The light-independent reaction of photosynthesis is where the ultimate product, glucose, is made. Given its name, the reactions involved in this step do not require light, since the reactants used are taken from the products of the light-dependent reaction. The LIR occurs in the stroma of chloroplasts (the space around thylakoid stacks which contains lots of enzymes involved in photosynthesis).
All LIR events can be viewed as a cycle termed the Calvin cycle. The starting point is carbon dioxide, CO2, and the ending point is glucose (C6H12O6). Before the carbon atoms in CO2 can be incorporated into glucose, a series of events must take place. As you can appreciate, turning a simple inorganic gas into a complex organic molecule which is at the heart of life today as we know it takes just a little bit of magic.
This magic has 3 chapters, as ordered in the Calvin cycle:
1. Carbon dioxide fixation
2. Carbon dioxide reduction
3. Ribulose bisphosphate regeneration
Firstly, carbon dioxide reacts with ribulose bisphosphate waaaaaaaaaaaaaaaaait. Ribulose bisphosphate. Say it out loud. Ribulose bisphosphate. What does it want from you? Nothing, just remember it’s a 5-carbon sugar. Carbon dioxide has 1 carbon, glucose has 6… catch the drift? 5+1…. 5+1=…! But those goodies are for later. For now, carbon dioxide reacts with RuBP (a reaction catalysed by the enzyme RuBisCO – sounds like a supermarket chain in Paraguay) to produce two 3-carbon molecules called glycerate 3-phosphate, GP. Here’s a carbon atom scheme:
C + CCCCC –> 2CCC
(CO2 + RuBP –> 2GP)
Next: The 2 GP molecules react further to produce triose phosphate, TP (3-carbon molecule also known as glyceraldehyde phosphate). This forms the building block for glucose and other organic compounds. To add up the carbon atoms, 2 TP are needed for 1 glucose.
Expenditure: 2 ATP molecules and 2 NADPH molecules from the LDR. Once NADPH is oxidised back to NADP, it can return to the LDR in the thylakoids. NADP is therefore recycled.
Finally: For the cycle to continue, a supply of RuBP must be kept constant to meet the incoming carbon dioxide back at the beginning of the Calvin cycle. This is actually achieved by most TP molecules produced in the previous step. A phosphate group from ATP is used to convert ribulose monophosphate into ribulose bisphosphate, RuBP.
Note that this process can also lead to the formation of other carbohydrates, as well as lipids and amino acids from triose phosphate.