Light-independent reaction
In photosynthesis, a light-independent reaction takes 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 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
- 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.
- 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.
- Some triose phosphate is converted (using ATP) back into Ribulose Biphosphate (which is why this is called a cycle).
- The rest left over is used to produce glucose.
The sum of reactions in the Calvin cycle is the following:
Light-independent Reaction Media
Calvin cycle step 1 (black circles represent carbon atoms)
Calvin cycle steps 2 and 3 combined
Regeneration stage of the Calvin cycle
The regeneration stage of Calvin's cycle. Substances and their parts are outlined in colors: green - carbon accepting aldoses, pink - ketoses-donors of three-carbon groups, yellow - parts of ketoses remaining after donation of two-carbon keto-groups highlighted in orange. Enzymes are also highlighted: aldolases in purple and transketolases in red.
Simplified C3 cycle with structural formulas
The overall equation of the Calvin cycle (black circles represent carbon atoms)
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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.