OCR A Level Biology A: Photosynthesis (H420) – Revision Notes

These notes condense 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. For full explanations go to the study guide, then test yourself with the practice set.

Related pages: the OCR A Level Biology hub, the printable checklist, Development of practical skills and the free diagnostics.

Key equation and link to respiration (5.2.1(a))

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, driven by light energy absorbed by chlorophyll.

	Photosynthesis	Aerobic respiration
Raw materials	CO₂, H₂O, light	Glucose, O₂
Products	Glucose, O₂	CO₂, H₂O, ATP
Energy	Stored in glucose	Released into ATP
Site in eukaryote	Chloroplast	Cytoplasm and mitochondrion

The products of one process are the raw materials of the other. When respiration and photosynthesis rates are equal, there is no net gas exchange.

Chloroplast (5.2.1(b))

Structure	Detail	Role
Outer membrane	One of two envelope membranes	Boundary
Thylakoid	Flattened sac, membrane holds pigments and carriers	Light-dependent stage
Granum	Stack of thylakoids	Large surface area
Lamellae	Membranes joining grana	Connects grana
Stroma	Fluid with enzymes, starch grains	Light-independent stage
DNA	Loop in the stroma (with ribosomes)	Codes for some chloroplast proteins

Magnification = image size ÷ actual size. Convert units first: 1 mm = 1000 µm.

Pigments and TLC (5.2.1(c))

• Absorption spectrum: light absorbed, plotted across wavelengths. Action spectrum: photosynthetic rate, plotted across wavelengths.
• Photosystem = light harvesting system: accessory pigments (chlorophyll b, carotenoids) pass energy to a reaction centre containing a primary pigment (chlorophyll a).
• PSII reaction centre 680 nm; PSI reaction centre 700 nm.
• Accessory pigments absorb wavelengths that chlorophyll a absorbs poorly, so more of the spectrum is used.

TLC method in steps

1. Draw a pencil origin line about 1.5 cm from the base of the plate.
2. Put on a small spot of leaf extract; let it dry; repeat so the spot is concentrated.
3. Stand the plate in solvent below the origin line. Cover the container.
4. Take the plate out before the front reaches the top and pencil-mark where the front stopped.
5. Measure the spot distances and the front distance, all from the origin (spot middles).
6. Rf = distance moved by spot ÷ distance moved by front. Compare with reference values.

A pigment with high Rf is more soluble in the mobile phase and moves further. Carotene has the highest Rf of the common leaf pigments.

Light-dependent stage (5.2.1(d))

Site: thylakoid membranes. Aim: make ATP and reduced NADP.

Method in steps (non-cyclic)

1. Light excites electrons in PSII.
2. Electrons pass down electron carriers; energy pumps protons into the thylakoid space.
3. Protons flow back through ATP synthase; ATP forms (photophosphorylation).
4. Light excites electrons in PSI; they reduce NADP.
5. Photolysis of water replaces PSII electrons: 2H₂O → 4H⁺ + 4e⁻ + O₂.

	Cyclic	Non-cyclic
Photosystem	PSI	PSII and PSI
Water split	No	Yes
ATP	Yes	Yes
Reduced NADP	No	Yes
O₂	No	Yes

Role of water: electron source, proton source, oxygen as by-product.

Calvin cycle (5.2.1(e) and (f))

Site: stroma. Needs ATP and reduced NADP.

Compound	Carbons	Role
RuBP	5	Combines with CO₂ (RuBisCO)
GP	3	First product; reduced to TP
TP	3	Leaves cycle or regenerates RuBP

Method in steps

1. CO₂ + RuBP → 2 GP (RuBisCO).
2. GP + ATP + reduced NADP → TP (NADP and ADP return to thylakoids).
3. TP → RuBP (uses ATP).
4. 1 in every 6 TP produced is taken out of the cycle; 5 in 6 are recycled.

Worked reminder. For every 3 CO₂ fixed, 3 RuBP (15 C) + 3 C = 18 C, giving 6 GP and 6 TP. One TP (3 C) leaves, and 5 TP (15 C) regenerate 3 RuBP.

Uses of TP: carbohydrates (glucose, sucrose, starch, cellulose); lipids; amino acids (with nitrate or another nitrogen source); recycling to RuBP.

Limiting factors and investigations (5.2.1(g))

Factor low	Rate	GP	TP	RuBP
Light intensity	Falls	Up	Down	Down
CO₂ concentration	Falls	Down	Down	Up
Stomata closed (water stress)	Falls	Down	Down	Up

Temperature: rate rises then falls as enzymes denature. Where RuBisCO is the slow step (cold, or hot enough to denature it) GP and TP fall and RuBP rises. Read the data given before choosing which intermediate changes.

Reading a graph. Where the curve rises, the factor on the x-axis is limiting. Where it is flat, another factor limits. If a second curve at a higher carbon dioxide level goes on rising, carbon dioxide was the limit on the first.

Practical points: change one factor at a time, control the others, use a heat shield (tank of water) between the lamp and plant, let the plant adjust to each condition, and repeat for a mean. Percentage error = uncertainty ÷ measured value × 100.

Short model answers

Explain why GP accumulates when light is switched off. Without light, no ATP or reduced NADP is made on the thylakoids. GP cannot be reduced to TP, so the GP level rises. TP falls, and so does RuBP because it is no longer regenerated. Fixation continues briefly, which adds to the GP.

Explain why the rate stops rising as light intensity increases. Another factor is now limiting, most often carbon dioxide concentration or temperature. Extra light makes more ATP and reduced NADP, but RuBisCO cannot fix carbon dioxide any faster.

Explain how a chloroplast is adapted. Many grana give a large membrane area for photosystems and electron carriers. Enzymes of the Calvin cycle lie in the stroma next to the thylakoids. DNA and ribosomes let the chloroplast make some of its own proteins.

Percentage change. (New − old) ÷ old × 100. If the rate goes from 15 to 24 units, the change is 9 ÷ 15 × 100 = 60% increase. Say “increase” or “decrease” and state the base value you used.

Rate from a graph. On a straight section, rate = change in y ÷ change in x. State the units, such as cm³ min⁻¹.

Interpreting spectra. If the action spectrum peaks in the same regions as the absorption spectrum, the pigments that absorb that light drive photosynthesis. If the action spectrum is higher than the absorption of chlorophyll a alone, accessory pigments are passing energy to the reaction centre.

Isolated chloroplasts and DCPIP. DCPIP is blue and loses its colour as it accepts electrons from the light-dependent stage. A tube in the dark or with boiled chloroplasts stays blue, which shows the colour change needs functioning thylakoids and light.

Must-know distinctions

• Photophosphorylation makes ATP using light; oxidative phosphorylation (respiration unit) uses energy from oxidation.
• Fixation = CO₂ joins RuBP. Reduction = GP becomes TP.
• Light-dependent stage makes oxygen from water, not from carbon dioxide.
• Limiting factor = the one in shortest supply, not the most important overall.

Quick self-test

1. Name the site of the Calvin cycle.
2. A granum is photographed at ×15 000. The image is 30 mm. Calculate the actual size in µm.
3. State the Rf of a spot that moved 3.6 cm when the solvent front moved 9.0 cm.
4. State the product of cyclic photophosphorylation, and name one thing it does not make.
5. State what happens to the electrons lost from PSII in non-cyclic photophosphorylation.
6. Name the enzyme that fixes carbon dioxide.
7. How many carbon atoms are in RuBP, GP and TP?
8. What happens to the GP level if the light is switched off?
9. What happens to the RuBP level if the carbon dioxide supply is reduced?
10. A gas syringe read to ±0.1 cm³ shows 2.5 cm³. Calculate the percentage error.
11. State two uses of TP.
12. Why do plants close stomata in a drought, and what effect does it have on photosynthesis?

Answers

1. The stroma.
2. 30 mm ÷ 15 000 = 0.0020 mm = 2.0 µm.
3. 3.6 ÷ 9.0 = 0.40.
4. ATP is made; reduced NADP and oxygen are not.
5. They are replaced by electrons from photolysis of water.
6. RuBisCO (ribulose bisphosphate carboxylase).
7. RuBP 5, GP 3, TP 3.
8. It rises, because less ATP and reduced NADP are available to reduce it.
9. It rises, because less is used up fixing carbon dioxide.
10. 0.1 ÷ 2.5 × 100 = 4.0%.
11. Any two: carbohydrates, lipids, amino acids, regenerating RuBP.
12. To reduce water loss; carbon dioxide entry also falls, so the rate of photosynthesis falls.

Where marks are usually lost

• Writing “oxygen comes from carbon dioxide” instead of water.
• Describing photolysis without saying it replaces electrons in PSII.
• Confusing the roles of GP (first product) and TP (product of reduction).
• Not saying that RuBP is regenerated, so the cycle continues.
• Stating that a limiting factor “stops” photosynthesis rather than limits the rate.
• Forgetting to convert mm to µm before using the magnification formula.
• Leaving out units, or giving Rf with units.
• Explaining temperature only as “enzymes work better”, with no link to RuBisCO or to denaturing.
• Describing a practical without saying which factors are controlled.

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.