Light-independent reaction

The Calvin cycle

Light-independent reactions take place in plant chloroplasts. In this process, sugars are made from carbon dioxide. The process, known as the Calvin cycle, uses products of the light-dependent reactions (ATP and NADPH) and various enzymes. Therefore, the light-independent reaction cannot happen without the light-dependent reaction. Sugars made in the light-independent reactions are then moved around the plant by translocation. Takes place in the grana region of the chloroplast.

The Calvin cycle

1. A sugar (Ribulose biphosphate or RuBP) made of 5 carbon atoms combines with carbon dioxide to form a 6-carbon sugar (phosphoglycerate). An enzyme called RuBisCO speeds this reaction up.
2. Phosphoglycerate is reduced with hydrogen atoms from the light-dependent reaction to form two molecules of triose phosphate (each has 3 carbon atoms). ATP is needed for this to occur.
3. Some triose phosphate is converted (using ATP) back into Ribulose Biphosphate (which is why this is called a cycle).
4. The rest left over is used to produce glucose.

The sum of reactions in the Calvin cycle is the following:

3 CO
₂

+ 6 NADPH + 5 H
2O
+ 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi   (Pi = inorganic phosphate)

Other websites

• The biochemistry of the Calvin cycle. Rensselaer Polytechnic Institute
• The Calvin cycle and the Pentose phosphate pathway from Berg J.M; Tymoczko J.L. & Stryer L. 2002. Biochemistry. 5th ed, W.H. Freeman.