How do plants capture light energy and use it to make organic molecules?
The light-dependent and light-independent reactions of photosynthesis, the role of chloroplast structure, the products of each stage, and the factors that limit the rate of photosynthesis.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on photosynthesis, covering the light-dependent and light-independent reactions, the structure of the chloroplast, the role of ATP and NADP, and limiting factors.
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What this dot point is asking
Edexcel wants you to describe the light-dependent and light-independent reactions of photosynthesis, relate them to chloroplast structure, state the products of each stage, and explain how light intensity, carbon dioxide concentration and temperature limit the rate. Limiting-factor graphs and the link between the two stages are the staple exam questions.
Chloroplast structure and the two stages
The light-dependent reactions
Light is absorbed by chlorophyll, exciting electrons. These pass along an electron transport chain, releasing energy used to make ATP. Water is split by photolysis (), releasing oxygen as a by-product and providing electrons and protons. NADP is reduced to reduced NADP.
The light-independent reactions (Calvin cycle)
In the stroma, carbon dioxide combines with the 5-carbon acceptor RuBP (ribulose bisphosphate), catalysed by the enzyme rubisco, to form two molecules of the 3-carbon glycerate 3-phosphate (GP). ATP (energy) and reduced NADP (hydrogen) from the light-dependent stage reduce GP to triose phosphate (TP). Some TP is used to make glucose, lipids and amino acids; most is used to regenerate RuBP (using more ATP) so the cycle continues. Six turns of the cycle fix six carbon dioxide molecules to make one glucose.
Limiting factors
The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity (provides energy for the light-dependent reactions), carbon dioxide concentration (the substrate for the Calvin cycle) or temperature (affects the enzymes such as rubisco). As you increase a limiting factor, the rate rises until another factor becomes limiting, at which point the line levels off. Above the optimum temperature, enzymes denature and the rate falls.
Examples in context
Example 1. Commercial greenhouses. Growers increase yields by raising the limiting factor. They burn fuel to add carbon dioxide and warmth, and use artificial lighting in winter, pushing each factor up until another becomes limiting. Tomato growers commonly enrich the air to around three times the normal carbon dioxide concentration, a direct application of limiting-factor theory that examiners use as context.
Example 2. Why the Calvin cycle stops in the dark. If you transfer an illuminated plant into darkness, GP rises and TP and RuBP fall, because no ATP or reduced NADP are being made to convert GP to TP, yet rubisco keeps fixing carbon dioxide onto RuBP for a short while. This classic experimental result shows the light-independent stage depends on the products of the light-dependent stage, a favourite exam deduction.
Try this
Q1. State the products of the light-dependent reactions. [3 marks]
- Cue. ATP, reduced NADP and oxygen.
Q2. Explain why increasing light intensity stops raising the rate of photosynthesis at high light levels. [2 marks]
- Cue. Another factor, such as carbon dioxide concentration or temperature, becomes limiting.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20186 marksDescribe the light-dependent reactions of photosynthesis and explain how their products are used in the light-independent reactions.Show worked answer →
Markers want the light-dependent events plus the link to the Calvin cycle.
In the thylakoid membranes, light is absorbed by chlorophyll, exciting electrons to a higher energy level. These electrons pass along an electron transport chain, releasing energy that pumps protons to create a gradient; protons then flow back through ATP synthase, making ATP (chemiosmosis or photophosphorylation). Water is split by photolysis (), replacing the lost electrons and releasing oxygen as a by-product. The protons and electrons reduce NADP to reduced NADP. In the light-independent reactions (Calvin cycle) in the stroma, the ATP provides energy and the reduced NADP provides hydrogen to reduce glycerate 3-phosphate to triose phosphate, used to make glucose and to regenerate RuBP.
Award marks for: light excites electrons; electron transport chain makes ATP; photolysis of water gives oxygen, electrons and protons; NADP reduced; ATP supplies energy and reduced NADP supplies hydrogen in the Calvin cycle.
Edexcel 20215 marksA greenhouse grower measured the rate of photosynthesis as the volume of oxygen released. At low light the rate rose steeply with light intensity, then levelled off at high light. Explain these results and deduce what factor limits the rate at high light intensity, suggesting how the grower could increase the rate.Show worked answer →
An analyse-and-deduce question on limiting factors.
At low light intensity, light is the limiting factor, so increasing it provides more energy for the light-dependent reactions and the rate rises steeply. At high light intensity the rate levels off, so light is no longer limiting; another factor (carbon dioxide concentration or temperature) must now be limiting because increasing light has no further effect. To increase the rate, the grower could raise the carbon dioxide concentration (for example by adding carbon dioxide to the greenhouse) and raise the temperature towards the optimum for the enzymes (such as rubisco).
Markers reward: light limiting at low intensity (steep rise); at the plateau light no longer limiting so carbon dioxide or temperature is limiting; correct suggestion to raise carbon dioxide or temperature.
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Sources & how we know this
- Pearson Edexcel A-Level Biology B (9BN0) specification — Pearson Edexcel (2015)