Free sample activity · Grades 6–8

Photosynthesis Practice

These questions revisit how green plants use light energy to make sugars. They focus on a simplified model of photosynthesis and do not cover every reaction or all of the ways plants use energy.

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Allow about five minutes. Sketch a plant with arrows for water, carbon dioxide, light, sugars, and oxygen. Decide which arrows represent matter and which represent energy.

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Energy input

What is the main energy source for photosynthesis in a sunlit green plant?

Sunlight Soil particles Oxygen gas
Answer: A.

Light supplies energy for photosynthesis. Soil supplies water and minerals, but it is not the main energy source for this process.

Matter inputs

Which pair supplies key starting materials for photosynthesis?

Sugar and oxygen Carbon dioxide and water Nitrogen and sugar
Answer: B.

Plants use carbon dioxide and water to form sugars during photosynthesis, with light providing the required energy.

Gas output

Which gas is released during photosynthesis in green plants?

Helium Nitrogen Oxygen
Answer: C.

Oxygen is released during photosynthesis. Plants also respire, so this answer describes the photosynthesis process rather than every gas exchange in a plant.

Carbon source

Where does the carbon in sugars made by a leaf mainly come from?

Carbon dioxide taken in from the air Sunlight entering the leaf Mineral grains from the soil
Answer: A.

Carbon dioxide supplies the carbon used to build sugars. Sunlight supplies energy; it does not supply carbon atoms.

Stored energy

What happens to some of the light energy used in photosynthesis?

It becomes extra soil. It is stored as chemical energy in sugars. It removes all need for plant respiration.
Answer: B.

Photosynthesis converts light energy into chemical energy stored in sugars. Plants can use those sugars in respiration and to build other materials.

Limiting light

A plant has enough water and carbon dioxide, but light is limiting photosynthesis. What would reducing the light usually do?

Make photosynthesis unlimited Remove the need for water Reduce the rate of photosynthesis
Answer: C.

When light is already limiting, reducing it further usually reduces the photosynthesis rate. The question holds other needs sufficient so the light change can be considered.

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All questions, answers & explanations

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  1. Energy input

    What is the main energy source for photosynthesis in a sunlit green plant?

    Answer: Sunlight

    Light supplies energy for photosynthesis. Soil supplies water and minerals, but it is not the main energy source for this process.

  2. Matter inputs

    Which pair supplies key starting materials for photosynthesis?

    Answer: Carbon dioxide and water

    Plants use carbon dioxide and water to form sugars during photosynthesis, with light providing the required energy.

  3. Gas output

    Which gas is released during photosynthesis in green plants?

    Answer: Oxygen

    Oxygen is released during photosynthesis. Plants also respire, so this answer describes the photosynthesis process rather than every gas exchange in a plant.

  4. Carbon source

    Where does the carbon in sugars made by a leaf mainly come from?

    Answer: Carbon dioxide taken in from the air

    Carbon dioxide supplies the carbon used to build sugars. Sunlight supplies energy; it does not supply carbon atoms.

  5. Stored energy

    What happens to some of the light energy used in photosynthesis?

    Answer: It is stored as chemical energy in sugars.

    Photosynthesis converts light energy into chemical energy stored in sugars. Plants can use those sugars in respiration and to build other materials.

  6. Limiting light

    A plant has enough water and carbon dioxide, but light is limiting photosynthesis. What would reducing the light usually do?

    Answer: Reduce the rate of photosynthesis

    When light is already limiting, reducing it further usually reduces the photosynthesis rate. The question holds other needs sufficient so the light change can be considered.

Take it one step further

Explain why sunlight supplies energy but is not the source of the carbon in a plant's sugars. Use a labeled diagram.

Background reading

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