Photosynthesis overview for A-Level Biology

Photosynthesis is the process by which plants, algae and some bacteria convert light energy into chemical energy stored in organic molecules, primarily glucose. It happens inside chloroplasts and takes place in two linked stages: The light-dependent reactions in the thylakoid membranes, and the light-independent reactions (the Calvin cycle) in the stroma.

This guide covers the overall equation, the two stages in detail, the role of chlorophyll and electron carriers, the limiting factors that affect rate, and the specific wording the AQA A-Level Biology mark scheme rewards in Section 3.5.1.

The overall summary equation for photosynthesis is: 6CO2 + 6H2O → C6H12O6 + 6O2. This sums up the inputs and outputs but hides every interesting step. At A-Level you are expected to break it into two stages and explain where each reactant goes and each product comes from.

Two facts the mark scheme cares about: The oxygen released comes from the splitting of water, not from carbon dioxide. The glucose carbon atoms come from carbon dioxide, fixed in the Calvin cycle.

The light-dependent reactions happen on the thylakoid membranes inside the chloroplast. Light is absorbed by chlorophyll in two photosystems (PSII and PSI), exciting electrons to a higher energy level. These electrons pass along an electron transport chain, releasing energy used to make ATP by chemiosmosis.

At the same time, water is split in a process called photolysis. This replaces the excited electrons in PSII, releases oxygen as a waste product, and provides hydrogen ions and electrons that ultimately reduce NADP to reduced NADP.

The light-independent reactions, also called the Calvin cycle, happen in the stroma of the chloroplast. They use the ATP and reduced NADP from the light stage to fix carbon dioxide into a three-carbon sugar called glycerate 3-phosphate (GP), which is then reduced to triose phosphate (TP).

The key enzyme is rubisco, which catalyses the reaction between carbon dioxide and ribulose bisphosphate (RuBP), a five-carbon molecule. Some triose phosphate leaves the cycle to make glucose and other useful organic compounds. The rest is used to regenerate RuBP so the cycle continues.

The rate of photosynthesis is controlled by whichever factor is in shortest supply at any moment. AQA expects you to know three limiting factors and how each affects the light-dependent or light-independent stage.

The three factors are light intensity, carbon dioxide concentration and temperature. A common exam graph shows rate plateauing because one of these is the limiting factor; you are expected to identify which one and explain why.

AQA examiner reports flag the same handful of errors every year. Most are about precise vocabulary, not biological understanding. Spending five minutes locking down the mark-scheme phrasing pays off across the whole paper.

A student investigates the rate of photosynthesis at increasing light intensities at 20°C and 0.04% CO2. The rate increases linearly, then plateaus. Explain why the rate plateaus.

Step 1: Identify what is on the x-axis. Light intensity is increasing.

Step 2: As light intensity increases, more photons are absorbed by chlorophyll, so the light-dependent reactions speed up and more ATP and reduced NADP are produced for the Calvin cycle.

Step 3: At the plateau, light is no longer the limiting factor. Either CO2 concentration or temperature is now limiting the rate. At 0.04% CO2 (atmospheric), CO2 is most likely the limiting factor because rubisco cannot fix carbon any faster.

Full mark answer: Light intensity is no longer the limiting factor; CO2 concentration limits the rate of the Calvin cycle, so rubisco fixes CO2 at its maximum rate and the overall rate cannot increase further.