Photosynthesis is the process plants use to make glucose from carbon dioxide and water, using light energy absorbed by chlorophyll. It is one of the most-examined topics in GCSE and A-Level Biology because it underpins ecosystems, food chains and the carbon cycle.
At GCSE (AQA, Edexcel, OCR) you need the word and symbol equations, the factors that affect the rate, and the required practical investigating light intensity. At A-Level you go further into the light-dependent and light-independent (Calvin cycle) reactions, including the role of NADP, ATP and RuBisCO.
At GCSE you must recall the word and symbol equations, know the role of chlorophyll, and use the required practical (light intensity vs distance of a lamp from pondweed) to apply the inverse-square law. Common 6-mark questions ask you to explain how limiting factors interact.
At A-Level you need the two-stage mechanism in detail: the light-dependent reactions in the thylakoid membranes (photolysis of water, ATP via chemiosmosis, reduced NADP) and the Calvin cycle in the stroma (CO2 fixation onto RuBP by RuBisCO, reduction of GP to TP, regeneration of RuBP). Calculations on net primary productivity and questions on chromatography of plant pigments are common.
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Q: Write the balanced symbol equation for photosynthesis.
A: 6CO2 + 6H2O → C6H12O6 + 6O2 (light energy, chlorophyll).
Q: Name three limiting factors of photosynthesis.
A: Light intensity, carbon dioxide concentration and temperature.
Q: Where in the chloroplast does the Calvin cycle take place?
A: In the stroma — the fluid-filled space surrounding the thylakoids.
Q: What does RuBisCO do?
A: It catalyses the fixation of CO2 onto ribulose bisphosphate (RuBP) in the Calvin cycle.
Light spreads out from a point source, so intensity falls with the square of distance (the inverse-square law). Doubling the distance quarters the light intensity reaching the leaf.
Light-dependent reactions occur in the thylakoid membranes, split water and produce ATP and NADPH. Light-independent reactions (Calvin cycle) occur in the stroma and use that ATP and NADPH to fix CO2 into glucose.
No — without light, the light-dependent reactions stop, so no ATP or NADPH is made and the Calvin cycle quickly runs out of substrate. Plants only respire at night.
Red (around 660 nm) and blue (around 450 nm), because these are absorbed most strongly by chlorophyll a and b. Green light is mostly reflected, which is why leaves look green.
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