Photosynthesis and respiration in plants

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Key points

  • is a chemical process in which green plants make their own food using energy from the sun - this food is made in the form of .

  • Photosynthesis occurs in chloroplasts found within cells. It is the chloroplasts that contain the green pigment chlorophyll which absorbs light.

  • The plant uses this glucose to grow as well as make other useful substances, such as cellulose found in the cell walls and starch used as energy storage.

  • The glucose can also be broken down in the plant by

Video - Photosynthesis

Can you answer these questions based on the video?

1. Which four things are needed for the plant to photosynthesise?

2. How do you test for oxygen?

  1. Sunlight, chlorophyll, carbon dioxide and water.

  2. If oxygen is present it will relight a glowing splint.

Photosynthesis is important to living organisms because it is the number one source of oxygen in the atmosphere. Fewer plants would mean less recycling of carbon dioxide and there would be less oxygen production. Without photosynthesis there would be no life as we know it on planet Earth.

How are plants adapted for photosynthesis?

Plants are to carry out the process of respiration and photosynthesis. Gases such as oxygen and carbon dioxide can travel through tiny holes underneath the leaf called . Carbon dioxide diffuses into the cells found within the leaf to help carry out the process of photosynthesis, whilst oxygen is released from these cells as a product of photosynthesis.

Diagram showing a cross section of a leaf
A cross-section through a leaf showing its main parts

Plants have adaptations to help them to photosynthesise and ultimately to survive.

Broad leaves - Provide a large surface area to trap sunlight. Thin leaves - Provide a short diffusion pathway for gases to move into and out of cells. Extensive roots - To absorb water from soil, needed for photosynthesis. Network of tubes (xylem and phloem) - To transport water, mineral ions and glucose (food) around the plant. Lots of chloroplast - Chloroplasts have a green substance called chlorophyll, which absorbs energy from the sun for photosynthesis.
The adaptations of a tree

Comparing photosynthesis and respiration in plants

Photosynthesis:

Carbon dioxide + water -> glucose + oxygen

Respiration:

Oxygen + glucose -> water + carbon dioxide

The gas carbon dioxide is needed for photosynthesis to take place in plants and is also produced when plants respire. Oxygen is a of photosynthesis - it is needed as a reactant during aerobic respiration within a plant. This means that the and products of photosynthesis and aerobic respiration are opposites.

If you memorise one of these equations, then you already know the other, as you can read it backwards.

When do plants respire?

Plants respire all the time, whether it is dark or light. However, they only photosynthesise when they are in the light.

The table shows the difference between photosynthesis and respiration at different times of the day.

TimePhotosynthesisRespirationGases into leafGases out of leaf
Day (light)Carbon dioxideOxygen, water vapour
Night (dark)No gas exchange as stomata close at nightNo gas exchange as stomata close at night
a tree in daytime - photosynthesis and, to a lesser extent, respiration are happening.

Photosynthesis and respiration in plants during the day.

1 of 2
  • During the day carbon dioxide diffuses into the leaf through the stomata which is needed for photosynthesis.
  • When cells in the leaf photosynthesise during the day, they will produce the oxygen needed for the plant to respire, therefore more oxygen is not required to diffuse into the leaf from the air.
  • Water vapour will leave the leaf through the stomata during the day as the stomata remain open. This allows water to move around the plant through the
  • The stomata close at night preventing gases from diffusing out of the leaf.
  • There are however exceptions to this. For example artificial light that is used at football pitches to help the grass grow during the night.

Light is one factor that affects the of photosynthesis. For example, if light intensity is decreased, this will result in a decrease in the rate of photosynthesis. Other factors such as the levels of carbon dioxide, presence of chlorophyll, water and temperature can also affect the rate.

Find out more about the factors which affect the rate of photosynthesis.

Test your knowledge

Quiz

Test questions

Write a paragraph to answer the following question. Tap 'Show answer' to see six points you could have included.

Describe and explain how plants are well-adapted for photosynthesis.

  • Plants have broad leaves to trap sunlight
  • Leaves are thin to provide a short diffusion path for oxygen and carbon dioxide
  • Long roots are good for absorbing more water for photosynthesis
  • Stomata allow gases into and out of the leaf
  • Guard cells to control the opening and closing of the stomata under different environmental conditions
  • Lots of chloroplast to absorb sunlight for photosynthesis

Write an answer to the following question. Tap 'Show answer' to see three points you could have included.

Describe and explain why photosynthesis does not occur in the roots of plants.

  • Roots generally grow underground where there is no light
  • Root hair cells don’t have chlorophyll
  • Chlorophyll is the green pigment required to trap sunlight needed for photosynthesis