What should I focus on when revising Photosynthesis?

Start with chlorophyll, chloroplast, endothermic reaction, limiting factor, inverse square law. Photosynthesis is endothermic. Light carries energy into the chloroplasts, where chlorophyll absorbs it.

Which mistakes should I check in Photosynthesis?

Writing that a limiting factor stops photosynthesis: it stops the rate from increasing, so always use the word rate. Saying a graph levels off because light becomes the limiting factor: at the plateau, light is no longer limiting; another factor is. Mixing up chlorophyll and chloroplasts, or calling chlorophyll an enzyme: chlorophyll is the green pigment inside chloroplasts that absorbs light. Writing carbon dioxide as Co2 or with a raised or full-size 2: write CO₂, with capital C, capital O and a small 2 written low. Using the wrong number or unit in a rate calculation: divide the amount the question describes by its time, and build the unit from that division. Vague practical answers: name the equipment and the measurement, and follow "do repeats" with "and calculate a mean".

How can I practise Photosynthesis without rereading?

Try this question from memory: Photosynthesis is an endothermic reaction. State what endothermic tells you about the energy change in photosynthesis. Then compare your answer with your course material and correct any missing steps or terms.

How can Aripsy help me revise Photosynthesis?

Revise photosynthesis for AQA and Pearson Edexcel GCSE Biology and Combined Science. The notes below cover both equations, limiting factors, the inverse square law and the light-intensity practical, then six practice questions you can get marked.

What does chlorophyll do?

It is a green pigment in chloroplasts that absorbs light energy for photosynthesis.

How does carbon dioxide reach the cells inside a leaf?

It diffuses in through the stomata, then through the air spaces in the spongy mesophyll.

Why are palisade cells near the top of the leaf?

They are packed with chloroplasts, and light is strongest near the upper surface.

What does the flat part of a light-intensity graph show?

Light is no longer the limiting factor. Another factor, such as carbon dioxide or temperature, is.

Why does the rate fall above the optimum temperature?

The enzymes that control photosynthesis denature.

The lamp is moved twice as far away. What happens to light intensity?

It falls to a quarter of what it was.

Why add sodium hydrogencarbonate in the pondweed practical?

It releases carbon dioxide, so carbon dioxide does not limit the rate.

Why put a beaker of water between the lamp and the pondweed?

It acts as a heat shield, so the temperature stays the same. An LED lamp does the same job.

Why is measuring gas volume better than counting bubbles?

Bubbles differ in size, so a volume reading is more accurate.

What do algal balls in hydrogencarbonate indicator show as they photosynthesise?

They remove carbon dioxide, so the indicator changes from orange-red towards purple.

Five uses of the glucose a plant makes (AQA)

Respiration; starch for storage; fats and oils for storage; cellulose for cell walls; amino acids for proteins.

What else do plants need to make proteins from glucose?

Nitrate ions, absorbed from the soil.

Why can heating a greenhouse waste money? (AQA Higher)

If light or carbon dioxide is the limiting factor, extra heat does not raise the rate, so the cost brings no extra crop.

Photosynthesis GCSE Revision Notes: AQA and Edexcel

Summary

Photosynthesis is how green plants and algae make glucose. It happens in chloroplasts, where chlorophyll absorbs light. It is endothermic, because light transfers energy into the reaction. Word equation: carbon dioxide + water → glucose + oxygen. Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The rate is limited by light intensity, carbon dioxide concentration and temperature. AQA also lists the amount of chlorophyll. Both boards test a light-intensity practical.

Checked 29 October 2026

Key takeaways

Photosynthesis is endothermic. Light carries energy into the chloroplasts, where chlorophyll absorbs it.
Learn both equations: carbon dioxide + water → glucose + oxygen, and 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
A limiting factor is the factor in shortest supply. It stops the rate from rising any further.
Higher tier only, on both boards: how the factors interact, and the inverse square law (light intensity ∝ 1/d²).
AQA adds the amount of chlorophyll, uses of glucose and (Higher only) greenhouse costs. Edexcel's practical names algal balls; AQA's names pondweed.

Notes

Photosynthesis is part of AQA topic 4.4 Bioenergetics and Pearson Edexcel Topic 6. For respiration and the rest of Bioenergetics, see the GCSE bioenergetics revision notes.

4.4.1.1 The photosynthetic reaction (AQA) / 6.1 and 6.2 (Edexcel)

Covered by: AQA 8461 and 8464, both tiers. Edexcel 1BI0 and 1SC0, both tiers.

Plants and algae make their own food. They take in carbon dioxide and water and turn them into glucose and oxygen.

The reaction needs energy. Light transfers energy from the surroundings to the chloroplasts. The products store more chemical energy than the reactants, so photosynthesis is an endothermic reaction.

Chloroplasts contain chlorophyll, a green pigment that absorbs light. Chlorophyll is not an enzyme. Enzymes inside the chloroplast control the steps of the reaction.

Word equation

carbon dioxide + water → glucose + oxygen (light energy needed)

Balanced symbol equation

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

AQA asks you to recognise the formulae CO₂, H₂O, O₂ and C₆H₁₂O₆. Edexcel describes the reactants and products. Learn the balanced version for either board: AQA has asked students to pick out the balanced equation (AQA 8464/B/1H report, November 2021).

Because they make glucose from simple molecules, photosynthetic organisms are the producers at the start of food chains. They make the biomass that other living things depend on. Edexcel puts this in 6.1. AQA covers it in Ecology (4.7.2.1, on Paper 2).

4.2.3.1 Where it happens: the leaf (AQA) / 6.9 and 6.11B (Edexcel)

Covered by: AQA 8461 and 8464, both tiers (plant tissues). Edexcel 1BI0 and 1SC0: 6.9 (stomata), both tiers. Edexcel 6.11B (leaf adaptations): GCSE Biology 1BI0 only.

A leaf is an organ built to catch light and exchange gases. Learn each tissue with its job.

4.2.3.1 Where it happens: the leaf (AQA) / 6.9 and 6.11B (Edexcel)
Part of the leaf	How it helps photosynthesis
Upper epidermis	Thin and see-through, so light reaches the cells below. A waxy cuticle cuts water loss.
Palisade mesophyll	Tall cells packed with chloroplasts, near the top where light is strongest. Most photosynthesis happens here.
Spongy mesophyll	Loosely packed cells with air spaces, so carbon dioxide and oxygen diffuse quickly.
Xylem	Brings water, a reactant, up from the roots.
Phloem	Carries sugar (as sucrose) to other parts of the plant.
Guard cells and stomata	Guard cells open and close the stomata, which are mostly on the lower surface. Open stomata let carbon dioxide in and oxygen out.

The whole leaf is broad and thin. Broad gives a large surface to absorb light. Thin keeps the diffusion distance short.

4.4.1.2 Rate of photosynthesis and limiting factors (AQA) / 6.3 (Edexcel)

Covered by: AQA 8461 and 8464, both tiers. Edexcel 1BI0 and 1SC0, both tiers. AQA lists four factors. Edexcel lists three (not the amount of chlorophyll).

A limiting factor is the factor in shortest supply. It stops the rate of photosynthesis from rising any further. Increase that factor and the rate goes up. Increase anything else and nothing changes.

Light intensity. Light supplies the energy. More light gives a faster rate, until another factor runs short.
Carbon dioxide concentration. Carbon dioxide is a reactant. More of it gives a faster rate, until another factor runs short. Air is only about 0.04% carbon dioxide, so it often limits the rate on bright, warm days.
Temperature. Enzymes control photosynthesis. Warmer molecules have more energy and collide more often, so the rate rises. Above the optimum, the enzymes denature and the rate falls quickly. Cold often limits photosynthesis in a UK winter.
Amount of chlorophyll (AQA). Less chlorophyll means less light is absorbed. Leaves can lose chlorophyll through disease, a shortage of magnesium ions, or age. The pale parts of variegated leaves have little chlorophyll.

How to read rate graphs

Rising section. The factor on the x-axis is limiting. Say so, then give the reason (light supplies energy; carbon dioxide is a reactant).
Plateau. The x-axis factor is no longer limiting. Something else is. Name a factor that was kept constant, such as carbon dioxide or temperature.
Temperature curves are different. They rise to a peak, then fall. They do not level off, because enzymes denature.
Quote numbers with units. For example: "the rate stops rising at 40 units of light, at 3.9 cm³ per hour".

You also need to measure and calculate rates, plot graphs with sensible scales, and move between tables and graphs. The worked examples below show how.

4.4.1.2 Higher tier: how limiting factors interact (AQA) / 6.4 (Edexcel)

Covered by: Higher tier only, on both boards. AQA 8461 and 8464. Edexcel 1BI0 and 1SC0.

The factors work together. At any moment, the one in shortest supply sets the rate. Exam graphs often show two or three lines against light intensity, each at a different carbon dioxide level or temperature.

To decide which factor is limiting:

Where all the lines overlap at low light, light intensity is limiting. Changing carbon dioxide or temperature makes no difference there.
Where a line levels off, light is no longer limiting.
If the higher-carbon-dioxide line levels off higher, carbon dioxide was limiting the lower line.
If the warmer line levels off higher, temperature was limiting.

Practice question 6 gives you a full data set to try this on.

Required practical 6 (AQA 8461) / 5 (AQA 8464) / 6.5 core practical (Edexcel): light intensity and rate

Covered by: AQA 8461 (required practical activity 6) and 8464 (required practical activity 5), both tiers. Edexcel 1BI0 and 1SC0 core practical 6.5, both tiers.

Both boards ask you to change light intensity and measure the rate of photosynthesis. AQA names an aquatic organism such as pondweed. Edexcel names algal balls (algae trapped in jelly beads), or a similar organism, placed at different distances from a lamp.

Method A: pondweed (common for AQA)

Fill a boiling tube with dilute sodium hydrogencarbonate solution. It releases carbon dioxide, so carbon dioxide does not limit the rate.
Add a fresh piece of pondweed, cut end up.
Put a beaker of water between the lamp and the tube, or use an LED lamp, so the lamp does not warm the water. Check with a thermometer.
Darken the room. Place the lamp 10 cm away, measured with a ruler. Wait 5 minutes for the pondweed to adjust.
Collect the gas for a fixed time in an upturned measuring cylinder or a gas syringe, and record its volume. Counting bubbles is quicker but less accurate.
Repeat at 20, 30, 40 and 50 cm. Do three runs at each distance and calculate a mean.

Method B: algal balls (Edexcel's named organism)

Put the same number of algal balls into bottles holding the same volume of hydrogencarbonate indicator.
Place the bottles at different distances from the lamp. Wrap one in foil as a control.
As the algae photosynthesise, they remove carbon dioxide. The indicator turns from orange-red towards purple.
Time how long each bottle takes to match the same colour on a colour chart. Rate = 1 ÷ time.

Variables

Independent: light intensity, changed by moving the lamp.
Dependent: rate of photosynthesis (gas volume per minute, bubbles per minute, or 1 ÷ time for the colour change).
Control: temperature, carbon dioxide concentration, type and size of pondweed or number of algal balls, the lamp, timing, and background light.

Risks

The lamp gets hot. Do not touch the bulb; let it cool.
Mains electricity near water. Keep the lamp, plug and your hands dry.
Glassware can break. Handle it and clear up carefully.
Living organisms. Never release pondweed into ponds or rivers, because some species spread fast. Dispose of it as your teacher says.
Indicator solution. Wear eye protection.

Typical results

The rate is highest close to the lamp and falls as the lamp moves away. At very short distances the rate may stop rising, because another factor is limiting. Without a heat shield, it may rise for the wrong reason: the lamp warms the water.

Common errors

Bubbles differ in size, so bubble counts are not accurate.
Some oxygen dissolves in the water or is used in respiration, so you measure less than the plant makes.
Light from windows adds to the lamp.
No time is left for the organism to adjust after each move.

4.4.1.2 Higher tier: the inverse square law (AQA) / 6.6 (Edexcel)

Covered by: Higher tier only, on both boards. AQA 8461 and 8464. Edexcel 1BI0 and 1SC0.

Light spreads out as it travels. Light intensity is inversely proportional to the square of the distance from the lamp:

light intensity ∝ 1 / distance²

Double the distance: intensity falls to a quarter.
Triple the distance: intensity falls to a ninth.
Halve the distance: intensity is four times greater.

In the practical, you can use 1 ÷ d² as a relative light intensity in arbitrary units. While light is the limiting factor, the rate of photosynthesis is directly proportional to light intensity. So the rate drops quickly as the lamp moves away. Moving the lamp from 10 cm to 20 cm changes the light far more than moving it from 40 cm to 50 cm.

4.4.1.2 Higher tier: greenhouse economics (AQA only)

Covered by: AQA 8461 and 8464, Higher tier only. Not in the Edexcel specification.

Greenhouse growers can raise the rate of photosynthesis to grow more crop:

Heat from heaters or boilers. Burning fuel also adds carbon dioxide.
Light from lamps, so plants can photosynthesise for longer in winter or at night.
Carbon dioxide, pumped in to raise its concentration.

Each costs money. It is only worth doing if the extra crop is worth more than the cost. Adding a factor that is not limiting wastes money.

Example: extra lighting costs £300 a week and raises crop sales by £450 a week, a profit of £150. Extra heating costs £500 a week and raises sales by £350, a loss of £150, because temperature was not the limiting factor. Exam questions give you data like this and ask which option is most cost-effective.

4.4.1.3 Uses of glucose (AQA only)

Covered by: AQA 8461 and 8464, both tiers. Not a named point in the Edexcel specification.

Plants use the glucose they make to:

Release energy in respiration.
Make starch for storage. Starch is insoluble, so it does not draw water into cells by osmosis.
Make fats and oils for storage, for example in seeds.
Make cellulose, which strengthens cell walls.
Make amino acids for proteins. For this, plants also need nitrate ions absorbed from the soil.

Worked example 1: the inverse square law (Higher)

A lamp is moved from 10 cm to 30 cm away from some pondweed. By what factor does the light intensity fall?

How many times further away is the lamp? 30 ÷ 10 = 3.
Square it: 3² = 9.
Light intensity falls to one ninth of its first value. It is 9 times lower.

Check with relative light intensity = 1 ÷ d²:

At 10 cm: 1 ÷ 10² = 1 ÷ 100 = 0.0100 arbitrary units.
At 30 cm: 1 ÷ 30² = 1 ÷ 900 = 0.00111 arbitrary units (3 significant figures).
0.0100 ÷ 0.00111 = 9.0. Same answer.

If a light meter read 900 lux at 10 cm, it would read 900 ÷ 9 = 100 lux at 30 cm.

Worked example 2: rate from a data table

A student counted the bubbles from some pondweed for 5 minutes at each distance.

Worked example 2: rate from a data table
Distance (cm)	Run 1	Run 2	Run 3
10	62	58	60
20	41	44	29
30	22	20	24
40	12	14	13

Scroll sideways to see the whole table.

Calculate the mean rate at 20 cm in bubbles per minute.

Spot the anomaly. 29 is far below 41 and 44, so leave it out.
Mean = (41 + 44) ÷ 2 = 42.5 bubbles in 5 minutes.
Rate = 42.5 ÷ 5 = 8.5 bubbles per minute.

The unit comes from the calculation: bubbles divided by minutes gives bubbles per minute. If you collected gas instead, the same method gives a volume rate. For example, 3.6 cm³ in 4 minutes is 3.6 ÷ 4 = 0.9 cm³ per minute.

What differs by board

Photosynthesis content by board: AQA 8461 and 8464 compared with Pearson Edexcel 1BI0 and 1SC0
Topic	AQA 8461 and 8464	Edexcel 1BI0 and 1SC0
Word equation, endothermic reaction	4.4.1.1, both tiers. Recognise CO₂, H₂O, O₂ and C₆H₁₂O₆	6.2, both tiers
Producers of biomass	4.7.2.1 (Ecology, Paper 2), both tiers	6.1, both tiers
Limiting factors	4.4.1.2, both tiers: temperature, light intensity, carbon dioxide, amount of chlorophyll	6.3, both tiers: temperature, light intensity, carbon dioxide
Interaction of factors	4.4.1.2, Higher only	6.4, Higher only
Inverse square law	4.4.1.2, Higher only	6.6, Higher only
Greenhouse economics	4.4.1.2, Higher only	Not in the specification
Uses of glucose	4.4.1.3, both tiers	Not a named point
Leaf structure	4.2.3.1, both tiers, both qualifications	6.9 stomata, both qualifications. 6.11B leaf adaptations, Biology only
Practical	Required practical 6 (8461) or 5 (8464): aquatic organism such as pondweed	Core practical 6.5: algal balls or similar, at different distances from a lamp
Exam paper	Paper 1 (8461); Biology Paper 1 (8464)	Paper 2 (1BI0); Paper 2: Biology 2 (1SC0)

Scroll sideways to see the whole table.

Key facts and equations

Key photosynthesis facts and equations with spec points
Fact or equation	Spec point	Meaning	Link
carbon dioxide + water → glucose + oxygen	AQA 4.4.1.1; Edexcel 6.2	Word equation. Light is a condition, not a reactant	Aerobic respiration has the reverse overall equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂	AQA 4.4.1.1 (recognise formulae)	Six of each small molecule, one glucose	Writing formulae correctly (CO₂, not Co2)
Photosynthesis is endothermic	AQA 4.4.1.1; Edexcel 6.2	Energy is taken in from the surroundings, as light	Energy changes in chemistry
Chlorophyll absorbs light	AQA 4.4.1.1 and 4.4.1.2; Edexcel 1.1 (chloroplasts)	A green pigment in chloroplasts, not an enzyme	Leaf tissues; magnesium ions
Limiting factor: the factor in shortest supply	AQA 4.4.1.2; Edexcel 6.3	Stops the rate rising any further	Reading graph plateaus
Above the optimum temperature, the rate falls	AQA 4.4.1.2; Edexcel 6.3	Enzymes denature	Enzymes
Light intensity ∝ 1/d²	AQA 4.4.1.2 (Higher); Edexcel 6.6 (Higher)	Double the distance, a quarter of the light	Maths: inverse proportion
Rate = amount ÷ time	AQA 4.4.1.2; Edexcel 6.5	The unit follows the division, such as cm³ per minute	Required and core practicals
Sodium hydrogencarbonate releases carbon dioxide	AQA practical 6 or 5; Edexcel 6.5	Stops carbon dioxide limiting the rate	Control variables
Glucose → starch, fats, cellulose, amino acids	AQA 4.4.1.3	Storage and building materials	Nitrate ions; food tests

Scroll sideways to see the whole table.

Flashcards

Say each answer out loud first, then flip the card to check it.

Practice questions

Six questions written by Aripsy in the style of AQA and Edexcel papers. They are not taken from any exam board. Try each one before you look at the mark scheme. To have your own answer marked, pick the question in the marking box below. It gives an estimated, exam-board-style mark and shows the points you made and missed.

In question 6 the mark scheme lists indicative content, so each point shows 0 marks. The examiner places the whole answer in a level and then picks a mark. An answer with no relevant content scores 0.

Question 1
Pearson Edexcel-style · 1 mark
Photosynthesis is an endothermic reaction. State what endothermic tells you about the energy change in photosynthesis.
- Command word: State
- Tier: Foundation and Higher
- Spec fit: Edexcel 6.2; also AQA 4.4.1.1
Show mark schemeHide mark scheme
Mark scheme
Energy is taken in from the surroundings (allow: light energy is transferred to the chloroplasts; the products store more energy than the reactants)1 mark
Model answer
Energy is taken in from the surroundings. In photosynthesis, the energy is transferred by light.
Question 2
AQA-style · 2 marks
Carbon dioxide enters a leaf through pores called stomata. Explain why the rate of photosynthesis falls if the stomata stay closed for a long time.
- Command word: Explain
- Tier: Foundation and Higher
- Spec fit: AQA 4.2.3.1 and 4.4.1.2; also Edexcel 6.3 and 6.9
Show mark schemeHide mark scheme
Mark scheme
Carbon dioxide cannot diffuse into the leaf when the stomata are closed1 mark
Carbon dioxide is a reactant in photosynthesis, so it runs short and becomes the limiting factor1 mark
Model answer
Carbon dioxide gets into the leaf by diffusing through open stomata. If the stomata stay closed, no more carbon dioxide can get in. Carbon dioxide is a reactant, so the leaf runs short of it and it becomes the limiting factor. The rate of photosynthesis falls.
Question 3
Pearson Edexcel-style · 3 marks
A student measured light intensity with a light meter. With a lamp 10 cm from the meter, the reading was 640 lux. The student moved the lamp to 40 cm from the meter. Use the inverse square law to calculate the new light meter reading. Give the unit.
- Command word: Calculate
- Tier: Higher only
- Spec fit: Edexcel 6.6; also AQA 4.4.1.2 (Higher)
Show mark schemeHide mark scheme
Mark scheme
The distance increases by a factor of 4 (40 ÷ 10 = 4)1 mark
Light intensity falls by a factor of 4² = 16 (allow: 640 × 10² = 64,000, then 64,000 ÷ 40²)1 mark
640 ÷ 16 = 40 lux (a correct answer with the unit scores 3 marks)1 mark
Model answer
40 ÷ 10 = 4, so the lamp is 4 times further away. Light intensity falls by 4² = 16 times. 640 ÷ 16 = 40 lux.
Question 4
AQA-style · 4 marks
A student measured the rate of photosynthesis of a plant at different light intensities. All other conditions were kept the same.
Question 4
Light intensity (arbitrary units) Rate of photosynthesis (cm³ of oxygen per hour)
0 0.0
10 1.2
20 2.4
30 3.5
40 3.9
50 3.9
Explain the results.
- Command word: Explain
- Tier: Foundation and Higher
- Spec fit: AQA 4.4.1.2; also Edexcel 6.3
Show mark schemeHide mark scheme
Mark scheme
From 0 to 30 units, the rate increases as light intensity increases (allow data quoted, e.g. 1.2 to 3.5 cm³ per hour)1 mark
Because light supplies the energy for photosynthesis, so light intensity is the limiting factor here1 mark
From 40 units, the rate stays the same at 3.9 cm³ per hour1 mark
Because another factor, such as carbon dioxide concentration or temperature, is now limiting the rate1 mark
Model answer
From 0 to 30 units, the rate rises as light intensity rises, from 0 to 3.5 cm³ per hour. Light supplies the energy for photosynthesis, so light intensity is the limiting factor. From 40 units, the rate stays at 3.9 cm³ per hour. Light is no longer limiting. Another factor, such as carbon dioxide concentration or temperature, is now limiting the rate.
Question 5
Pearson Edexcel-style · 4 marks
A student investigated the effect of light intensity on photosynthesis using algal balls. She put 10 algal balls into a bottle of hydrogencarbonate indicator, 10 cm from a filament lamp. She timed how long the indicator took to change from orange-red to purple. She repeated this with new bottles at 20 cm, 30 cm and 40 cm. She did one test at each distance, next to a sunny window. Explain two ways she could improve her method.
- Command word: Explain
- Tier: Foundation and Higher
- Spec fit: Edexcel core practical 6.5; also AQA required practical 6 or 5
Show mark schemeHide mark scheme
Mark scheme
Improvement 1, e.g. place a beaker of water (heat shield) between the lamp and the bottle, or use an LED lamp1 mark
Reason for improvement 1, e.g. so heat from the lamp does not change the temperature, which would also change the rate1 mark
Improvement 2, e.g. block light from the window or work in a darkened room (allow: repeat each distance three times and calculate a mean; compare the colour against a colour chart)1 mark
Reason linked to improvement 2, e.g. so the only light reaching the algae comes from the lamp at a known distance (allow: repeats show up anomalies and make results more repeatable; judging colour by eye is subjective)1 mark
Model answer
First, she should put a beaker of water between the lamp and the bottle, or use an LED lamp. A filament lamp gives off heat, and a rise in temperature would also speed up photosynthesis, so the test would not be fair. Second, she should block out the sunlight from the window. Then the only light reaching the algae comes from the lamp, so light intensity depends only on the distance she sets.
Question 6
AQA-style, level-marked · 6 marks
A student measured the rate of photosynthesis of pondweed at different light intensities under three sets of conditions. Rates are in arbitrary units.
Question 6
Light intensity (arbitrary units) 10 20 30 40 50
A: 0.04% carbon dioxide, 20 °C 8 16 19 19 19
B: 0.10% carbon dioxide, 20 °C 8 16 24 27 27
C: 0.10% carbon dioxide, 30 °C 8 16 24 32 38
Scroll sideways to see the whole table.
Explain the results. Use ideas about limiting factors in your answer.
- Command word: Explain
- Tier: Higher only
- Spec fit: AQA 4.4.1.2 (Higher); also Edexcel 6.4
Show mark schemeHide mark scheme
Mark scheme
At 10 and 20 units of light the rate is the same in A, B and C, so light intensity is the limiting factor0 marks
The rate rises with light intensity because light supplies the energy for photosynthesis0 marks
A levels off at 19 from 30 units, so light is no longer limiting in A0 marks
B levels off higher than A (27 compared with 19), so carbon dioxide concentration was limiting A0 marks
Carbon dioxide is a reactant, so more carbon dioxide allows a faster rate0 marks
B levels off at 27 but C keeps rising at the higher temperature, so temperature was limiting B0 marks
Below the optimum, a higher temperature gives molecules more energy, so they collide more often and the enzyme-controlled reactions go faster0 marks
C is still rising at 50 units, so light is still the limiting factor for C0 marks
Level descriptors
Level 3 · Marks 5-6
Explains the pattern in all three conditions. Uses the data to name the limiting factor in each part, with correct reasons linked to light energy, carbon dioxide as a reactant and temperature. A clear, logical answer.
Level 2 · Marks 3-4
Names at least two limiting factors correctly with some explanation, or explains one condition in detail. May not use the data, or may leave out one condition.
Level 1 · Marks 1-2
Simple statements, for example that more light, more carbon dioxide or a higher temperature increases the rate. Little or no correct use of limiting factors.
Model answer
At 10 and 20 units of light, the rate is the same in all three conditions. Changing carbon dioxide or temperature makes no difference, so light intensity is the limiting factor. The rate rises because light supplies the energy for photosynthesis. In A, the rate levels off at 19 from 30 units, so light is no longer limiting. B has more carbon dioxide and reaches 27, so carbon dioxide concentration was limiting A. Carbon dioxide is a reactant, so more of it lets the reaction go faster. B levels off at 27 from 40 units. C has the same carbon dioxide but is 10 °C warmer, and its rate keeps rising to 38, so temperature was limiting B. A higher temperature gives molecules more energy, so they collide more often and the enzymes controlling photosynthesis work faster. C is still rising at 50 units, so light is still the limiting factor for C.

Question 4
Light intensity (arbitrary units)	Rate of photosynthesis (cm³ of oxygen per hour)
0	0.0
10	1.2
20	2.4
30	3.5
40	3.9
50	3.9

Question 6
Light intensity (arbitrary units)	10	20	30	40	50
A: 0.04% carbon dioxide, 20 °C	8	16	19	19	19
B: 0.10% carbon dioxide, 20 °C	8	16	24	27	27
C: 0.10% carbon dioxide, 30 °C	8	16	24	32	38

How examiners mark this

Command words. Both boards publish what their command words mean: see AQA's command words for the sciences and Appendix 2 of the Pearson Edexcel 1BI0 specification. In our words:

Describe: say what happens or what something is like. Reasons are not needed.
Explain: give the reason or cause. Each point needs a "because" or "so".
Evaluate: weigh up the evidence for and against, then give a judgement and back it up.
Calculate: work out a number from the data. Show your working and give the unit.

AQA's June 2024 Paper 1H report found that misreading command words cost students marks, including on a question explaining a limiting-factors graph.

Units and significant figures. Use the unit the question gives, or build it from your calculation: cm³ ÷ minutes gives cm³ per minute. If the question asks for 2 or 3 significant figures, round only at the very end. AQA's published marking guidance gives full marks for a correct final answer even without working, but awards marks step by step for working. So working is what saves marks when your final number is wrong.

6-mark questions are level-marked. The examiner does not count points. They read the whole answer and place it in one of three levels: Level 1 (1 to 2 marks) for simple, separate statements; Level 2 (3 to 4 marks) for relevant points with some explanation; Level 3 (5 to 6 marks) for a clear, logical answer that explains each step and links it to the question. Then they choose the mark within the level. A long list of facts with no reasons stays low, and an answer that skips part of the question cannot reach the top. Pearson marks its extended open-response questions, shown with an asterisk, in levels too.

Find each board's mark schemes and reports here: AQA past papers and mark schemes and Pearson past papers.

Common mistakes

Mistake: "A limiting factor stops photosynthesis."
Fix: It stops the rate from increasing. Always use the word rate.
Reported in: AQA 8461/1H report, June 2024, Q07.7
Mistake: "The graph levels off because light becomes the limiting factor."
Fix: It is the other way round. At the plateau, light is no longer limiting; another factor is.
Reported in: AQA 8461/1H report, June 2024, Q07.8
Mistake: Mixing up chlorophyll and chloroplasts, or calling chlorophyll an enzyme.
Fix: Chloroplasts are the organelles. Chlorophyll is the green pigment inside them that absorbs light.
Reported in: AQA 8461/1H report, June 2024, Q07.3; June 2025, Q04.3 and Q04.8
Mistake: Writing carbon dioxide as Co2, with a raised 2, or with a full-size 2.
Fix: Capital C, capital O, and a small 2 written low: CO₂.
Reported in: AQA 8461/1H report, June 2024, Q07.1; June 2025, Q04.4
Mistake: Using the wrong number or unit in a rate calculation.
Fix: Divide the amount the question describes by its time, and build the unit from that division, such as bubbles per minute. Made-up short forms like "b/s" do not count.
Reported in: AQA 8461/1H report, June 2025, Q04.2; AQA 8464/B/1H report, November 2021, Q1
Mistake: Vague practical answers.
Fix: Name the equipment and the measurement: "collect the gas in an upturned measuring cylinder and measure its volume". "Do repeats" needs "and calculate a mean". Know that the beaker of water is a heat shield that keeps the temperature constant.
Reported in: AQA 8461/1H report, June 2025, Q04.5; AQA 8464/B/1H report, November 2021, Q1; Pearson 1BI0 2H examiners' report, June 2023, Q5(a)(i)

FAQ

Is photosynthesis on Paper 1 or Paper 2?

It depends on your board. For AQA, it is on Paper 1, with the rest of Bioenergetics, in both GCSE Biology (8461) and Combined Science: Trilogy (8464). For Pearson Edexcel, it is in Topic 6, which is examined on Paper 2 in both GCSE Biology (1BI0) and Combined Science (1SC0, Biology 2).

Do I need to learn the balanced symbol equation?

AQA expects you to recognise the formulae CO₂, H₂O, O₂ and C₆H₁₂O₆, and it has asked students to pick out the balanced equation. Edexcel's specification describes the reactants and products. Learning 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ takes a few minutes, so learn it whichever board you sit.

Is the inverse square law on the Foundation paper?

No. On both AQA and Edexcel, the inverse square law and the interaction of limiting factors are Higher tier only. Foundation students still need the single limiting factors, the light-intensity practical, and how to read a graph with one factor. If you are not sure which tier you are sitting, ask your teacher.

Why does the rate fall at high temperature but level off at high light?

Too much heat damages the enzymes that control photosynthesis. Above the optimum they denature, so the rate falls. Extra light does not damage the enzymes. Once another factor, such as carbon dioxide or temperature, becomes limiting, more light simply stops helping. So light graphs level off, while temperature graphs rise to a peak and fall.

How should I revise photosynthesis?

Learn the equations and the leaf first. Then practise graphs and tables, because many questions ask you to explain data. Test yourself with the flashcards above, answer the practice questions and get them marked. Add photosynthesis to a revision timetable with spaced reviews before your GCSE exams.

How this page was made

Drafted with AI assistance and checked against the AQA (8461, 8464) and Pearson Edexcel (1BI0, 1SC0) specifications for spec numbers and tiers on 29 October 2026.

All explanations, data and questions are original, and no exam-board questions, mark schemes or answers are reproduced.

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Sources

AQA GCSE Biology 8461 specification: 4.4 Bioenergetics (web) · checked 29 October 2026
AQA GCSE Biology 8461 specification PDF, version 1.0 · checked 29 October 2026
AQA GCSE Combined Science: Trilogy 8464 specification PDF, version 1.1 · checked 29 October 2026
Pearson Edexcel GCSE Biology 1BI0 specification, Issue 4 · checked 29 October 2026
Pearson Edexcel GCSE Combined Science 1SC0 specification, Issue 6 · checked 29 October 2026
AQA command words (GCSE sciences) · checked 29 October 2026
AQA GCSE Biology 8461/1H mark scheme, June 2024 (marking guidance) · checked 29 October 2026
AQA GCSE Biology 8461/1H report on the examination, June 2024 · checked 29 October 2026
AQA GCSE Biology 8461/1H report on the examination, June 2025 · checked 29 October 2026
AQA GCSE Combined Science: Trilogy 8464/B/1H report on the examination, November 2021 · checked 29 October 2026
Pearson Edexcel GCSE Biology 1BI0 2H examiners' report, June 2023 · checked 29 October 2026
AQA past papers and mark schemes finder · checked 29 October 2026
Pearson past papers, mark schemes and examiners' reports · checked 29 October 2026

Photosynthesis GCSE Revision Notes: AQA and Edexcel

Summary

Key takeaways

Notes

4.4.1.1 The photosynthetic reaction (AQA) / 6.1 and 6.2 (Edexcel)

4.2.3.1 Where it happens: the leaf (AQA) / 6.9 and 6.11B (Edexcel)

4.4.1.2 Rate of photosynthesis and limiting factors (AQA) / 6.3 (Edexcel)

How to read rate graphs

4.4.1.2 Higher tier: how limiting factors interact (AQA) / 6.4 (Edexcel)

Required practical 6 (AQA 8461) / 5 (AQA 8464) / 6.5 core practical (Edexcel): light intensity and rate

Method A: pondweed (common for AQA)

Method B: algal balls (Edexcel's named organism)

Variables

Risks

Typical results

Common errors

4.4.1.2 Higher tier: the inverse square law (AQA) / 6.6 (Edexcel)

4.4.1.2 Higher tier: greenhouse economics (AQA only)

4.4.1.3 Uses of glucose (AQA only)

Worked example 1: the inverse square law (Higher)

Worked example 2: rate from a data table

What differs by board

Key facts and equations

Flashcards

Practice questions

Question 1

Mark scheme

Model answer

Question 2

Mark scheme

Model answer

Question 3

Mark scheme

Model answer

Question 4

Mark scheme

Model answer

Question 5

Mark scheme

Model answer

Question 6

Mark scheme

Level descriptors

Model answer

Write your answer, get it marked

How examiners mark this

Common mistakes

FAQ

Is photosynthesis on Paper 1 or Paper 2?

Do I need to learn the balanced symbol equation?

Is the inverse square law on the Foundation paper?

Why does the rate fall at high temperature but level off at high light?

How should I revise photosynthesis?

How this page was made

Sources

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