Cambridge International AS & A Level Biology 9700: Photosynthesis – Study Guide

This study guide teaches Topic 13, Photosynthesis, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers sections 13.1 (Photosynthesis as an energy transfer process) and 13.2 (Investigation of limiting factors). This is A Level content, not AS Level content: it is examined in Paper 4 (A Level Structured Questions), and the chromatography, redox-indicator and whole-plant investigations are the kind of practical work that Paper 5 (Planning, Analysis and Evaluation) builds on.

Useful links: the Cambridge A Level Biology hub, the printable Biology checklist, the photosynthesis revision notes and the photosynthesis practice questions. To find your gaps across the A Level topics, try the A Level diagnostic.

What this topic covers

Syllabus ref	What you must be able to do
13.1.1, 13.1.3	Relate chloroplast structure (diagrams and electron micrographs) to function; state the sites of the two stages
13.1.2	Explain that ATP and reduced NADP from the light-dependent stage are used in the Calvin cycle
13.1.4–5	Describe the roles of chlorophyll a, chlorophyll b, carotene and xanthophyll; interpret absorption and action spectra
13.1.6	Use chromatography to separate pigments and identify them with Rf values
13.1.7–10	Explain cyclic and non-cyclic photophosphorylation and ATP synthesis by chemiosmosis
13.1.11–12	Outline the three main stages of the Calvin cycle and the uses of GP and TP
13.2.1–2	State and explain the effects of light intensity, carbon dioxide concentration and temperature
13.2.3–4	Investigate limiting factors with redox indicators and chloroplast suspensions, and with whole plants

The guide assumes the plant cell work from Topic 1 and leaf structure from Topic 7, as the syllabus says. Respiration uses the same chemiosmosis idea: see the energy and respiration study guide.

Chloroplast structure and function

A chloroplast has a double membrane (envelope). Inside are:

• Thylakoids: flattened membrane sacs, each with a thylakoid membrane enclosing a thylakoid space. They occur in stacks called grana. The thylakoids are the site of the light-dependent stage.
• Stroma: the fluid around the thylakoids, the site of the light-independent stage (Calvin cycle).

Link structure to function:

• Many stacked thylakoids give a large membrane surface area for pigments, electron carriers and ATP synthase.
• The thylakoid membrane keeps protons in the small thylakoid space, so a proton gradient builds.
• The stroma contains the enzymes of the Calvin cycle, including rubisco. It also holds circular DNA, ribosomes and starch grains.

The two stages and how they connect

The light-dependent stage transfers light energy into chemical energy as ATP and reduced NADP. The light-independent stage (Calvin cycle) uses that ATP and reduced NADP to turn carbon dioxide into complex organic molecules. If the light-dependent stage stops, the Calvin cycle soon stops too.

Pigments and spectra

Chloroplast pigments sit in the thylakoid membranes, grouped in photosystems. The four named pigments are:

• Chlorophyll a: the main pigment. Energy reaching it causes electrons to be emitted.
• Chlorophyll b, carotene and xanthophyll: accessory pigments. They absorb light at wavelengths chlorophyll a absorbs less well and pass the energy on to chlorophyll a. Carotene and xanthophyll are carotenoids.

An absorption spectrum shows how much light a pigment absorbs at each wavelength. An action spectrum shows the rate of photosynthesis at each wavelength.

Interpreting them:

• The chlorophylls absorb strongly in the blue and red parts of the spectrum; carotenoids absorb mainly blue.
• Little green light is absorbed, so it is reflected or transmitted. That is why leaves look green.
• The action spectrum closely follows the combined absorption spectra. This is evidence that the light absorbed by these pigments is the light used in photosynthesis.

Chromatography and Rf values

Pigments are extracted by grinding leaves with a solvent, then a small concentrated spot is placed on a pencil origin line and the paper (or thin-layer plate) stands in a solvent below the line. Each pigment moves a different distance.

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

Both distances are measured from the origin; measure to the centre of each spot. Rf has no units and is always less than 1. You identify a pigment by matching its Rf to reference values for the same solvent, because Rf changes with the solvent.

Worked example

The solvent front is 84 mm from the origin. Four spots are at 80 mm, 55 mm, 50 mm and 42 mm.

1. 80 ÷ 84 = 0.95
2. 55 ÷ 84 = 0.65
3. 50 ÷ 84 = 0.60
4. 42 ÷ 84 = 0.50

If the reference table for this solvent lists carotene 0.95, xanthophyll 0.65, chlorophyll a 0.60 and chlorophyll b 0.50, the spots are carotene, xanthophyll, chlorophyll a and chlorophyll b. In an exam the reference values will be given to you.

The light-dependent stage: photophosphorylation

Both types happen in the thylakoids. In both, light causes photoactivation of chlorophyll: an electron is raised to a higher energy level and emitted.

Cyclic photophosphorylation

• Only photosystem I (PSI) is involved.
• Electrons emitted from PSI pass along an electron transport chain and return to PSI.
• ATP is made. No reduced NADP, no photolysis and no oxygen.

Non-cyclic photophosphorylation

• Both PSII and PSI are involved.
• Electrons emitted from PSII pass along an electron transport chain to PSI.
• Electrons emitted from PSI are used to reduce NADP, together with protons.
• PSII replaces its lost electrons from water. The oxygen-evolving complex catalyses the photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂. Oxygen is released as a by-product.
• ATP and reduced NADP are both made.

How ATP is made: chemiosmosis

1. Energetic electrons release energy as they pass along the electron transport chain.
2. The energy is used to transfer protons across the thylakoid membrane from the stroma into the thylakoid space.
3. Protons return to the stroma by facilitated diffusion through ATP synthase, providing energy to make ATP.

This is the same mechanism as in mitochondria, but protons move into the thylakoid space rather than the intermembrane space. You do not need the names of the carriers or details of ATP synthase.

The light-independent stage: the Calvin cycle

It happens in the stroma and has three main stages:

1. Fixation: rubisco catalyses the combination of carbon dioxide with ribulose bisphosphate (RuBP, 5C) to give two molecules of glycerate 3-phosphate (GP, 3C).
2. Reduction: GP is reduced to triose phosphate (TP, 3C), using reduced NADP and ATP.
3. Regeneration: most TP is used to regenerate RuBP, in reactions that use ATP.

Carbon bookkeeping. For three turns: 3 CO₂ + 3 RuBP = 3 + 15 = 18 carbons, which become 6 GP (18 carbons), then 6 TP. Five TP (15 carbons) regenerate 3 RuBP (15 carbons). One TP (3 carbons) is the net gain.

Uses of intermediates (limited by the syllabus to these):

• GP is used to make some amino acids.
• TP is used to make carbohydrates, lipids and amino acids.

Predicting changes. If carbon dioxide concentration falls, RuBP is still regenerated but less is fixed, so RuBP rises and GP falls. Always explain both the “made from” and “used for” sides.

13.2 Limiting factors

Light intensity, carbon dioxide concentration and temperature are examples of limiting factors. When several factors affect a process, the rate is limited by the one nearest its minimum; raising that factor raises the rate.

• Light intensity limits the light-dependent stage: more light gives more ATP and reduced NADP, until another factor limits.
• Carbon dioxide concentration limits fixation by rubisco, so less GP and TP are made.
• Temperature affects the enzymes of the Calvin cycle, such as rubisco. The rate rises towards the optimum; above it, enzymes denature and the rate falls.

Reading a graph. On a curve of rate against light intensity, the rising part means light is limiting. Where the curve levels off, something else (carbon dioxide or temperature) is limiting. If a second curve at higher carbon dioxide levels off higher, carbon dioxide was limiting on the first plateau.

Investigations

Redox indicators and chloroplast suspensions

Grind leaves in a cold buffer solution, filter, and centrifuge to get a chloroplast suspension. Add DCPIP or methylene blue. In light, electrons from the light-dependent stage reduce the dye, turning it from blue to colourless. Time the colour change and use rate = 1 / time.

• Light intensity: vary the lamp distance. Light intensity falls with distance: moving the lamp from 10 cm to 20 cm reduces it to about a quarter.
• Light wavelength: use coloured filters, keeping light intensity equal.
• Controls: a tube kept in the dark, and a tube without chloroplasts.

Whole plants, including aquatic plants

Use an aquatic plant such as pondweed in a solution of sodium hydrogencarbonate (a carbon dioxide source).

• Measure oxygen released: count bubbles (less reliable) or collect gas in a capillary tube or gas syringe.
• Vary light intensity (lamp distance), carbon dioxide (hydrogencarbonate concentration) or temperature (water bath). Keep the other two constant.
• Let the plant acclimatise for a few minutes at each new condition.

Worked example. A gas syringe collects 0.36 cm³ of oxygen in 6 minutes. Rate = 0.36 ÷ 6 = 0.060 cm³ min⁻¹. This is the net rate: some oxygen is used in respiration, and some dissolves.

Common errors

• Placing the light-dependent stage in the stroma, or the Calvin cycle on the grana.
• Saying cyclic photophosphorylation makes reduced NADP or releases oxygen.
• Writing that light “splits water”. The syllabus wording is: the oxygen-evolving complex catalyses the photolysis of water.
• Saying rubisco produces one GP. Each CO₂ fixed gives two GP.
• Stating that the Calvin cycle needs no light. It does not use light directly, but it stops without ATP and reduced NADP.
• Measuring Rf from the bottom of the paper instead of the origin, or to the top of a spot.
• Changing lamp distance without controlling heat from the lamp, so temperature changes too.

Next steps

Condense this with the photosynthesis revision notes, then work through the photosynthesis practice questions.

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.