ATP and NADPH from the light reactions are spent in the stroma to build sugar. Here you count the turns of the Calvin cycle, follow carbon through a maize leaf, see why RuBisCO sometimes makes a costly mistake, and learn which factors set the pace of photosynthesis.
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Created by StemPy LabThe Calvin cycle runs in the stroma and builds sugar from carbon dioxide, using the ATP and NADPH of the light reactions. In carboxylation, RuBisCO joins carbon dioxide to RuBP, a 5-carbon compound, to give two molecules of 3-PGA. Reduction uses ATP and NADPH to make triose phosphate. Regeneration rebuilds RuBP with ATP. One glucose needs 6 turns, 18 ATP and 12 NADPH.
C4 plants such as maize and sugarcane have Kranz anatomy: large bundle sheath cells with many chloroplasts form a wreath round each vascular bundle. In the mesophyll, PEP carboxylase joins carbon dioxide to PEP, a 3-carbon compound, to give OAA, a 4-carbon compound. The carbon dioxide is then released in the bundle sheath and enters the Calvin cycle. C4 plants lack photorespiration and do well at high temperature.
RuBisCO can also bind oxygen. When the carbon dioxide is low and the oxygen is high, RuBP is oxidised and photorespiration follows. It makes no sugar, no ATP and no NADPH, and it releases carbon dioxide. C4 plants avoid it. Blackman's law of limiting factors says that the factor nearest its minimum sets the rate. Light, carbon dioxide, temperature and water affect photosynthesis.
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