Carbon Fixation

The Calvin cycle is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar. In the Calvin cycle, carbon atoms from CO2 in the atmosphere are bonded, or "fixed," into organic compounds. This incorporation of CO2 into organic compounds is called carbon fixation. A total of three CO2 molecules must enter the Calvin cycle to produce each three-carbon sugar that will be used to make the organic compounds. The Calvin cycle occurs within the stroma of the chloroplast. Figure 6-9 shows the events that occur when three CO2 molecules enter the Calvin cycle.

In step Q, CO2 diffuses into the stroma from the surrounding cytosol. An enzyme combines each CO2 molecule with a five-carbon molecule called ribulose bisphosphate (RuBP). The six-carbon molecules that result are very unstable, and they each immediately split into two three-carbon molecules. These three-carbon molecules are called 3-phosphoglycerate (3-PGA).

The Calvin cycle, in which carbon is fixed into organic compounds, takes place in the stroma of the chloroplast.

C 3 CO2


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