaloacetate is permanently destroyed in every complete turn
    • Oxaloacetate is regenerated during each complete turn
    • Oxaloacetate never reacts and retains every original carbon atom unchanged

  3. Per acetyl group entering as acetyl CoA, which cycle yield account is correct? Oxaloacetate is regenerated.

    • 1 CO2, 2 NADH, freely released FADH2 and 2 ATP/GTP per acetyl group
    • 2 CO2, 3 NADH, freely released FADH2 and no ATP/GTP in any tissue
    • 2 CO2, 3 NADH, bound-FAD electron transfer and 1 ATP/GTP

  4. Which reaction adds oxaloacetate directly to the intermediate pool?

    • Lactate dehydrogenase converting pyruvate to lactate while oxidising NADH
    • Pyruvate carboxylase converting pyruvate to oxaloacetate using bicarbonate and ATP
    • ATP synthase producing ATP from ADP without adding carbon to oxaloacetate

  5. A liver cell withdraws oxaloacetate for glucose production faster than net refill replaces it. The pool becomes limiting. Other conditions stay fixed. Why can cycle flux fall?

    • Less available oxaloacetate limits entry of acetyl groups
    • Glucose production directly poisons every cycle enzyme even when intermediates remain available
    • Any glucose production blocks acetyl CoA entry regardless of oxaloacetate availability

  6. In comparable cells, loss and existing net refill are measured per minute. Which needs the largest extra refill to match loss?

    • A resting cell with loss 0 and existing net refill 0
    • An active cell with loss 4 and existing net refill 4
    • A growing cell with loss 9 and existing net refill 3

  7. A cell has sustained net intermediate loss and no anaplerotic input. Increasing acetyl CoA alone can replace that loss because each ordinary cycle turn regenerates oxaloacetate.

    Circle one:   True   False

  8. A mutant loses pyruvate carboxylase and has sustained oxaloacetate withdrawal. Its intermediate pool approaches a limiting level. Which change could prevent further depletion?

    • Increase withdrawal because losing more oxaloacetate automatically speeds its net replacement
    • Rely on ordinary cycle regeneration to replace every net withdrawal without any new input
    • Provide another net refilling reaction that matches the loss
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Answer key

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

Keeping the cycle supplied with intermediates W1-mt_BPPfI-vnab-s1

  1. Alpha-ketoglutarate for amino acids and oxaloacetate for glucose production · Alpha-ketoglutarate and oxaloacetate are cycle intermediates with the named biosynthetic uses.
  2. Oxaloacetate is regenerated during each complete turn · Cycle reactions regenerate oxaloacetate. That does not mean its original carbon atoms remain together or that only incoming acetyl carbons leave immediately.
  3. 2 CO2, 3 NADH, bound-FAD electron transfer and 1 ATP/GTP · The net account includes two CO2, three NADH, bound-FAD electron transfer and one ATP or GTP.
  4. Pyruvate carboxylase converting pyruvate to oxaloacetate using bicarbonate and ATP · Pyruvate carboxylase makes oxaloacetate from pyruvate, providing a net intermediate input.
  5. Less available oxaloacetate limits entry of acetyl groups · The prompt states uncompensated depletion and limiting oxaloacetate, which can constrain acetyl-group entry.
  6. A growing cell with loss 9 and existing net refill 3 · The extra refill requirements are 0, 0 and 6 units per minute. The growing cell has the largest deficit in these stated measurements.
  7. False · Acetyl-group entry uses oxaloacetate that is later regenerated. This is not a net addition that replaces lost intermediates.
  8. Provide another net refilling reaction that matches the loss · Another adequate net input could replace the missing refill and balance sustained withdrawal.
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