The light-independent stage of photosynthesis is a series of reactions that can take place even when light is not present. It uses ATP and NADP from the light-dependent stage to synthesize carbohydrates from carbon dioxide.
» The light-independent stage is made up a cycle of reactions known as the Calvin
cycle.
» It takes place in the stroma of the chloroplast, where the enzyme ribulose bisphosphate carboxylase, usually known as rubisco, is found.
» Carbon dioxide diffuses into the stroma from the air spaces within the leaf.
» The carbon dioxide enters the active site of rubisco, which combines it with a 5C compound called ribulose bisphosphate, RuBP. The products of this reaction are two 3C molecules of glycerate 3-phosphate, GP.
» The combination of carbon dioxide with RuBP is called carbon fixation.
» Energy from ATP and hydrogen from reduced NADP are then used to reduce the GP
to triose phosphate, TP.
» Triose phosphate is the first carbohydrate produced in photosynthesis.
» Most of the triose phosphate is used to regenerate ribulose bisphosphate, so that
more carbon dioxide can be fixed. This series of reactions requires the use of ATP.
» The rest of the TP is used to make glucose or whatever other organic substances the plant cell requires. These include polysaccharides, such as starch for energy storage and cellulose for making cell walls, sucrose for transport, amino acids for making proteins, lipids for energy storage and nucleotides for making DNA and RNA. Some of the GP is used directly to produce some forms of amino acid.
Practice questions
Outline/ describe the main features/reactions of the Calvin Cycle.
- RuBP 5C;
- combines with carbon dioxide;
- rubisco;
- to form an unstable 6C compound;
- which forms 2 X GP (PGA);
- ATP;
- energy source;
- and reduced NADP;
- forms TP (GALP);
- TP used to form glucose / carbohydrates 1 lipids / amino acids;
- TP used in regeneration of RuBP;
- requires ATP;
- as source of phosphate;
- light independent;
- Grana increase surface area for light absorption