Cambridge International AS & A Level Biology 9700: Photosynthesis – Revision Notes

These revision notes condense Topic 13, Photosynthesis, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. They cover sections 13.1 (Photosynthesis as an energy transfer process) and 13.2 (Investigation of limiting factors). This is A Level content, examined in Paper 4; the investigations also feed the planning and analysis skills tested in Paper 5. For full explanations and worked examples, use the photosynthesis study guide.

Links: Cambridge A Level Biology hub · printable checklist · photosynthesis practice questions · A Level diagnostic · energy and respiration revision notes

13.1 Photosynthesis as an energy transfer process

Chloroplast: structure to function (13.1.1, 13.1.3)

Structure	Function
Thylakoids (membranes + thylakoid spaces), stacked in grana	Site of the light-dependent stage; large surface area for pigments, electron carriers, ATP synthase
Thylakoid space	Small volume, so protons accumulate and a gradient forms
Stroma	Site of the light-independent stage (Calvin cycle); contains rubisco and other enzymes
Double membrane (envelope)	Separates chloroplast reactions from the cytoplasm

How the stages link (13.1.2)

Light-dependent stage → ATP + reduced NADP → used in the Calvin cycle to make complex organic molecules from CO₂.

Pigments (13.1.4)

Pigment	Role
Chlorophyll a	Main pigment; emits electrons when photoactivated
Chlorophyll b	Accessory pigment; passes absorbed energy to chlorophyll a
Carotene	Accessory pigment (carotenoid)
Xanthophyll	Accessory pigment (carotenoid)

Spectra (13.1.5)

• Absorption spectrum: light absorbed by a pigment against wavelength.
• Action spectrum: rate of photosynthesis against wavelength.
• Chlorophylls absorb blue and red; carotenoids mainly blue; little green is absorbed.
• Action spectrum ≈ combined absorption spectra → the absorbed light drives photosynthesis.

Chromatography (13.1.6)

Rf = distance moved by pigment ÷ distance moved by solvent front

Method in steps

1. Draw the origin line in pencil.
2. Build up a small, concentrated spot of pigment extract, drying between applications.
3. Stand the paper in solvent with the origin above the solvent surface.
4. Remove before the solvent reaches the top; mark the front straight away.
5. Measure from the origin to the centre of each spot and to the solvent front.
6. Calculate Rf and match to reference values for the same solvent.

Photophosphorylation (13.1.7–10)

Feature	Cyclic	Non-cyclic
Photosystems	PSI only	PSII and PSI
Photoactivation of chlorophyll	Yes	Yes
Electron path	PSI → chain → back to PSI	PSII → chain → PSI → NADP
Photolysis of water	No	Yes (oxygen-evolving complex)
O₂ released	No	Yes
Products	ATP	ATP and reduced NADP

Photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂, catalysed by the oxygen-evolving complex in PSII.

Chemiosmosis in three lines

1. Energetic electrons release energy along the electron transport chain.
2. Energy transfers protons from the stroma into the thylakoid space.
3. Protons return to the stroma by facilitated diffusion through ATP synthase → ATP.

Calvin cycle (13.1.11–12)

Stage	What happens	Needs
Fixation	CO₂ + RuBP (5C) → 2 GP (3C), catalysed by rubisco	—
Reduction	GP → TP (3C)	Reduced NADP and ATP
Regeneration	TP → RuBP	ATP

Carbon check for 3 turns: 3 CO₂ + 3 RuBP (18 C) → 6 GP → 6 TP; 5 TP → 3 RuBP; 1 TP net gain.

Uses: GP → some amino acids. TP → carbohydrates, lipids and amino acids.

Predicting concentration changes

Change	GP	RuBP	Reason
Light reduced	Rises	Falls	Less ATP and reduced NADP, so GP is not reduced and RuBP is not regenerated; fixation continues for a while
CO₂ reduced	Falls	Rises	Less fixation, so less RuBP used and less GP made; regeneration continues for a while

13.2 Investigation of limiting factors

The three named limiting factors (13.2.1–2)

Factor	Stage mainly affected	Why
Light intensity	Light-dependent	Less photoactivation → less ATP and reduced NADP
CO₂ concentration	Light-independent	Less fixation by rubisco → less GP and TP
Temperature	Light-independent	Enzymes such as rubisco; slower below optimum, denatured well above it

Graph rule: rising section → the factor on the x-axis is limiting. Plateau → another factor is limiting. A higher plateau when CO₂ (or temperature) is raised shows that factor was limiting on the lower plateau.

Investigations (13.2.3–4)

Chloroplast suspension + redox indicator

• DCPIP or methylene blue: blue → colourless when reduced by electrons from the light-dependent stage.
• Rate = 1 / time to decolourise.
• Light intensity: lamp distance. Light wavelength: coloured filters at equal intensity.
• Controls: dark tube; tube without chloroplasts.
• Keep the suspension cold and buffered during preparation.

Whole plants (aquatic plants)

• Measure O₂: bubble count, capillary tube, or gas syringe.
• CO₂ source: sodium hydrogencarbonate solution at set concentrations.
• Temperature: water bath. Heat from the lamp: heat shield or water bath.
• Acclimatise before each reading; repeat and take a mean.

Method in steps: effect of light intensity on an aquatic plant

1. Cut a length of pondweed and place it, cut end up, in a boiling tube of sodium hydrogencarbonate solution.
2. Stand the tube in a water bath at a fixed temperature, with a heat shield between lamp and tube.
3. Set the lamp at the first distance and let the plant acclimatise for a few minutes.
4. Collect the gas released over a fixed time in a capillary tube or gas syringe.
5. Read the volume (for a capillary tube, volume = πr² × length of the gas bubble).
6. Repeat three times at each distance, then change the distance and repeat.
7. Calculate mean rates and plot rate against light intensity (or against distance, if intensity cannot be measured).

Method in steps: explaining a limiting-factor graph

1. Describe the shape: where the rate rises and where it levels off, with values read from the graph.
2. Name the limiting factor on the rising part: the variable on the x-axis.
3. Name what may limit on the plateau: one of the other factors held constant.
4. Link to the stage affected: light → ATP and reduced NADP; CO₂ and temperature → the Calvin cycle.
5. If two curves are given, use the difference in plateau height as evidence.

Small worked reminders

• Coloured filters change light intensity as well as wavelength. Check intensity with a light meter and make it equal before comparing wavelengths.
• Oxygen collected from a whole plant is a net rate: some oxygen is used in respiration, so the true rate is higher.
• A carbon dioxide source must be kept constant when light is the independent variable, or it may run out and become limiting.

Must-know distinctions

• Thylakoid space vs stroma: protons are pumped into the thylakoid space and return to the stroma.
• Absorption vs action spectrum: what pigments absorb vs what drives photosynthesis.
• GP vs TP: GP is made by fixation; TP is made by reducing GP.
• Cyclic vs non-cyclic: only non-cyclic makes reduced NADP and releases oxygen.
• Photosynthesis chemiosmosis vs respiration chemiosmosis: thylakoid membrane and space vs inner mitochondrial membrane and intermembrane space.

Quick self-test

1. State the site of the light-dependent stage.
2. A pigment spot is 36 mm from the origin and the solvent front is 60 mm from the origin. Calculate the Rf value.
3. Which photosystem is involved in cyclic photophosphorylation?
4. List the three products of non-cyclic photophosphorylation.
5. DCPIP in a chloroplast suspension goes colourless in 80 s. Calculate the rate as 1000/t.
6. An aquatic plant gives 14 bubbles per minute; after the hydrogencarbonate concentration is raised it gives 21. Calculate the percentage increase.
7. Name the enzyme that fixes carbon dioxide and the product of fixation.
8. State the number of carbon atoms in RuBP, GP and TP.
9. The carbon dioxide concentration is suddenly increased. State what happens to GP and RuBP.
10. Give two uses of triose phosphate.
11. Explain why leaves look green.

Answers

1. The thylakoids (thylakoid membranes), stacked in grana.
2. Rf = 36 ÷ 60 = 0.60.
3. Photosystem I only.
4. ATP, reduced NADP and oxygen (oxygen as a by-product of photolysis).
5. 1000 ÷ 80 = 12.5 s⁻¹.
6. (21 − 14) ÷ 14 × 100 = 50%.
7. Rubisco; two molecules of glycerate 3-phosphate (GP).
8. RuBP 5, GP 3, TP 3.
9. More fixation: GP rises and RuBP falls, because RuBP is used faster than it is regenerated.
10. Any two: making carbohydrates, lipids, amino acids, or regenerating RuBP.
11. Chlorophylls absorb little green light, so green is reflected or transmitted.

Where marks are usually lost

• Writing “chlorophyll” when the question asks which pigments; name chlorophyll a, chlorophyll b, carotene, xanthophyll.
• Saying the stroma is inside the thylakoids, or that protons are pumped into the stroma.
• Missing “two” when describing fixation: one RuBP plus one CO₂ gives two GP.
• Saying GP is reduced by “NADP” rather than reduced NADP.
• Forgetting that the regeneration of RuBP also uses ATP.
• Describing a plateau without naming which other factor now limits.
• Explaining temperature effects on the light-dependent stage instead of on the enzymes of the Calvin cycle.
• Measuring Rf to the top edge of a spot, or quoting it with units.
• Giving bubble counts as a precise measure of oxygen volume; bubbles vary in size.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, published by Cambridge Assessment International Education (Cambridge University Press & Assessment, September 2022). Topic 13, Photosynthesis, sections 13.1 and 13.2.