Concept 8

A Closer Look at the Calvin Cycle

The ATP and NADPH produced by the light reactions power sugar synthesis in the Calvin cycle. In the animation of the Calvin cycle, three molecules of CO2 are added to three molecules of ribulose bisphosphate (RuBP), a 5-carbon sugar already present in the stroma. We now have a total of eighteen carbons in the cycle (three from CO2 and fifteen from the three molecules of RuBP). As the three RuBP molecules accept a molecule of carbon dioxide, they immediately break down into six 3-carbon molecules of phosphoglyceric acid (PGA).

The Calvin cycle then spends the chemical energy generated by the light reactions: ATP phosphorylates (adds phosphate to) the PGA; and the resulting compound is then reduced by NADPH. The product is a 3-carbon sugar named glyceraldehyde-3-phosphate (abbreviated here as GP). Of the six molecules of GP formed, only one represents net sugar output. The other five GP molecules are used to regenerate the three 5-carbon molecules of RuBP required to sustain the cycle.

Thus, the Calvin cycle uses ATP and NADPH to convert three molecules of CO2 to one molecule of a 3-carbon sugar. The plant can then use this small sugar to make larger sugars such as glucose and many other organic compounds. The main role of the light reactions is to restock the stroma with the ATP and NADPH required for the Calvin cycle.