How does a chloroplast capture light energy and use it to fix carbon dioxide into sugars?
5.2.1 Photosynthesis: the structure of the chloroplast; the light-dependent stage (photolysis of water, photophosphorylation and the reduction of NADP); the light-independent stage (the Calvin cycle, fixing carbon dioxide using RuBP, forming GP and TP and regenerating RuBP); and the effect of limiting factors (light intensity, carbon dioxide concentration and temperature).
A focused answer to the OCR H420 5.2.1 dot point on photosynthesis. Covers chloroplast structure, the light-dependent stage (photolysis, photophosphorylation and reduced NADP), the light-independent stage (the Calvin cycle with RuBP, GP and TP), and the effect of limiting factors.
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What this dot point is asking
OCR wants you to describe the structure of the chloroplast, explain the light-dependent stage (photolysis, photophosphorylation and the reduction of NADP), explain the light-independent stage (the Calvin cycle), and explain how limiting factors affect the rate of photosynthesis.
The answer
Chloroplast structure
The chloroplast is adapted to its two stages:
- Thylakoids (flattened sacs, stacked into grana) contain chlorophyll and the electron carriers, and are the site of the light-dependent stage. Their stacking gives a large surface area for light absorption.
- The stroma is the fluid surrounding the thylakoids, containing the enzymes for the light-independent stage (the Calvin cycle).
The light-dependent stage (in the thylakoid membranes)
Light energy is converted into chemical energy in ATP and reduced NADP:
- Light is absorbed by chlorophyll, exciting electrons that leave the chlorophyll and pass along an electron transport chain in the thylakoid membrane.
- As electrons move, protons are pumped into the thylakoid space; they flow back out through ATP synthase, making ATP (photophosphorylation by chemiosmosis).
- Photolysis of water splits water using light energy: . This replaces the electrons lost from chlorophyll, releases oxygen as a by-product and provides protons.
- At the end of the chain, electrons and protons reduce NADP to reduced NADP.
ATP and reduced NADP then pass to the stroma for the next stage.
The light-independent stage (the Calvin cycle, in the stroma)
The Calvin cycle fixes carbon dioxide into sugars using the ATP and reduced NADP from the light-dependent stage:
- Carbon fixation. Carbon dioxide combines with the 5-carbon RuBP (ribulose bisphosphate), catalysed by the enzyme rubisco, forming two molecules of the 3-carbon GP (glycerate 3-phosphate).
- Reduction. GP is reduced to TP (triose phosphate) using reduced NADP and energy from ATP.
- Regeneration. Most TP is used to regenerate RuBP (using more ATP), so the cycle can continue; some TP is used to make glucose and other organic molecules.
It takes several turns of the cycle to make one glucose molecule.
Limiting factors
The rate of photosynthesis is limited by whichever factor is in shortest supply:
- Light intensity: more light means more photolysis and more ATP and reduced NADP, up to a point.
- Carbon dioxide concentration: more carbon dioxide means more fixation in the Calvin cycle (often the limiting factor in the field).
- Temperature: the reactions are enzyme-controlled, so the rate rises with temperature up to an optimum, then falls as enzymes (such as rubisco) denature.
On a graph, the rate rises with the factor then plateaus when another factor becomes limiting.
Examples in context
Example 1. Greenhouse carbon dioxide enrichment. Commercial growers raise the carbon dioxide concentration (and light and temperature) in greenhouses to lift the limiting factor and increase the rate of photosynthesis, boosting crop yield.
Example 2. The Calvin lollipop experiment. Calvin traced radioactive carbon-14 through the cycle, showing the order GP then TP then RuBP, which is why we know the structure of the light-independent stage.
Try this
Q1. State the products of the light-dependent stage that are used in the Calvin cycle. [2 marks]
- Cue. ATP and reduced NADP.
Q2. Explain where the oxygen released in photosynthesis comes from. [2 marks]
- Cue. From the photolysis (splitting) of water in the light-dependent stage, which produces protons, electrons and oxygen.
Q3. Name the enzyme that catalyses the fixation of carbon dioxide onto RuBP. [1 mark]
- Cue. Rubisco (RuBP carboxylase).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/01 20196 marksDescribe the light-dependent stage of photosynthesis and explain how it produces ATP and reduced NADP.Show worked answer →
Sequence photolysis, the electron transport chain and the two products.
Light is absorbed by chlorophyll in the thylakoid membranes, exciting electrons that leave the chlorophyll (photoionisation) and pass along an electron transport chain. As they do, protons are pumped into the thylakoid space, and the electrons' energy is used; the protons flow back through ATP synthase, making ATP (photophosphorylation, by chemiosmosis).
Photolysis of water splits water using light: , replacing the lost electrons, releasing oxygen as a by-product and providing protons. At the end of the chain, electrons and protons reduce NADP to reduced NADP.
Markers reward light exciting electrons, photolysis replacing them and releasing oxygen, ATP made by chemiosmosis through ATP synthase, and reduced NADP formed.
OCR H420/01 20214 marksExplain what happens to the concentrations of RuBP and GP in the Calvin cycle if a plant is suddenly deprived of carbon dioxide.Show worked answer →
Reason from which reaction stops to the knock-on effect on each intermediate.
Carbon dioxide combines with RuBP to form GP (catalysed by rubisco). If carbon dioxide is removed, this reaction stops.
So GP is no longer being produced, but GP continues to be converted to TP (using ATP and reduced NADP), so the concentration of GP falls.
Meanwhile RuBP is no longer being used up (no carbon dioxide to react with), but it is still being regenerated from TP, so the concentration of RuBP rises.
Markers reward GP falling because it is not made but is still used, and RuBP rising because it is not used but is still regenerated.
Related dot points
- 5.2.2 Respiration: the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation); the role of decarboxylation, dehydrogenation, reduced NAD and FAD, the electron transport chain, chemiosmosis and ATP synthase; the synthesis of ATP and the role of oxygen as the final electron acceptor; and anaerobic respiration in animals (lactate) and in yeast (ethanol).
A focused answer to the OCR H420 5.2.2 dot point on respiration. Covers the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation), chemiosmosis and ATP synthase, the role of oxygen, and anaerobic respiration producing lactate or ethanol.
- 5.1.5 Plant and animal responses: tropisms and the role of auxin (IAA) in phototropism; the structure and function of the mammalian nervous system (central and peripheral, voluntary and autonomic), the reflex arc and the fight-or-flight response; and the structure and the sliding filament mechanism of skeletal muscle contraction.
A focused answer to the OCR H420 5.1.5 dot point on plant and animal responses. Covers tropisms and the role of auxin in phototropism, the organisation of the mammalian nervous system, the reflex arc and fight-or-flight, and the sliding filament mechanism of muscle contraction.
- 3.1.3 Transport in plants: the structure and function of xylem and phloem; the cohesion-tension theory of water transport in the xylem and the factors affecting transpiration; the mass flow hypothesis of translocation in the phloem from source to sink; and the adaptations of xerophytes for reducing water loss.
A focused answer to the OCR H420 3.1.3 dot point on transport in plants. Covers xylem and phloem structure, the cohesion-tension theory of transpiration, the factors affecting transpiration rate, the mass flow hypothesis of translocation, and xerophyte adaptations.
- 2.1.4 Enzymes: the role of enzymes as biological catalysts in metabolic reactions; the mechanism of enzyme action including the lock-and-key and induced-fit models; the effects of temperature, pH, enzyme and substrate concentration on the rate of reaction; the action of competitive and non-competitive inhibitors; the roles of cofactors, coenzymes and prosthetic groups.
A focused answer to the OCR H420 2.1.4 dot point on enzymes. Covers enzymes as catalysts, the lock-and-key and induced-fit models, activation energy, the effects of temperature, pH and concentration, competitive and non-competitive inhibition, and cofactors.
- 2.1.1 Cell structure: the ultrastructure of eukaryotic and prokaryotic cells, the function of organelles including the role of the rough endoplasmic reticulum and Golgi apparatus in producing and secreting proteins; the use, calibration and resolution of light and electron microscopes.
A focused answer to the OCR H420 2.1.1 dot point on cell structure and microscopy. Covers every required eukaryotic and prokaryotic organelle, the protein secretory pathway, the three microscopes, eyepiece-graticule calibration and the magnification equation.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)