Key Stage 3 science · Biology – Material cycles and energy

KS3 photosynthesis: a guide for parents

Photosynthesis is the process plants and algae use to turn light energy into stored chemical energy. In Key Stage 3 children learn what goes into the process and what comes out, how to write it as a word equation, how leaves are built to carry it out, and why nearly all life on Earth depends on it, both for food and for the oxygen and carbon dioxide levels in the air.

Photosynthesis, explained

The core idea is that green plants make their own food. Carbon dioxide from the air and water from the soil are the reactants, the substances that go in. Using energy from sunlight, they are turned into glucose, a sugar, and oxygen, the products that come out. The word equation is: carbon dioxide + water → glucose + oxygen, with light energy needed and chlorophyll, the green pigment in chloroplasts, absorbing that light. Children are expected to know this equation and to be able to say what is a reactant and what is a product. The symbol equation is not required at Key Stage 3; it comes later, at GCSE.

Glucose is the plant’s energy store, and it is used in several ways. Some is broken down in respiration to release energy for growth. Some is turned into starch for storage, or into cellulose to build cell walls. Some is combined with minerals such as nitrates from the soil to make proteins. This is why plants need minerals as well as light, water and carbon dioxide. Plants and algae are the producers at the start of nearly every food chain, so almost every animal, including us, depends on the energy they capture from sunlight.

Photosynthesis also keeps the air in balance. It removes carbon dioxide from the atmosphere and releases oxygen, and living things then use that oxygen in respiration. Children are taught that this includes algae in the oceans and ponds, not only trees and grass. They also hear that the balance of gases in the atmosphere is affected when large areas of forest are cleared or when more carbon dioxide is released by burning fuels.

Leaves are the main organs of photosynthesis, and the National Curriculum asks children to explain how their structure suits the job. A leaf is broad and flat to catch light and thin so gases do not have far to travel. Most chloroplasts sit in the palisade cells near the top, where the light is strongest. Tiny holes called stomata, opened and closed by guard cells, let carbon dioxide in and oxygen out. Air spaces in the spongy layer help gases move around, a waxy layer on the surface cuts water loss, xylem vessels bring water in and phloem carries sugars away. Many schools back these ideas up with practical work, for example testing leaves for starch with iodine in a supervised lesson to show that a leaf needs light and chlorophyll to make it.

Teachers also like to link photosynthesis with respiration. The two word equations look like mirror images, but that does not mean a plant does one instead of the other. Photosynthesis happens only in cells with chloroplasts and only when there is light, whereas respiration happens in every living cell all the time, plants included.

Words your child will hear

Reactant: A substance that goes into a chemical reaction. For photosynthesis these are carbon dioxide and water.
Product: A substance made in a reaction. Photosynthesis makes glucose and oxygen.
Chlorophyll: The green substance in chloroplasts that absorbs light for photosynthesis.
Chloroplast: The part of a plant or algal cell where photosynthesis happens.
Stomata: Tiny holes, mostly on the underside of a leaf, that let gases in and out. One hole is a stoma.
Glucose: A sugar made in photosynthesis that stores energy and is used for respiration, starch and growth.
Producer: An organism, such as a plant or alga, that makes its own food at the start of a food chain.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about the reactants in, and products of, photosynthesis, and a word summary for photosynthesis
Pupils should be taught about the dependence of almost all life on Earth on the ability of photosynthetic organisms, such as plants and algae, to use sunlight in photosynthesis to build organic molecules that are an essential energy store and to maintain levels of oxygen and carbon dioxide in the atmosphere
Pupils should be taught about the adaptations of leaves for photosynthesis

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

Try it: explaining an observation

A healthy pot plant is kept in a dark cupboard for two weeks. Its new shoots are thin, weak and pale yellow, and the plant looks unhealthy. Explain why.

Name the process and the word equation: carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll.
Say what has changed. In the cupboard there is no light, so the plant cannot absorb the energy it needs to carry out the reaction. The plant still has water and carbon dioxide, so a shortage of reactants is not the cause. Light is the missing factor.
Link to the product. Without photosynthesis the plant makes little or no glucose, so it has no new energy store and no new material for strong cell walls. It has to live on the starch it had already stored, so any new growth is thin and weak.
Explain the colour. Leaves grown in the dark do not make chlorophyll, and the green pigment in old leaves can break down, so the leaves lose their green colour and look pale yellow.
Predict. If the plant is moved back to a bright windowsill and watered, it should start photosynthesising again and gradually turn greener, as long as it has not already used up its stores.

The plant could not photosynthesise because there was no light, so it made no new glucose and had to use up its stored starch, which is why its new growth was weak. Leaves grown in the dark do not make chlorophyll, which is why they were pale and yellow.

Common mistakes and how to help

Thinking plants take in their food from the soil

Gardeners feed plants with compost and fertiliser, and roots clearly take in something from the ground, so it seems that soil must be the food.

What helps: Soil gives water and minerals, but the glucose that builds the plant is made in the leaves. Most of a tree’s mass comes from carbon dioxide and water, which is a surprise to many adults. Plants grown in water with dissolved minerals and no soil at all show that soil itself is not the food.

Believing photosynthesis is the opposite of breathing, so plants only do one of them

“Plants take in carbon dioxide and give out oxygen” is a handy summary that sounds like a swap with animals.

What helps: Plants respire all the time, day and night, to release energy from glucose, as animals do. In daylight they usually photosynthesise faster than they respire, so overall they release oxygen. At night there is only respiration.

Putting the substances on the wrong side of the equation

Children remember the four substance names but not which side of the arrow they sit on.

What helps: Use the idea of a recipe. The ingredients go in on the left (carbon dioxide, water) and the cake comes out on the right (glucose, oxygen). Ask your child which two things the plant takes in from the air and soil (the left side) and which two it makes (the right side).

Saying light is a substance that is “used up” like water

Because light is listed alongside the reactants, it looks like another ingredient.

What helps: Light supplies the energy, which is stored in the glucose. It is not a material that becomes part of the products. This idea comes back when children study energy transfers in physics.

How to help at home

Compare light and dark. Grow cress on damp kitchen paper, one tray on a windowsill and one inside a closed cupboard. After a week compare height, colour and sturdiness. Ask your child to explain the difference using the word equation.
Look under a garden object. Leave an upturned plant pot, plate or tile on a patch of your own lawn for a week, then lift it. The grass underneath will be pale or yellow. Ask your child to explain why, and to predict what will happen over the next week now it is back in the light.
Look at leaf shapes. Collect fallen leaves in autumn and use a magnifying glass to look at the veins and surface. Ask your child to point out what helps the leaf catch light, let in air and bring in water.
Trace a meal back to the sun. Pick a meal, such as chicken with rice and peas, and ask where the energy in each part first came from. Following the chain back to a plant and then to sunlight makes the big idea behind the topic concrete.