The gate of the leaf

The idea

Stomata are tiny leaf pores for gas exchange; guard cells swell with water to open the pore and shrink to close it, so the plant closes stomata to reduce water loss when it needs no carbon dioxide.

A leaf with tiny pores

The surface of a leaf has tiny pores. Each pore has a guard cell on each side.

What do you think makes the pore open?

• The guard cells fill with water
• The leaf gets cold
• The guard cells fall off

Work the pore

Use the slider to put water into the guard cells, then take it out.

Swell to open, shrink to close

Stomata are tiny pores on the surface of leaves, where gases are exchanged. The guard cells decide whether a pore is open. When water flows in, they swell and the pore opens. When they shrink, the pore closes. Large amounts of water can also be lost through stomata, so the plant closes them when it does not need carbon dioxide for photosynthesis.

Stomata are tiny leaf pores for gas exchange; guard cells swell with water to open the pore and shrink to close it, so the plant closes stomata to reduce water loss when it needs no carbon dioxide.

Notes

Guard cells swell with water to open a stoma and shrink to close it; the plant closes stomata to reduce water loss when it needs no carbon dioxide.

Check yourself

What makes the pore of a stoma open?

When does a plant close its stomata?

Why is closing the stomata useful to the plant?

Which cells control the opening and closing of a stomatal pore?

• The guard cells shrink. When the guard cells shrink, the pore closes.
• The guard cells swell as water flows into them — correct. Yes!
• Water flows out of the guard cells. If water leaves the guard cells they shrink, and then the pore closes.
• When the guard cells are swollen with water. Swollen guard cells open the pore.
• When it wants to lose more water. Large amounts of water can be lost through stomata, so closing them reduces the loss.
• When it does not need carbon dioxide for photosynthesis — correct. Yes!
• It reduces the loss of water — correct. Yes!
• It lets more carbon dioxide in. A closed pore does not let gases pass, so it cannot let more carbon dioxide in.
• It makes the guard cells swell. Guard cells shrink when the pore closes.
• Chloroplasts. Chloroplasts are the green cell organelles that contain chlorophyll.
• Guard cells — correct. Yes!
• Companion cells. Companion cells help the sieve tubes of the phloem; they do not control the pores.