3.3 The light independent reaction
The Calvin Cycle
1. carbon dioxide from the atmosphere diffuses into the leaf through stomata and dissolves in water around the walls of the mesophyll cells. It then diffuses through the plasma membrane, cytoplasm and chloroplast membranes into the stroma of the chloroplasts.
2. In the stroma, carbon dioxide comobines with the 5-carbon compound ribulose biphosphate (RuBP) using an enzyme
3.The combination of CO2 and RuBP produces two molecules of 3-carbon gylcerate 3-phosphate (GP)
4.ATP and NADP from the light-dependant reaction are used to recude the activated gylcerate 3-phosphate to triose phosphate (TP)
5. The NADP is reformed and goes back to the light-dependant reaction to be reduced again by accepting more hydrogen.
The Calvin Cycle cont...
6. Some trios phosphate molecules are converted to useful organic substances, such as glucose.
7. Most trios phosphate molecules are used to regenerate ribulose bisphosphate using ATP from the light-dependent reaction.
Site of light-independent reaction
The light independent reaction of photosynthesis takes place in the stroma of the chloroplasts.
The chloroplasts is adapted to carrying out the light-independent reaction of photosynthesis in the following ways.
- The fluid of the stroma contains all the enzymes needed to carry out the light-independent reaction (reduction of carbon dioxide)
- The stroma fluid surrounds the grana and so the products of the light-dependent reaction in the grana can readily diffuse into the stroma.
- It contains both DNA and ribosomes so it can quickly and easily manufacture some of the proteins needed for the light-independant reaction.