In the last article we left photosynthesis at a point where ATP, reduced NADP, and oxygen had been produced in the light-dependent reaction. Oxygen is a waste product of photosynthesis, however the ATP and reduced NADP are used for the light-independent reaction. So although light energy isn’t specifically needed for any reaction in this next part, the light-dependent reaction is essential for allowing the Calvin cycle to keep turning.
The Light-Independent Reaction (The Calvin Cycle)
This reaction is also called the Calvin cycle, which is easily confused with the Krebs cycle in respiration so be careful not to let that catch you out. You might also see it referred to as carbon dioxide fixation, because carbon from CO2 is fixed into organic molecules. It happens in the stroma of chloroplasts where the necessary enzymes are contained. Reduced NADP and ATP were produced in the stroma in the light-independent reaction, so everything is ready to go.
- CO2 combines with a molecule of ribulose bisphosphate (RuBP) to form an unstable 6-carbon compound in a reaction catalysed by ribulose bisphosphate carboxylase (RuBisCo). The unstable 6-carbon compound immediately splits into two molecules of glycerate 3-phosphate (GP).
- Each GP molecule is converted into a triose phosphate (TP) molecule. Each reaction requires ATP and reduced NADP. Reduced NADP becomes oxidised again by donating a H+ ion to the reaction, so it is regenerated for the light-dependent reaction.
- For every six molecules of TP produced, five are converted back to RuBP which requires ATP (RuBP is regenerated).
- For every six molecules of TP produced, one is used to synthesise organic compounds such as glucose, ribose, and fructose.
With a little bit of maths, we can work out how many rounds of the Calvin cycle are needed to produce one molecule of glucose. Because TP only contains three carbon atoms, and we know that glucose contains six carbon atoms, we need two TP molecules to make one glucose. Only a sixth of the TP from each turn of the Calvin cycle can be used to produce glucose, meaning that six turns are required for one molecule of glucose (remember there are two TP molecules produced in each turn). Therefore we need 18ATP, 12 reduced NADP, and 6CO2 to synthesise one molecule of glucose. This fits nicely with the photosynthesis equation.
TP isn’t the only Calvin cycle molecule used to produce organic compounds. GP can be used to synthesise fatty acids and some amino acids. Note that sometimes triose phosphate (TP) is called glyceraldehyde 3-phosphate (GALP).
The light-independent reaction can be difficult to learn, so practise drawing out the Calvin cycle until it becomes second nature. To summaries today’s article:
- The light-independent reaction (Calvin cycle) fixes the carbon in CO2 into organic molecules.
- It takes place in the stroma and requires ATP and reduced NADP.
- Five out of the six TP molecules produced goes into regeneration of RuBP, the other one goes into synthesis of glucose and other molecules such as ribose.