Photosynthesis and respiration: the links CCEA examiners test
Photosynthesis and respiration are usually revised as separate chapters and examined as one idea: energy carried by ATP and reduced coenzymes, made in one place and spent in another.
The two questions that unlock both processes
For any stage, answer two things: what goes in, and what is produced that the next stage needs. Learn the processes as a supply chain and the recall questions become mechanical.
Light-dependent stage: light energy is absorbed by chlorophyll, water is photolysed to give protons, electrons and oxygen, and the products are ATP and reduced NADP. Light-independent stage (Calvin cycle): carbon dioxide is fixed by combining with ribulose bisphosphate using rubisco, and the ATP and reduced NADP from the light stage are spent reducing glycerate 3-phosphate to triose phosphate.
Why the two stages fail together
A classic CCEA question removes light and asks what happens to the concentration of a named intermediate. Reason it through the supply chain: without light there is no ATP or reduced NADP, so GP cannot be reduced to TP. GP accumulates, TP falls, and RuBP falls because it is still being consumed by carbon fixation but is no longer being regenerated.
Remove carbon dioxide instead and the logic reverses: RuBP accumulates because it is not being used, and GP falls.
Practise both versions until you can state the direction of change for GP, TP and RuBP without hesitating. It is a recurring two- or three-mark gift.
Respiration: where the ATP actually comes from
- Glycolysis, in the cytoplasm: glucose to two pyruvate, net gain of 2 ATP by substrate-level phosphorylation, plus reduced NAD.
- Link reaction, in the mitochondrial matrix: pyruvate is decarboxylated and dehydrogenated to acetyl coenzyme A.
- Krebs cycle, in the matrix: carbon dioxide released, reduced NAD and reduced FAD produced, small direct ATP yield.
- Oxidative phosphorylation, on the inner mitochondrial membrane: electrons from the reduced coenzymes pass along the electron transport chain, protons are pumped into the intermembrane space, and the resulting gradient drives ATP synthase. Oxygen is the final electron acceptor.
Anaerobic conditions
Without oxygen, the electron transport chain backs up because there is no final electron acceptor. Reduced NAD cannot be reoxidised, so the Krebs cycle and link reaction stop. Only glycolysis continues, and only if NAD is regenerated — in mammals by reducing pyruvate to lactate.
That is why the anaerobic yield is tiny compared with the aerobic yield: substrate-level phosphorylation in glycolysis is all that remains.
Limiting factors, done properly
For a photosynthesis rate graph, describe the shape with figures first, then explain each region. The rising portion is limited by the factor being changed; the plateau is limited by something else — commonly temperature, carbon dioxide concentration or the amount of chlorophyll.
State explicitly which factor is limiting in each region. Answers that say 'it levels off because it reaches a maximum' describe the graph without explaining it and score nothing on an explain question.