Key Stage 4 science · Biology – Photosynthesis

GCSE photosynthesis and limiting factors: parent guide

Photosynthesis is the process by which plants, algae and some bacteria capture light energy and use it to make glucose from carbon dioxide and water. Almost every food chain starts here, which is why the biomass of living things depends on it. At GCSE, students learn the word and symbol equations, where photosynthesis happens in the cell, what the glucose is used for, and how light, carbon dioxide and temperature can limit the rate.

Photosynthesis and its rate, explained

Photosynthesis is the key process for food production. Green plants and algae are producers: they make their own food, and everything that eats them, directly or indirectly, relies on the energy they capture. The glucose they make becomes biomass, the mass of living material, which passes along food chains. Photosynthesis also releases oxygen, and it is the source of the oxygen in the air we breathe. The reaction is endothermic, meaning it takes in energy from the surroundings, which in this case is light.

The reaction can be written as a word equation: carbon dioxide plus water gives glucose plus oxygen, using light energy. In symbols it is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. It takes place in chloroplasts, the small structures in plant cells that contain the green pigment chlorophyll. Chlorophyll absorbs light. Carbon dioxide enters the leaf through the stomata, and water is carried up from the roots in the xylem. Leaves are shaped to make this work: they are broad and thin, with a large surface area to catch light, and have air spaces inside and many chloroplasts in the upper cells.

The glucose is used in several ways. Some is used straight away for respiration, to release energy. Some is turned into starch, an insoluble store, or into fats and oils for storage in seeds. Some becomes cellulose for cell walls. Plants also combine glucose with nitrate ions absorbed from the soil to make amino acids, then proteins. This is why plants need minerals from the soil even though they do not take their food from it.

The rate of photosynthesis depends on conditions. Light intensity, the concentration of carbon dioxide and temperature can each be limiting factors, and so can the amount of chlorophyll in the leaf, meaning the one in shortest supply sets the speed. Raising a limiting factor increases the rate until something else becomes limiting. Too high a temperature denatures the enzymes involved, so the rate falls. Students read graphs of these effects, and many courses also link this to growers’ practice, for example using a greenhouse or polytunnel, extra lighting, or carbon dioxide enrichment to increase yield while weighing up the cost.

Words your child will hear

Producer: An organism, such as a plant or alga, that makes its own food by photosynthesis.
Chlorophyll: The green pigment in chloroplasts that absorbs light for photosynthesis.
Chloroplast: The part of a plant cell where photosynthesis takes place.
Biomass: The mass of living material in an organism or a group of organisms.
Endothermic: A reaction that takes in energy from the surroundings. Photosynthesis takes in light energy.
Limiting factor: The factor in shortest supply, which sets the maximum rate of a process.
Stomata: Tiny pores, mostly on the underside of leaves, that let gases in and out.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about photosynthesis as the key process for food production and therefore biomass for life
Students should be taught about the process of photosynthesis
Students should be taught about factors affecting the rate of photosynthesis

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A model exam answer

A student measures the rate of photosynthesis in pondweed at different light intensities, with the temperature and carbon dioxide concentration kept the same. Explain the shape of the results.

Look at the data first. The rate rises in even steps and then stops rising at a light intensity of 30 units.

Light       Rate (units)
  0          0
 10          6
 20         12
 30         18
 40         18
 50         18

From 0 to 30 units, the rate is directly proportional to light intensity: each extra 10 units adds 6 to the rate. Light is the limiting factor here, because the rate rises whenever more light is supplied.
From 30 to 50 units the rate stays at 18. Increasing light has no effect, so something else has become limiting. Light is no longer the limiting factor.
Suggest what the new limiting factor could be. In this experiment, temperature and carbon dioxide concentration were fixed, so one of them is now too low, or the enzymes or chlorophyll available are working at their maximum.

The rate increases in proportion to light intensity up to 30 units because light is limiting. Above 30 units the graph is flat because another factor, such as carbon dioxide concentration or temperature, is now limiting. Photosynthesis is still happening, just not any faster.

Common mistakes and how to help

Plants take their food from the soil

Plants are in soil and are fed with fertiliser, and children are often told that plants need soil to grow.

What helps: Plants make glucose from carbon dioxide and water. Most of a tree’s dry mass comes from carbon dioxide in the air and from water. The soil supplies water and minerals such as nitrates.

Plants photosynthesise and animals respire, so plants don’t respire

Photosynthesis and respiration are often taught as opposites, and the equations look like mirror images.

What helps: Plants respire all the time, in every living cell. In bright light they photosynthesise faster than they respire, so there is a net release of oxygen and a net gain of glucose.

More light, carbon dioxide or heat always speeds up photosynthesis

The rising part of a graph suggests that increasing a factor always helps.

What helps: The rate only increases while that factor is limiting. Past this, another factor is limiting. Heat also denatures enzymes, so very high temperatures reduce the rate.

A flat graph means photosynthesis has stopped

A plateau looks like “nothing is happening”.

What helps: The reaction continues at its maximum rate for the conditions. A flat line means increasing that particular factor no longer makes a difference.

How to help at home

Trace your lunch back to a leaf. Pick a meal and follow each ingredient back to a plant, directly or through an animal that ate plants. It makes the idea of producers and biomass concrete.
Grow cress in light and dark. Sow cress on damp kitchen paper in two saucers, one on a windowsill and one in a dark cupboard. After a few days compare height and colour. The dark one is usually taller, thinner and pale yellow: without light it makes little chlorophyll and uses up its stored food stretching upwards. Ask why it would not survive for long.
Vary the light. Place identical seedlings or cress at different distances from a window or lamp and measure growth over a week. Ask what must be kept the same for a fair test, and which factor might become limiting.
Sketch a limiting-factor graph. Ask your teenager to draw the shape for light, then for temperature, and to explain the difference. The temperature curve falls at the end, while the light curve flattens.

Everything in Key Stage 4 science

Updated 20 October 2026.