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Gas exchange in leaves
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Leaves exchange carbon dioxide and oxygen by diffusion. Which way the gases move depends on the light.
Diffusion through stomata
Inside, they diffuse through the air spaces of the spongy mesophyll to and from the cells.
Leaves are thin with a large surface area and many stomata, so diffusion is quick. Guard cells open and close the stomata.
Respiration and photosynthesis
In the light they also photosynthesise: they take in carbon dioxide and give out oxygen.
Bright light: photosynthesis is faster than respiration, so there is a net uptake of carbon dioxide and net release of oxygen.
Dark: only respiration, so there is a net release of carbon dioxide.
Dim light: the two rates can be equal, so there is no net exchange.
Hydrogencarbonate indicator practical
Cover one tube with foil (dark), one with gauze (dim light) and leave one uncovered (bright light).
A leaf in a tube of hydrogencarbonate indicator is left in bright light. The indicator turns purple. Explain why.
- In bright light, photosynthesis is faster than respiration.
- The leaf takes in more carbon dioxide than it gives out.
- Lower carbon dioxide makes the indicator turn purple.
Answer: Net uptake of carbon dioxide by photosynthesis lowers the carbon dioxide level, so the indicator turns purple.
Gases diffuse through stomata and air spaces. Plants respire all the time and photosynthesise in light. In bright light there is net uptake of carbon dioxide, in the dark net release. Hydrogencarbonate indicator: yellow for more carbon dioxide, purple for less.
The interactive lesson includes the diagrams for this topic.
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