Six turns of the Calvin cycle take in 6 molecules of RuBP and 6 molecules of carbon dioxide…
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Six turns of the Calvin cycle take in molecules of RuBP and molecules of carbon dioxide. They make molecules of triose phosphate. (a) How many carbon atoms are in the RuBP? (b) How many carbon atoms enter in all? (c) How many carbon atoms are in the triose phosphates? (d) Two of the triose phosphates leave to make glucose. How many carbon atoms leave? (e) How many triose phosphates are left to rebuild the RuBP?
Need a reminder? The key ideaShow
Calvin cycle (C3 pathway, in the stroma): carboxylation (RuBP, 5C, plus carbon dioxide, by RuBisCO, gives two 3-PGA, 3C), reduction (2 ATP and 2 NADPH for each carbon dioxide, gives triose phosphate), regeneration (1 ATP for each carbon dioxide). Each carbon dioxide: 3 ATP and 2 NADPH. One glucose: 6 turns, 18 ATP, 12 NADPH. Six turns: 6 RuBP (30 C) and 6 CO2 (6 C) give 12 triose phosphate (36 C); 2 leave as glucose (6 C) and 10 (30 C) rebuild 6 RuBP.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. RuBP. carbon atoms.
Step 2. In all. carbon atoms.
Step 3. Triose phosphates. carbon atoms. The carbon is conserved.
Step 4. Leaving. carbon atoms, which is exactly one glucose.
Step 5. Left. molecules. They hold carbon atoms, which rebuild RuBP ().
Answer: (a) ; (b) ; (c) ; (d) ; (e) triose phosphates.
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