Resources for teachers: BIOLOGY

How do plants make food?

Discipline: BIOLOGY

Key Stage: 3 or 4

Key idea:


How do plants make food?


You should understand that during photosynthesis plants release oxygen as a waste product.


All living organisms need food. They need it as a source of raw materials to build new cells and tissues as they grow.
Plants are unique organisms since they are able to make their own food by using a chemical contained in their leaves called chlorophyll to absorb sunlight and transform it into chemical energy. This chemical energy is used to transform carbon dioxide and water into glucose (food) and oxygen. This process is called photosynthesis.
When leaves carry out photosynthesis they give off oxygen that passes to the atmosphere. If water plants are placed in sunlight you can collect the oxygen produced.


  • For this experiment you will need pond weed (Elodea canadensis), a jar, a test tube, a short-stemmed funnel and splints.
  • Fill in a sink with water and submerge the jar.
  • Put the pondweed in the jar and cover it with the inverted funnel.
  • Submerge the test tube in the sink, fill it with water and invert over the funnel, making sure there are no air bubbles trapped.
  • Take the apparatus out of the sink and place it under a table lamp or in a sunny windowsill.
  • Bubbles of gas will collect in the inverted test tube. When the tube is half filled with gas, light a splint and blow it out. Quickly place the glowing splint into the test tube and observe what happens.
  • Repeat the same test but with ordinary air, compare what happens.


    1. How did you test the gas in the test tube?
    2. What gas do you think was released by the pondweed into the test tube?
    3. What would have happened if you had placed the apparatus in a dark cupboard?
    4. Use the idea of photosynthesis to explain the results from this experiment.
    5. Why do you think a pondweed, rather than a land plant is used in this experiment?

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    February 2007