4.1 Photosynthesis
Summary: Unlike animals, plants do not have to eat other organisms to survive. Instead they make their own food using sunlight. How do they do this? Tags: science oxford-science student-book-7 biology Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe the process of Photosynthesis
- give the word equation for photosynthesis
- describe how to test a leaf for starch.
Think back
- What substances does a plant need to grow?
- Why are plants green?
- Name three cell parts found in plant Cells but not in animal cells.
Key idea Photosynthesis is the process where plants make their own food using sunlight. Plants take in carbon dioxide and water and change them into glucose and oxygen.
Key words algae, producer, glucose, consumer, photosynthesis, chlorophyll
Unlike animals, plants do not have to eat other organisms to survive. Instead they make their own food using sunlight. How do they do this?
What is a producer?
Plants and algae are called producers because they make their own food. They change materials found in their environment into glucose, a carbohydrate, using sunlight.
Figure 1 shows algae. Algae are like plants because they are green organisms that make their own food. However, they are different from plants in the following ways:
- Algae can be unicellular or multicellular organisms.
- Algae live underwater while most plants live on land.
- Algae do not have leaves, stems, or roots.
Figure 1Algae live in water.
Animals are called consumers because they have to eat other organisms to survive. These can be plants or other animals. They break down the organism during digestion. This releases nutrients, which are then used by the body.
A State whether a human is a producer or a consumer.
What is photosynthesis?
Plants make food through the process of photosynthesis. Photosynthesis is a chemical reaction where plants take in carbon dioxide and water and change them into glucose. This provides the plant with food. Oxygen is also made. Oxygen is a waste product of the reaction and it returns to the atmosphere. Plants need to use Light from the Sun in this chemical reaction.
The word equation for photosynthesis is:
B Name the useful product of photosynthesis for plants.
Where does photosynthesis take place?
Photosynthesis mainly takes place inside chloroplasts in the leaf cells. Figure 2 shows chloroplasts. However, a small amount happens in the stem. Leaves and stems are green because they contain the green pigment (dye) chlorophyll. Chlorophyll uses light from the Sun. The Energy transferred from the Sun is needed for the plant to change carbon dioxide and water into glucose and oxygen.
Figure 2Photosynthesis takes place inside chloroplasts.
C Name the part of the cell where chlorophyll is found.
Testing a leaf for starch
During photosynthesis, a plant produces glucose. It stores the glucose as starch in the leaf.
You can check that a plant has carried out photosynthesis by testing it with iodine.
Full text alternative to the content
Step 1 – Boil the leaf in water to remove the waxy layer.
Step 2 – Put the leaf into a test tube of ethanol. Place the test tube into the boiling water. The ethanol dissolves the chlorophyll, removing it from the leaf.
Step 3 – Wash the leaf in water to remove the ethanol and soften the leaf (as the ethanol makes the leaf hard).
Step 4 – Put the leaf on a white tile. Add a few drops of iodine. If the leaf turns black, it contains starch.
Working scientifically A hypothesis is an idea about why something happens. Look at the word equation for photosynthesis. Write a hypothesis for what would happen to the plant if you put it in a dark cupboard for a week.
Stretch zone
- Copy and complete the sentences. Plants and algae/animals are consumers/producers. They use digestion/photosynthesis to make their own food. They use carbon dioxide/nitrogen and water to make ethanol/glucose and oxygen using light/thermal energy.
- State and explain whether photosynthesis would occur in these situations: a bright, sunny dayat nightin the root hair cells
Stretch zone
- Explain why photosynthesis is important for all life on Earth.