Topics · Primary Science · Systems: Photosynthesis
Photosynthesis: how a plant makes its own food
A plant makes food in its leaves using light. Four things go in, two come out, and nearly every question is about removing one of the four and saying what stops.
- 180 exam papers analysed
- appears in 86% of P6 Science papers
- ~3 min read
01 What goes in and what comes out
In: water, carbon dioxide, light and chlorophyll. Out: food (sugar or starch) and oxygen.
Chlorophyll is the green substance in the leaves. It is not used up — it captures the light energy, so it is a requirement rather than an ingredient.
Light is needed, but not necessarily sunlight. A bright lamp works, which is a distinction papers ask about.
02 Where each thing comes from
Papers ask this directly, and mixing up the routes is a common error.
- Water — taken in by the roots and carried up the stem to the leaves.
- Carbon dioxide — taken in from the air, through tiny openings in the leaf.
- Light — falls on the leaf from outside.
- Chlorophyll — already in the leaf, in the chloroplasts.
- Oxygen — leaves the plant through the same openings the carbon dioxide came in by.
03 Remove one thing
The standard experiment covers part of a leaf, or removes one requirement, then tests for starch. Say which requirement is missing and that photosynthesis stopped there.
Covered with black paper — no light reaches that part, so no starch forms there. The rest of the leaf still makes it.
A variegated leaf — only the green parts have chlorophyll, so only they make starch. The white parts test negative.
Kept in the dark first — the plant is destarched, so any starch found afterwards must have been made during the experiment. Skipping this step is why an experiment fails.
No carbon dioxide — nothing is made even in full light.
04 Why it matters beyond the plant
Photosynthesis is where almost all food on Earth begins. Animals cannot make their own, so every food chain starts with a plant.
It also puts oxygen into the air, which animals need to breathe.
So a question about cutting down a forest wants both: less food at the base of the food chains, and less oxygen produced.
Plants respire too, day and night, using oxygen. They photosynthesise only in light, which is why they release more oxygen than they use during the day and none at night.
05 How to say them
These are words most children have never said out loud, and a word you cannot say is a word you avoid using. Capitals mark the syllable you lean on. Tap the speaker to hear it.
- photosynthesissayfoh-toh-SIN-thuh-siss
The making of food in a plant's leaves using light, water and carbon dioxide.
- chlorophyllsayKLOR-uh-fil
The green substance in leaves that captures light energy.
- carbon dioxidesayKAR-buhn dy-OK-side
The gas taken in from the air for photosynthesis.
- starch
The food a plant stores. Testing a leaf for starch shows whether photosynthesis happened.
- variegatedsayVAIR-ee-uh-gay-tid
A leaf with green and white parts. Only the green parts have chlorophyll.
- destarch
To keep a plant in the dark first, so any starch found later must be new.
06 What each level is tested on
The same topic, asked differently as it goes up. The percentage is the share of papers at that level in which it appears.
- P5
What a plant needs to make food, and that food is made in the leaves.
Practise P5 Systems: Photosynthesis — 10 questions →29% of P5 papers
- P6
The full process, starch experiments, and photosynthesis inside food chains and environment questions.
Practise P6 Systems: Photosynthesis — 10 questions →86% of P6 papers
07 Mistakes worth knowing about
Each of these is wrong. Decide why before you open it.
“Plants get their food from the soil.”
They make it in their leaves. From the soil they take water and mineral salts, which are not food.
“Photosynthesis needs sunlight specifically.”
It needs light. A bright lamp works just as well.
“Plants only respire at night.”
They respire all the time. They photosynthesise only in light, which is why they give out more oxygen than they use during the day.
“The white part of the leaf made no starch because it got no light.”
It got the same light. It has no chlorophyll, which is the missing requirement.
“Chlorophyll is used up making food.”
It captures the light energy and is not consumed, which is why a leaf stays green while photosynthesising.
Now try it
20 questions across 2 levels, marked as you go with the working shown.