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. 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 reacts with a molecule of ribulose bisphosphate (RuBP) to form two molecules of GP. This reaction is catalysed by an enzyme called Rubisco.
- Each GP molecule is reduced into a triose phosphate molecule. Each reaction requires hydrolysis of ATP and use of reduced NADP. Reduced NADP becomes oxidised again by donating a H+ ion and electron to the reaction, so it is regenerated for the light-dependent reaction.
- For every six molecules of triose phosphate produced, five are converted back to RuBP which requires hydrolysis of ATP (RuBP is regenerated).
- For every six molecules of triose phosphate 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 triose phosphate only contains three carbon atoms, and we know that glucose contains six carbon atoms, we need two triose phosphate molecules to make one glucose. Only a sixth of the triose phosphate from each turn of the Calvin cycle can be used to produce glucose, meaning that six turns are required to make one molecule of glucose (remember there are two triose phosphoate 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.
Note that sometimes triose phosphate (TP) is called glyceraldehyde 3-phosphate (GALP) in some A-level biology specifications. Make sure you fully understand the requirements of your specification.
Summary
The light-independent reaction can be difficult to learn, so practice drawing out the Calvin cycle until it becomes second nature. To summarise:
- 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 triose phosphate molecules produced go into regeneration of RuBP, the other one goes into synthesis of glucose and other organic compounds.
