The light-independent reaction (Calvin cycle)AQA A-Level Biology: Revision notes
Section 1
Where it happens and what it needs
The light-independent reaction takes place in the stroma of the chloroplast. It does not use light directly, but it depends on the products of the light-dependent reaction: reduced NADP (hydrogen/electron supply) and ATP (energy supply, released by hydrolysis of ATP to ADP and Pi). The product is a simple sugar, via a cycle of reactions known as the Calvin cycle.
The Calvin cycle is not simply a dark reaction. It needs ATP and reduced NADP, so it stops soon after light stops.
Section 2
Carbon fixation
Carbon dioxide diffuses into the stroma and reacts with the five-carbon compound ribulose bisphosphate (RuBP). The reaction is catalysed by the enzyme rubisco. This forms an unstable six-carbon intermediate that immediately splits into two molecules of glycerate 3-phosphate (GP), a three-carbon compound. This step is carbon dioxide fixation.
Do not say carbon dioxide combines with GP. Carbon dioxide combines with RuBP and GP is the product.
Section 3
Reduction of GP
GP is reduced to triose phosphate (TP), a three-carbon sugar phosphate. This step uses ATP (as energy) and reduced NADP (as the source of hydrogen). The reduced NADP is oxidised back to NADP, and ATP is hydrolysed to ADP + Pi, which return to the light-dependent reaction.
Section 4
Regeneration of RuBP
Most of the triose phosphate is used to regenerate RuBP. This needs ATP. Regenerating RuBP means the cycle can continue to fix carbon dioxide. If all the triose phosphate were removed, the cycle would stop because there would be no RuBP.
Section 5
Useful organic substances
Some triose phosphate leaves the cycle (about one in six molecules). It is converted to useful organic substances: hexose sugars such as glucose, then starch and cellulose; lipids (from glycerol and fatty acids); and amino acids (with a nitrogen source) for proteins. Plants also use these as respiratory substrates.
Section 6
Linking the cycle and predicting changes
If light is removed, ATP and reduced NADP fall, so GP rises and RuBP falls. If carbon dioxide is removed, GP falls and RuBP rises (still regenerated, no longer used). Use these patterns to interpret graphs and radioactive-tracer data, in which labelled carbon appears first in GP, then triose phosphate, then RuBP and other products.
In a change question, ask: what is no longer made, what is no longer used, and which compound is next in the cycle?
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The light-independent reaction (Calvin cycle)
- A plant biochemist is studying the first step of the Calvin cycle in an extract of chloroplasts, using carbon dioxide and the five-carbon compound ribulose bisphosphate (RuBP).Explain, using the number of carbon atoms in each compound, how one carbon dioxide molecule reacting with RuBP leads to the formation of two molecules of GP.2 marks
- A culture of algae is kept in bright light with a constant supply of carbon dioxide. The concentrations of glycerate 3-phosphate (GP), reduced NADP and ribulose bisphosphate (RuBP) in the cells are steady. The carbon dioxide supply is then suddenly stopped while the light stays on.Explain the change in the concentration of RuBP after the carbon dioxide supply is stopped.2 marks
- A herbicide that inhibits the enzyme catalysing the reduction of glycerate 3-phosphate (GP) to triose phosphate (TP) is sprayed on a field of wheat in bright sunlight.Predict and explain the effect of the herbicide on the concentrations of GP, triose phosphate and RuBP in the wheat leaves.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).