Also known as the carbon-fixing reactions, the light-independent reactions take place in the stroma. The pathway is also cyclical and is called the Calvin Cycle.
First of all, carbon dioxide binds to ribulose bisphosphate (RuBP), a 5-carbon sugar. This creates two molecules of glycerate 3-phosphate (GP), a 3-carbon compound. The process is catalysed by the enzyme ribulose bisphosphate carboxylase or rubisco. It’s not a very efficient enzyme so a lot is needed. In fact, it takes up about 50% of a chloroplast’s mass and is created made here from chloroplast DNA.
GP is an acid so when’s it’s reduced it forms triose phosphate, a 3-carbon sugar also created in glycolysis. The energy required for this step uses the ATP and NADPH created in the light-dependent reactions. The ADP and NADP left over then return to the thylakoid membrane to be used again.
Most of the triose phosphate continues on through the cycle to create RuBP: five triose phosphate molecules combine with three RuBP molecules. However, every three turns of the cycle three CO2 molecules combine to create another triose phosphate molecule. This molecule leaves the cycle and when it joins with another triose phosphate molecule and glycolysis enzymes glucose is formed. The glucose can then be used as needed to create:
- starch
- sucrose
- amino acids
- lipids
- cellulose