How do plants capture light energy and use it to build organic molecules?
Photosynthesis as a two-stage process: the light-dependent reactions in the thylakoid membranes (photoionisation of chlorophyll, photolysis of water, the production of ATP by photophosphorylation, the production of reduced NADP, and the role of the electron transport chain); the light-independent reactions in the stroma (the Calvin cycle: fixation of carbon dioxide by RuBP to form GP, reduction of GP to TP using reduced NADP and ATP, and regeneration of RuBP); the effect of light intensity, carbon dioxide concentration and temperature as limiting factors.
A focused answer to the AQA 3.5 dot point on photosynthesis. Covers the light-dependent reactions (photoionisation, photolysis, the electron transport chain and photophosphorylation), the Calvin cycle in the stroma, and how light, carbon dioxide and temperature act as limiting factors.
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What this dot point is asking
AQA wants you to describe photosynthesis as two linked stages, name every intermediate in the Calvin cycle, explain exactly how the light-dependent products power the light-independent reactions, and use limiting-factor reasoning to interpret graphs.
The answer
Photosynthesis converts light energy into chemical energy stored in organic molecules. The overall equation is:
It happens in two stages in the chloroplast: the light-dependent reactions in the thylakoid membranes, and the light-independent reactions (Calvin cycle) in the stroma.
The light-dependent reactions
These take place on the thylakoid membranes, which hold chlorophyll in photosystems. They require light and produce ATP, reduced NADP and oxygen.
- Photoionisation of chlorophyll. Light excites electrons in chlorophyll to a higher energy level; these excited electrons leave the chlorophyll (the chlorophyll is photoionised).
- Electron transport chain. The excited electrons pass along a chain of electron carriers in the thylakoid membrane. As they pass, energy is released and used to pump protons (H plus) into the thylakoid space, creating a proton gradient.
- Chemiosmosis and photophosphorylation. Protons flow back into the stroma through ATP synthase, and this drives the synthesis of ATP from ADP and Pi.
- Photolysis of water. Light splits water to replace the electrons lost from chlorophyll:
The electrons replace those lost by chlorophyll, the protons contribute to the gradient and reduce NADP, and the oxygen is released as a waste product.
5. Reduction of NADP. At the end of the chain, electrons and protons reduce NADP to reduced NADP, which carries hydrogen to the Calvin cycle.
The light-independent reactions (Calvin cycle)
These take place in the stroma and do not directly need light, but they depend on the ATP and reduced NADP made by the light-dependent stage. The cycle has three steps.
- Fixation. Carbon dioxide combines with the 5-carbon ribulose bisphosphate (RuBP), catalysed by the enzyme rubisco, to form two molecules of the 3-carbon glycerate 3-phosphate (GP).
- Reduction. GP is reduced to triose phosphate (TP) using hydrogen from reduced NADP and energy from ATP. Some TP leaves the cycle to make glucose, amino acids, lipids and other organic molecules.
- Regeneration. Most TP is used, with energy from more ATP, to regenerate RuBP so the cycle can continue.
To make one molecule of glucose (a 6-carbon sugar), the cycle must turn six times, fixing six molecules of carbon dioxide.
Limiting factors
The rate of photosynthesis is set by whichever factor is in shortest supply. The three you must know are light intensity, carbon dioxide concentration and temperature.
- Light intensity. Provides the energy for the light-dependent reactions. At low light, increasing light raises the rate; eventually another factor becomes limiting and the curve plateaus.
- Carbon dioxide concentration. The substrate for fixation. In most natural conditions, CO2 (about 0.04 percent of air) is the main limiting factor.
- Temperature. Affects enzyme activity (especially rubisco). Rate rises with temperature up to an optimum, then falls as enzymes denature. Above about 25 degrees Celsius, the stomata may also close to reduce water loss, lowering CO2 supply.
Examples in context
Example 1. Commercial glasshouse tomato production. Growers enrich the air to roughly 0.1 percent carbon dioxide, add supplementary lighting in winter and hold the temperature near 24 degrees Celsius. Each of these removes a different limiting factor, so the rate of the Calvin cycle stays high and yields can be two to three times those of an unmanaged crop, a direct application of limiting-factor theory.
Example 2. Shade plants on a forest floor. Plants such as ferns growing under a canopy have a larger surface area of thylakoid membrane and more chlorophyll per chloroplast. This raises the efficiency of the light-dependent reactions at low light intensity, letting them photosynthesise where light is the permanent limiting factor.
Try this
Q1. State precisely where in the chloroplast the light-dependent and light-independent reactions occur. [2 marks]
- Cue. Light-dependent: thylakoid membranes. Light-independent (Calvin cycle): stroma.
Q2. Explain why the oxygen released in photosynthesis comes from water and not carbon dioxide. [2 marks]
- Cue. Water is split by photolysis, releasing oxygen, protons and electrons; carbon dioxide is reduced and its carbon is incorporated into organic molecules, not released as oxygen.
Q3. A plant is moved from bright light into darkness. Explain what happens to the concentrations of GP and RuBP in the next few seconds. [3 marks]
- Cue. GP rises (it is still made by fixation but cannot be reduced without ATP and reduced NADP); RuBP falls (it is still used in fixation but cannot be regenerated, which needs ATP).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 AQA5 marksDescribe how the products of the light-dependent reaction are used in the light-independent reaction (Calvin cycle).Show worked answer →
A 5-mark answer needs to link the two stages by name.
- The light-dependent reaction produces ATP and reduced NADP.
- In the Calvin cycle, carbon dioxide combines with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP).
- Reduced NADP provides hydrogen to reduce GP to triose phosphate (TP).
- ATP provides energy for this reduction and for the regeneration of RuBP.
- The oxidised NADP and ADP plus Pi return to the thylakoid for the light-dependent reaction.
Markers reward naming GP, TP and RuBP correctly and stating which product does which job.
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