OCR A Level Biology A: Photosynthesis (H420)

This guide covers learning outcomes 5.2.1(a) to 5.2.1(g) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. It sits in module 5, Communication, homeostasis and energy. It teaches the link with respiration, chloroplast structure, pigments and chromatography, the light-dependent stage, the Calvin cycle, the uses of triose phosphate and limiting factors. Respiration (5.2.2) is a separate unit.

Use it with the revision notes and the practice set. Practical language is in Development of practical skills; the course is on the OCR A Level Biology hub. Tick off outcomes on the printable checklist, and test yourself with the free diagnostics.

Outcomes covered

Outcome	You must be able to
5.2.1(a)	Link photosynthesis and respiration, including raw materials and products
5.2.1(b)	Describe chloroplast structure and place the two main stages
5.2.1(c)	Explain the importance of pigments, light harvesting systems and photosystems; use thin layer chromatography (TLC)
5.2.1(d)	Explain the light-dependent stage: ATP, reduced NADP, electron carriers, cyclic and non-cyclic photophosphorylation, the role of water
5.2.1(e)	Explain carbon dioxide fixation and the Calvin cycle: RuBP, RuBisCO, GP, TP
5.2.1(f)	State the uses of TP and the recycling of TP to RuBP
5.2.1(g)	Explain limiting factors and water stress, the effect on GP, RuBP and TP, and investigate rates

5.2.1(a) Photosynthesis and respiration

Photosynthesis uses light energy to build large organic molecules from inorganic ones. The overall equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy, chlorophyll)

Respiration is close to the reverse. Glucose and oxygen are the raw materials of aerobic respiration, and carbon dioxide and water are its products. So the products of each process are the raw materials of the other. The difference is the energy: photosynthesis stores light energy in chemical bonds, while respiration releases it to make ATP.

In the light, a plant respires and photosynthesises at once. When photosynthesis is faster than respiration there is a net uptake of carbon dioxide and a net release of oxygen. At the light intensity where the two rates are equal there is no net gas exchange.

Worked example. A crop takes up 52.8 g of carbon dioxide. Calculate the mass of glucose made and the mass of oxygen released. (Relative molecular masses: CO₂ 44, glucose 180, O₂ 32.)

1. Moles of CO₂ = 52.8 ÷ 44 = 1.20 mol.
2. The equation gives 6 mol CO₂ per 1 mol glucose, so glucose = 1.20 ÷ 6 = 0.200 mol.
3. Mass of glucose = 0.200 × 180 = 36.0 g.
4. Oxygen is 6 mol per 6 mol CO₂, so 1.20 mol O₂ = 1.20 × 32 = 38.4 g.

5.2.1(b) Chloroplast structure

A chloroplast is surrounded by an envelope of two membranes; the outer one is the outer membrane. Inside is the stroma, a fluid containing the enzymes of the light-independent stage, starch grains, and a loop of DNA with ribosomes. This DNA codes for some chloroplast proteins, so the organelle makes part of its own machinery.

Suspended in the stroma are flattened sacs called thylakoids. Stacks of thylakoids are grana (singular granum). Lamellae are thylakoid membranes that join one granum to the next.

Region	Stage that happens there
Thylakoid membranes (grana)	Light-dependent stage: pigments in photosystems, electron carriers, ATP synthesis
Thylakoid space	Protons build up here
Stroma	Light-independent stage (Calvin cycle)

The large membrane area of many grana holds many photosystems and carriers. Calvin cycle enzymes in the stroma sit next to the thylakoids that supply ATP and reduced NADP.

Worked example. An electron micrograph shows a chloroplast 48 mm long. The magnification is ×8000. Actual length = image ÷ magnification = 48 mm ÷ 8000 = 0.0060 mm = 6.0 µm.

5.2.1(c) Pigments, photosystems and chromatography

A pigment absorbs some wavelengths and reflects or transmits others. Chlorophyll absorbs mostly red and blue-violet light and reflects green. An absorption spectrum plots the light a pigment takes in across the wavelengths. An action spectrum plots the rate of photosynthesis at each wavelength. The two match closely, which is evidence that chlorophyll drives photosynthesis. The action spectrum is not zero in green light, because accessory pigments absorb some of it.

The pigments are grouped in two light harvesting systems called photosystems. Each photosystem has many accessory pigment molecules (chlorophyll b and carotenoids) around a reaction centre. The accessory pigments absorb light and pass the energy inwards to the reaction centre, which contains a primary pigment, a form of chlorophyll a. Photosystem II (PSII) has its reaction centre peak at 680 nm and photosystem I (PSI) at 700 nm. Many pigments widen the range of wavelengths used.

Thin layer chromatography (TLC)

Pigments are extracted from leaves in a solvent. A small spot of extract is placed on a pencil line (the origin) on a TLC plate, and the base of the plate dips into a shallow pool of solvent. The solvent is the mobile phase, and the plate coating is the stationary phase. The solvent front rises, carrying each pigment a different distance depending on its solubility in the solvent and its attraction to the plate. The solvent must not touch the origin line, and the plate is removed and the front marked before the solvent runs off the top.

Rf is the pigment’s distance from the origin divided by the solvent front’s distance from the origin. Measure to the middle of each spot.

Worked example. The solvent front moves 7.5 cm. Four spots have centres 7.2, 5.1, 3.9 and 2.4 cm from the origin. Rf values: 7.2 ÷ 7.5 = 0.96; 5.1 ÷ 7.5 = 0.68; 3.9 ÷ 7.5 = 0.52; 2.4 ÷ 7.5 = 0.32. The spot with Rf 0.96 is carotene (very soluble in the solvent, so it travels furthest). Compare Rf values with reference values obtained in the same solvent to identify the others.

Safety and reliability: ventilate, keep solvent from flames, use pencil because ink would run, and apply small concentrated spots.

5.2.1(d) The light-dependent stage

This stage happens in the thylakoid membranes. Light energy is used to make ATP and reduced NADP, which are energy sources for the Calvin cycle.

Non-cyclic photophosphorylation

1. Light is absorbed by PSII and energy passes to the reaction centre. Two electrons are excited and leave the chlorophyll a molecule.
2. The electrons pass along a chain of electron carriers in the thylakoid membrane, losing energy at each step.
3. The energy released is used to pump protons into the thylakoid space. Protons then diffuse back through the enzyme ATP synthase, which joins ADP and Pᵢ to form ATP. Making ATP using light energy is photophosphorylation.
4. Light absorbed by PSI excites its electrons, which pass to NADP. With protons, NADP is reduced (reduced NADP).
5. The electrons lost from PSII are replaced by photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂.

Role of water. Water supplies electrons to replace those lost from PSII. It supplies protons, which contribute to the gradient and to reducing NADP. The oxygen released is a waste product (some is used by the plant’s own respiration, the rest leaves through stomata).

Cyclic photophosphorylation. Only PSI is involved. Excited electrons leave PSI, pass along electron carriers and return to PSI. ATP is made, but no reduced NADP and no oxygen are formed, and water is not split.

Feature	Non-cyclic	Cyclic
Photosystems used	PSII and PSI	PSI only
Electron source	Water	PSI itself
Products	ATP, reduced NADP, O₂	ATP only
Final electron acceptor	NADP	PSI (electrons return)

5.2.1(e) and (f) The Calvin cycle and uses of TP

The light-independent stage takes place in the stroma. Carbon dioxide is fixed when it combines with ribulose bisphosphate (RuBP), a five-carbon compound. The enzyme is ribulose bisphosphate carboxylase (RuBisCO). The six-carbon product is unstable and splits into two molecules of glycerate 3-phosphate (GP), each with three carbons.

1. Fixation: CO₂ + RuBP → 2 GP, catalysed by RuBisCO.
2. Reduction: GP gains hydrogen and loses oxygen to become triose phosphate (TP); ATP and reduced NADP from the light-dependent stage supply the energy and hydrogen. The NADP and ADP return to the thylakoids.
3. Regeneration: Most TP is used, with more ATP, to regenerate RuBP so the cycle continues.

Worked example. Six carbon dioxide molecules are fixed in six turns, each with one RuBP.

• 6 RuBP (30 carbons) + 6 CO₂ (6 carbons) = 36 carbons, giving 12 GP (3 × 12 = 36 carbons).
• 12 GP are reduced to 12 TP.
• 2 TP (6 carbons) leave the cycle, enough for one hexose.
• The remaining 10 TP hold 30 carbons, exactly the 6 × 5 needed to rebuild 6 RuBP.

So 10 of 12 TP, or 83.3%, are recycled. Carbon in equals carbon out.

Uses of TP. TP is the starting material for carbohydrates (glucose, then sucrose for transport, starch for storage, cellulose for cell walls), for lipids (the glycerol and fatty acid parts) and for amino acids (which also need a nitrogen source such as nitrate). TP that is not removed is recycled to make RuBP.

5.2.1(g) Limiting factors and investigations

A limiting factor is whichever condition is closest to its minimum and so holds the rate down. Raising it increases the rate until another factor becomes limiting.

Factor	Why it limits	Effect when it is low (stage affected)
Light intensity	Less energy for ATP and reduced NADP	GP rises (not reduced), TP falls, RuBP falls (not regenerated)
Carbon dioxide concentration	Less fixation by RuBisCO	GP falls, TP falls, RuBP rises (not used)
Temperature	Enzymes of both stages, including RuBisCO, are temperature sensitive	Rate rises with kinetic energy, then falls as enzymes denature; when RuBisCO is the slow step (cold, or hot and denaturing) GP and TP fall and RuBP rises

Water stress. When a plant loses water faster than it takes it up, guard cells close the stomata. This reduces water loss but also reduces carbon dioxide entry, so carbon dioxide becomes limiting. The effect is similar to low carbon dioxide: GP and TP fall and RuBP rises.

Worked example (rates). With light and carbon dioxide not limiting, a pondweed’s rate was 4.0 units at 15 °C, 9.0 at 25 °C, 13.5 at 35 °C and 3.0 at 45 °C. Percentage increase from 15 to 25 °C = (9.0 − 4.0) ÷ 4.0 × 100 = 125%. From 25 to 35 °C = 4.5 ÷ 9.0 × 100 = 50%. The rise slows because enzymes approach their optimum. At 45 °C the rate falls by (3.0 − 13.5) ÷ 13.5 × 100 = −77.8%, because enzymes denature.

Practical investigations

Typical methods: count bubbles or collect gas from aquatic pondweed, or measure the movement of a gas bubble in a capillary tube. Vary light by distance from a lamp, carbon dioxide by the concentration of hydrogencarbonate solution, and temperature with a water bath. Keep the other two factors constant. A heat shield, such as a tank of water, stops the lamp changing the temperature. Repeat readings for a mean.

Percentage error. Percentage error = uncertainty ÷ measured value × 100. A graduated tube read to ±0.01 cm³ at the start and end has a total uncertainty of 0.02 cm³. For 0.64 cm³ collected: 0.02 ÷ 0.64 × 100 = 3.1%.

Isolated chloroplasts can also be used: DCPIP is a blue dye that loses its colour when it accepts electrons from the light-dependent stage, so decolourising in light shows electrons are being passed on.

Common errors

• Saying carbon dioxide is split in the light-dependent stage. The oxygen comes from water.
• Writing that light “is used in the Calvin cycle”. Light is not needed directly; the stage depends on ATP and reduced NADP.
• Swapping the way GP and RuBP change when carbon dioxide or light falls.
• Giving Rf as a distance rather than a ratio.

Official syllabus

OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025, published by OCR. Section 5.2.1 Photosynthesis.