6.3 - Leaf Structure

Guard cells and the stomata

• Each stoma (pore) is bordered by a pair of guard cells, which open and close the pore to control gas exchange and water loss.
• A guard cell has a kidney shape, with:
  • A thicker, less flexible wall on the inner side (next to the pore)
  • A thinner wall on the outer side
• This uneven thickening is what makes the opening and closing work.

Opening and closing

• Plenty of water - Guard cells take in water and become turgid (plump and swollen). The uneven wall thickness makes the pair curve apart, so the stoma opens - letting carbon dioxide and oxygen be exchanged for photosynthesis.
• Short of water - Guard cells lose water and become flaccid (limp and floppy). They straighten and close together, so the stoma closes - helping to stop too much water vapour escaping from the leaf.

Sensitivity to light

• Guard cells are also sensitive to light.
• They close the stomata at night, when photosynthesis cannot happen.
• This saves water without any loss of photosynthesis.