Photosynthesis Revision Notes | A-Level Biology (All Boards)

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Overview Key Terms Chloroplast Light Reaction Calvin Cycle Compare Stages Limiting Factors Exam Boards Common Mistakes FAQ

Why Photosynthesis Separates the A and the B Grade

Photosynthesis is one of the most heavily examined topics in A-Level Biology, and it is also one of the most misunderstood. The questions are rarely about reciting an equation. They ask you to explain a mechanism: how light energy ends up trapped in a molecule of glucose, where each stage happens inside the chloroplast, and what happens to the rate when you change a limiting factor. Students who can describe the two stages in the correct order, using the correct molecules, walk away with full marks. Students who blur the light-dependent and light-independent reactions together lose marks they have the knowledge to earn.

Having taught and examined this topic for over two decades, I can tell you that the single biggest reason students drop marks here is sequence and location. They know that ATP and reduced NADP are involved, but they cannot say which stage produces them and which stage uses them. They write that “carbon dioxide is split” when it is in fact water that undergoes photolysis. These are not knowledge gaps – they are precision gaps, and precision is exactly what the mark scheme rewards.

On this page I will take you through every part of photosynthesis your specification requires, in the order examiners want to see it. I will show you where each reaction happens, which molecules carry energy from one stage to the next, and the exact phrases that earn marks in the longer answers.

Board check: Photosynthesis is examined in AQA 3.5.1, OCR A 5.2.1, OCR B (Module 4, Energy for Biological Processes), Edexcel A (Salters-Nuffield) 5.5–5.9, Edexcel B Topic 5, and WJEC/Eduqas (A2 Unit 3 / Component 2, “Photosynthesis uses light energy to synthesise organic molecules”). WJEC, Eduqas and OCR A go furthest – they require you to name Photosystems I and II and distinguish cyclic from non-cyclic photophosphorylation. AQA does not use those exact terms, so check your spec before memorising them.

Key Terminology – The Words That Earn Marks

Photosynthesis has a dense vocabulary, and examiners have precise accept and reject criteria for each term. Learn these definitions exactly before you attempt any question.

Photosynthesis The process by which light energy is used to synthesise complex organic molecules (such as glucose) from carbon dioxide and water. The overall word equation is: carbon dioxide + water → glucose + oxygen, driven by light energy and chlorophyll.

Photophosphorylation The production of ATP from ADP and inorganic phosphate (P_i) using light energy, occurring in the thylakoid membranes during the light-dependent reaction.

Photolysis The splitting of water molecules using light energy, producing protons (H⁺), electrons, and oxygen. This is the source of the oxygen released by photosynthesis.

Photoionisation The loss of electrons from chlorophyll when it absorbs light. The excited electrons are raised to a higher energy level and leave the chlorophyll molecule.

Reduced NADP (NADPH) The reduced form of the coenzyme NADP. It carries hydrogen (and the electrons that come with it) from the light-dependent reaction to the light-independent reaction, where it is used to reduce GP to triose phosphate.

RuBP (ribulose bisphosphate) The 5-carbon (5C) carbon dioxide acceptor molecule in the Calvin cycle. It combines with CO₂ to begin carbon fixation.

Rubisco The enzyme (ribulose bisphosphate carboxylase) that catalyses the reaction between carbon dioxide and RuBP. It is often described as the most abundant enzyme on Earth.

Limiting factor A factor that, when in short supply, restricts the rate of a process. In photosynthesis the main limiting factors are light intensity, carbon dioxide concentration, and temperature.

Examiner reject list: Do NOT write that “carbon dioxide is split” in photolysis – it is water. Do NOT confuse NADP (photosynthesis) with NAD (respiration). Do NOT say light is “made into” ATP – light energy is used to produce ATP. Do NOT call GP and TP “glucose” – they are 3-carbon intermediates; glucose is built from triose phosphate later.

Chloroplast Structure – Linking Structure to Function

You cannot explain photosynthesis without referring to the structure of the chloroplast, because each stage happens in a specific location. Every board requires you to relate chloroplast structure to its role (AQA 3.5.1, OCR A 5.2.1b, Edexcel A 5.9).

Thylakoid membranes (stacked into grana) Site of the light-dependent reaction. The membranes hold the chlorophyll and electron carriers, and their large surface area maximises light absorption. Stacking into grana increases the surface area available for the light-dependent stage.

Thylakoid space (lumen) The space enclosed by the thylakoid membrane, where protons accumulate to create the proton gradient used to make ATP by chemiosmosis.

Stroma The fluid-filled matrix surrounding the grana. Site of the light-independent reaction (Calvin cycle). It contains the enzyme rubisco and all the other enzymes of the cycle.

Photosynthetic pigments Chlorophyll a, chlorophyll b, and the carotenoids (e.g. carotene, xanthophyll) are held in the thylakoid membranes. Different pigments absorb different wavelengths, broadening the range of light the chloroplast can use.

Practical link – separating pigments: OCR A (5.2.1c), WJEC and Eduqas require knowledge of thin-layer chromatography (TLC) or paper chromatography to separate chloroplast pigments. Each pigment travels a different distance up the plate; the R_f value (distance moved by pigment ÷ distance moved by solvent front) identifies it. This is a common 4–6 mark practical question.

The Light-Dependent Reaction

The light-dependent reaction takes place in the thylakoid membranes. Its job is to use light energy to produce the ATP and reduced NADP that the next stage needs, and to split water by photolysis (releasing oxygen as a by-product).

The Steps Examiners Want to See

Light excites electrons in chlorophyll (photoionisation) Chlorophyll absorbs light, raising a pair of electrons to a higher energy level. These excited electrons leave the chlorophyll molecule – the chlorophyll is now photoionised.

Electrons pass down the electron transfer chain The excited electrons are picked up by electron carriers in the thylakoid membrane and passed along the chain in a series of redox reactions, releasing energy at each step.

ATP is produced by chemiosmosis Energy released as electrons move down the chain is used to pump protons (H⁺) into the thylakoid space. The protons then diffuse back through ATP synthase down their electrochemical gradient, and this is coupled to the synthesis of ATP from ADP and P_i (photophosphorylation).

Water is split by photolysis Light energy splits water: 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons replace those lost from chlorophyll, the protons are used to reduce NADP, and the oxygen is released (or used in respiration).

NADP is reduced At the end of the chain, electrons and protons combine with NADP to form reduced NADP. Both the ATP and the reduced NADP now pass to the stroma for the light-independent reaction.

Photosystems and the two types of photophosphorylation (WJEC, Eduqas, OCR A): These boards require Photosystem I (PSI) and Photosystem II (PSII). In non-cyclic photophosphorylation, electrons flow from PSII (where photolysis replaces lost electrons) along the chain to PSI and finally to NADP – producing ATP, reduced NADP and oxygen. In cyclic photophosphorylation, only PSI is involved: electrons return to PSI rather than passing to NADP, so only ATP is made (no reduced NADP, no oxygen). AQA does not require the PSI/PSII terminology.

The mark-losing mistake: Students routinely write that “ATP synthase makes ATP from light.” ATP synthase does not use light directly – it uses the proton gradient set up by the electron transfer chain. The full marking chain is: light excites electrons → electrons move down the chain → protons pumped into the thylakoid space → protons diffuse back through ATP synthase → ATP is synthesised. Skip a link and you lose the mark.

The Light-Independent Reaction (Calvin Cycle)

The light-independent reaction takes place in the stroma. It does not need light directly, but it depends entirely on the ATP and reduced NADP made by the light-dependent reaction – which is why it stops in the dark once those run out. Its job is to fix carbon dioxide into organic molecules.

The Three Stages of the Calvin Cycle

Carbon fixation Carbon dioxide combines with the 5-carbon acceptor RuBP, catalysed by the enzyme rubisco. The unstable 6-carbon intermediate immediately breaks down into two molecules of glycerate 3-phosphate (GP), a 3-carbon compound.

Reduction of GP to triose phosphate GP is reduced to triose phosphate (TP), a 3-carbon sugar. This step uses reduced NADP (as the reducing agent) and energy from the hydrolysis of ATP – both supplied by the light-dependent reaction.

Regeneration of RuBP Most of the triose phosphate is used to regenerate RuBP, using more ATP, so the cycle can continue to fix more carbon dioxide. Only a small proportion of TP leaves the cycle to build useful products.

The numbers examiners reward: To make one molecule of glucose, the cycle must turn six times, fixing six CO₂. This uses 18 ATP and 12 reduced NADP. Of the 12 TP produced, only 2 leave to form glucose; the other 10 regenerate RuBP. Quoting these figures correctly is the difference between a good answer and a full-mark answer.

What triose phosphate becomes: The TP that leaves the cycle is used to synthesise glucose and other carbohydrates (starch, cellulose, sucrose), as well as lipids (glycerol and fatty acids) and, with the addition of nitrogen, amino acids. This links photosynthesis to the rest of the plant’s metabolism.

Common error: Do not say “CO₂ is reduced to glucose in one step.” Carbon fixation produces GP, not glucose. GP is then reduced to TP, and only later is TP built up into glucose. Naming GP and TP in the correct order is a frequent marking point.

Comparing the Two Stages of Photosynthesis

Comparison questions between the two stages are common and require direct paired comparisons. Writing features in isolation scores zero – compare like with like.

Feature	Light-dependent reaction	Light-independent reaction (Calvin cycle)
Location	Thylakoid membranes	Stroma
Requires light directly?	Yes	No (uses ATP & reduced NADP from stage 1)
Raw materials	Light, water, ADP + P_i, NADP	Carbon dioxide, ATP, reduced NADP, RuBP
Products	ATP, reduced NADP, oxygen	Triose phosphate (→ glucose etc.), regenerated RuBP, ADP + P_i, NADP
Key process	Photophosphorylation & photolysis	Carbon fixation & reduction
Key enzyme	ATP synthase	Rubisco

Limiting Factors – The Rate of Photosynthesis

Every board requires you to explain how light intensity, carbon dioxide concentration and temperature affect the rate of photosynthesis (AQA 3.5.1, OCR A 5.2.1, WJEC/Eduqas). This is also where the maths and graph-reading marks live.

The Three Main Limiting Factors

Light intensity Provides the energy for the light-dependent reaction. As light intensity increases, the rate increases – up to a point. Beyond that, another factor becomes limiting and the rate plateaus.

Carbon dioxide concentration The raw material for carbon fixation. CO₂ is often the limiting factor in normal field conditions because atmospheric levels (~0.04%) are low relative to what the plant could use.

Temperature Affects the enzyme-controlled reactions, especially in the Calvin cycle. Rate increases with temperature up to an optimum, then falls as enzymes (including rubisco) begin to denature.

Reading the graph – what examiners want: On a rate-against-light-intensity graph, the steep linear region shows light as the limiting factor. The plateau shows that light is no longer limiting – something else (CO₂ or temperature) now caps the rate. To prove which, you raise the suspected factor: if the rate increases, that factor was limiting. You should be able to identify limiting factors from a graph and explain commercial applications such as glasshouses.

Commercial application (AQA & others): Growers raise yields by removing limiting factors in glasshouses – using artificial lighting, paraffin heaters or CO₂ enrichment to keep light, temperature and CO₂ above their limiting levels. You may be asked to evaluate the costs and benefits of these practices.

Exam Board Comparison – What Your Board Requires

This is the table no other revision site provides. Use it to check exactly what your board requires – do not waste time learning content your specification does not examine.

Subtopic	AQA	OCR A	OCR B	Edexcel A	Edexcel B	WJEC / Eduqas
Chloroplast structure linked to function	✔	✔	✔	✔	✔	✔
Pigments & chromatography (R_f)	❌	✔	✔	❌	✔	✔
Photoionisation of chlorophyll	✔	✔	✔	✔	✔	✔
Photosystems I & II named	❌	✔	✔	❌	✔	✔
Cyclic vs non-cyclic photophosphorylation	❌	✔	✔	❌	✔	✔
Photolysis of water	✔	✔	✔	✔	✔	✔
Chemiosmosis / ATP synthase	✔	✔	✔	Implicit	✔	✔
Calvin cycle (RuBP, GP, TP, rubisco)	✔	✔	✔	✔	✔	✔
Hill reaction practical	❌	❌	❌	✔	❌	❌
Limiting factors & commercial use	✔	✔	✔	✔	✔	✔

Edexcel A note – the Hill reaction: Edexcel A (Salters-Nuffield) uniquely requires Core Practical 11: investigating photosynthesis using isolated chloroplasts (the Hill reaction), using a redox indicator such as DCPIP that changes colour as it is reduced by electrons from the light-dependent reaction.

8 Common Mistakes from Examiner Reports

These are the errors I see again and again, both as an examiner and as a tutor. Every one of them costs marks.

#	The mistake	The correction
1	“Carbon dioxide is split in photolysis”	It is water that is split by photolysis, releasing oxygen. Carbon dioxide is fixed in the Calvin cycle, not split.
2	Confusing NADP with NAD	Photosynthesis uses NADP; respiration uses NAD. Using the wrong coenzyme loses the mark.
3	“ATP synthase makes ATP from light”	ATP synthase uses the proton gradient, not light directly. Light drives the electron chain that builds the gradient.
4	Calling GP and TP “glucose”	GP and TP are 3-carbon intermediates. Glucose is built from triose phosphate later in the pathway.
5	Saying the Calvin cycle “does not use light”	It needs no light directly, but it depends on ATP and reduced NADP from the light-dependent reaction, so it stops in prolonged darkness.
6	Putting the light reaction in the stroma	Light-dependent reaction = thylakoid membranes. Calvin cycle = stroma. Mixing up locations is a frequent error.
7	Forgetting RuBP regeneration	Most triose phosphate regenerates RuBP. If you omit this, the cycle “runs out” of acceptor – examiners look for the regeneration step.
8	Vague limiting-factor answers	Don’t just say “light helps photosynthesis.” State which factor is limiting in the region of the graph and explain why the rate plateaus.

Struggling to Keep the Two Stages Straight?

Photosynthesis is the topic where precision earns the grade. If the light-dependent and light-independent reactions blur into one in your mind, or you can never remember which stage makes ATP and which one uses it, tutoring will fix that fast. I have examined this exact topic and I teach it in the order the mark scheme rewards.

Tyrone John • CBiol MRSB • Former WJEC/Eduqas & Edexcel Examiner • 25+ Years Teaching A-Level Biology

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Frequently Asked Questions – Photosynthesis

Where exactly do the two stages of photosynthesis take place?

The light-dependent reaction takes place in the thylakoid membranes (the stacked grana) of the chloroplast, where chlorophyll and the electron carriers are located. The light-independent reaction, also called the Calvin cycle, takes place in the stroma – the fluid-filled matrix surrounding the grana – which contains the enzyme rubisco. Naming the correct location for each stage is a frequent marking point.

What is the difference between photolysis and photophosphorylation?

Photolysis is the splitting of water using light energy, producing protons, electrons and oxygen. The electrons replace those lost from photoionised chlorophyll, and the oxygen is released. Photophosphorylation is the production of ATP using light energy, driven by a proton gradient across the thylakoid membrane and catalysed by ATP synthase. Both happen during the light-dependent reaction, but they are different processes – one splits water, the other makes ATP.

Why does the Calvin cycle stop in the dark if it is “light-independent”?

The Calvin cycle does not use light directly, but it depends completely on the ATP and reduced NADP made by the light-dependent reaction. In the dark, the light-dependent reaction stops, so ATP and reduced NADP are no longer supplied. Without them, GP cannot be reduced to triose phosphate, and the cycle grinds to a halt. So “light-independent” means it needs no light directly – not that it can run indefinitely without light.

How many times does the Calvin cycle turn to make one glucose?

The Calvin cycle must turn six times to produce one molecule of glucose, fixing six molecules of carbon dioxide. This uses 18 ATP and 12 reduced NADP. Each turn produces two molecules of triose phosphate (TP); over six turns that is 12 TP, but only 2 of them leave the cycle to build glucose. The other 10 are used to regenerate RuBP so the cycle can continue.

What is the difference between cyclic and non-cyclic photophosphorylation?

In non-cyclic photophosphorylation, electrons flow from Photosystem II along the electron transfer chain to Photosystem I and finally to NADP. It produces ATP, reduced NADP and oxygen (from photolysis, which replaces the electrons lost by PSII). In cyclic photophosphorylation, only Photosystem I is involved: the electrons are returned to PSI rather than passed to NADP, so only ATP is produced – no reduced NADP and no oxygen. This distinction is required by OCR A, WJEC and Eduqas, but not by AQA.

What is rubisco and why is it important?

Rubisco (ribulose bisphosphate carboxylase) is the enzyme that catalyses the first step of the Calvin cycle – the reaction between carbon dioxide and RuBP. This is the point at which inorganic carbon dioxide is “fixed” into an organic molecule, so rubisco effectively controls the entry of carbon into the living world. It is often described as the most abundant enzyme on Earth. Because it works relatively slowly, it is frequently a limiting factor on the overall rate of photosynthesis.

How do limiting factors affect the rate of photosynthesis?

A limiting factor is whichever factor is in shortest supply and therefore caps the rate. The three main limiting factors are light intensity, carbon dioxide concentration and temperature. On a graph, the steep region shows that the factor on the x-axis is limiting (increasing it increases the rate); the plateau shows it is no longer limiting because something else now restricts the rate. Growers exploit this in glasshouses by adding light, heat and extra carbon dioxide to push all three above their limiting levels and increase yield.

How should I structure a 6-mark answer on the light-dependent reaction?

For a top-level answer, work through the steps in order using precise terminology: chlorophyll absorbs light and is photoionised, releasing excited electrons; the electrons pass down the electron transfer chain in the thylakoid membrane; energy released pumps protons into the thylakoid space; protons diffuse back through ATP synthase, driving the synthesis of ATP (photophosphorylation); water is split by photolysis to give protons, electrons and oxygen; and NADP is reduced. Make clear that the products – ATP and reduced NADP – then pass to the stroma for the Calvin cycle.

Written by Tyrone John

CBiol MRSB • Former WJEC/Eduqas & Edexcel Examiner • PGCE • 25+ Years Teaching A-Level Biology • Published Scientific Research

Tyrone has over 25 years of experience teaching A-Level Biology and is a Chartered Biologist and member of the Royal Society of Biology. As a former examiner for WJEC/Eduqas and Edexcel, he has first-hand knowledge of how mark schemes are applied and what examiners look for in student answers. Learn more →