THE CALVIN CYCLE Section 7.10

Carbon Fixation by the Calvin Cycle The Second set of reactions in photosynthesis involves a biochemical pathway known as the Calvin Cycle The Calvin Cycle produces organic compounds, using energy stored in ATP and NADPH

Calvin Cycle’s 4 Steps

Step 1: Carbon Fixation Atoms from CO 2 are bonded or “fixed” into organic compounds Incorporation of CO 2 into organic compounds is known as carbon fixation Catalyzed by the enzyme rubisco Rubisco combines CO2 with the five carbon sugar ribulose bisphosphate Splits into two three-carbon acids called 3- phosphoglyceric acid (3-PGA) Three CO2 go in, Six 3-PGA come out

Step 2: Reduction Two chemical reactions take place They use energy from 6 ATP and 6 NADPH’s are oxidized The six molecules of 3-PGA are reduced and six molecules of energy rich G3P are produced

Step 3: Release of One G3P Five G3P’s remain in the cycle from step 2 The molecule that leaves is the net product of photosynthesis The plant cell will use two of these “released” G3P’s to make one molecule of glucose

Step 4: Regeneration of RuBP A number of chemical reactions occur, using energy from 3 ATP to rearrange the atoms in the five G3P molecules that remain This forms three RuBP molecules each consisting of five carbons These can be used to start another turn of the cycle