The Calvin Cycle and Carbon Fixation · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Turning thin air into sugar

Science · Organisms & Life Processes · ages 16-18
Name ______________________   Date ____________

  1. What is RuBP?

    • A light-trapping pigment
    • The finished three-carbon sugar
    • The five-carbon acceptor for carbon dioxide

  2. Which enzyme fixes carbon dioxide in the cycle?

    • ATP synthase
    • Rubisco
    • DNA polymerase

  3. Most triose phosphate is spent rebuilding RuBP.

    Circle one:   True   False

  4. One carbon enters as carbon dioxide. Where does it go first?

    • Straight into glucose
    • Into released oxygen
    • Joined onto RuBP

  5. What are the ATP and reduced NADP used for inside the cycle?

    • They split water molecules to release oxygen in the stroma
    • Both reduce GP to triose; ATP also rebuilds RuBP
    • They trap photons in chlorophyll to excite more electrons

  6. Why do three turns net only one spare triose?

    • Rubisco fixes three carbon dioxide molecules on every single turn
    • Each turn fixes one carbon, and a triose holds three carbons
    • ATP runs out after each turn, so only every third turn finishes

  7. A student says the cycle traps its own light energy. What is wrong?

    • It uses carriers, not light
    • It makes its own ATP from glucose
    • It fixes oxygen instead of carbon

  8. Why does the cycle stall within minutes of darkness?

    • Its carrier stocks drain away
    • Rubisco is destroyed by darkness
    • The stroma runs out of water
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Answer key

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

Turning thin air into sugar W1-mt_zGOCx9mNrx-s1

  1. The five-carbon acceptor for carbon dioxide · RuBP is the acceptor that carbon dioxide joins.
  2. Rubisco · Rubisco bolts carbon dioxide onto RuBP.
  3. True · Only a small spare leaves; the rest rebuilds the acceptor.
  4. Joined onto RuBP · Fixation onto RuBP is the entry step.
  5. Both reduce GP to triose; ATP also rebuilds RuBP · ATP and reduced NADP together reduce GP to triose phosphate. Rebuilding RuBP spends more ATP but no reduced NADP.
  6. Each turn fixes one carbon, and a triose holds three carbons · Each turn fixes one carbon dioxide. A triose phosphate holds three carbons, so three turns are needed before one triose can leave while the rest rebuild RuBP.
  7. It uses carriers, not light · Light work happens in the thylakoids, not here.
  8. Its carrier stocks drain away · No light means no refill of the imported stocks.
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