Light-independent reaction

In photosynthesis, a light-independent reaction takes place in plant chloroplasts. In this process, sugars are made from carbon dioxide.

The Calvin cycle

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 moved around the plant (translocation). This takes place in the grana region of the chloroplast. It is anaerobic (does not use oxygen).

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 P_i   (P_i = inorganic phosphate)

Other websites

• The biochemistry of the Calvin cycle. Rensselaer Polytechnic Institute Archived 2005-03-16 at the Wayback Machine
• The Calvin cycle and the Pentose phosphate pathway from Berg J.M; Tymoczko J.L. & Stryer L. 2002. Biochemistry. 5th ed, W.H. Freeman.