The light-independent stage and limiting factorsEdexcel A-Level Biology B: Revision notes
Section 1
The stroma and the Calvin cycle
The light-independent stage (the Calvin cycle) takes place in the stroma, which contains the enzymes needed. It uses ATP and reduced NADP from the light-dependent stage to build organic molecules from carbon dioxide. It needs no light directly but stops soon after the light is switched off.
Section 2
Carbon fixation
Carbon dioxide combines with the five-carbon compound ribulose bisphosphate (RuBP), catalysed by the enzyme RUBISCO (ribulose bisphosphate carboxylase). This is carbon fixation.
The unstable six-carbon product splits at once into two molecules of glycerate 3-phosphate (GP), a three-carbon compound.
GP is the first stable product, not GALP and not glucose.
Section 3
Reduction and regeneration
GP is reduced to glyceraldehyde phosphate (GALP), using hydrogen from reduced NADP and energy from ATP. Most of the GALP (five-sixths) is used with more ATP to regenerate RuBP, so the cycle can go on.
If light stops, ATP and reduced NADP run out. GP then accumulates because it cannot be reduced, and RuBP falls because it cannot be regenerated. Conversely, with low carbon dioxide RuBP accumulates and GP falls.
Link changes in GP and RuBP to the cause: light low means GP up and RuBP down; carbon dioxide low means GP down and RuBP up.
Section 4
Uses of GALP
The GALP that leaves the cycle is the raw material for other molecules. Two GALP join to make a hexose such as glucose, which can be polymerised to starch or cellulose. GALP is converted to glycerol and, via other compounds, to fatty acids to make lipids. Other carbon compounds made from GALP gain nitrogen (from nitrate) to become amino acids, and GALP also supplies carbon for nucleotides and other molecules.
Section 5
Limiting factors
A limiting factor is the factor in shortest supply that limits the rate of photosynthesis.
- Light intensity: low light means less ATP and reduced NADP, so the cycle slows.
- Carbon dioxide concentration: low carbon dioxide means less fixation by RUBISCO.
- Temperature: the Calvin cycle is enzyme-controlled; the rate rises with temperature up to an optimum of about 25 to 35 °C, then falls as enzymes denature.
On a graph the rate rises, then levels off when another factor becomes limiting. Example: rate 8 units at 0.04% carbon dioxide rising to 14 at 0.08% is a (14 − 8) ÷ 8 × 100 = 75% increase.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The light-independent stage and limiting factors
- Cells of the green alga Chlorella were kept in bright light with a constant supply of carbon dioxide, so that the Calvin cycle was running steadily. The light was then switched off while the carbon dioxide supply continued, and the concentrations of ribulose bisphosphate (RuBP) and glycerate 3-phosphate (GP) in the cells were measured.Explain the changes in the concentrations of GP and RuBP after the light was switched off.2 marks
- A grower produces tomatoes in a glasshouse. In winter she adds carbon dioxide to the air, uses artificial lighting and heats the glasshouse, but keeps the temperature below 35 °C.The grower finds that raising the temperature from 15 °C to 25 °C increases the rate of photosynthesis when light and carbon dioxide are plentiful. Explain why.2 marks
- A leaf was illuminated at a constant light intensity and a constant temperature of 25 °C. The rate of photosynthesis was 8 arbitrary units at 0.04% carbon dioxide, 14 arbitrary units at 0.08% and 15 arbitrary units at 0.12%.Calculate the percentage increase in the rate of photosynthesis when the carbon dioxide concentration is raised from 0.04% to 0.08%.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).