Stomata are tiny pores on the surfaces of leaves that control the movement of gases between a plant and the air. They let carbon dioxide enter for photosynthesis and allow oxygen to leave as a product. At the same time, open stomata also let water vapor escape, so plants must balance making sugar with avoiding dehydration.
This balance is especially important in hot, dry, or windy environments.
Key Facts
- CO2 enters leaves through stomata and is used in photosynthesis: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2.
- O2 produced by photosynthesis diffuses out through stomata when its concentration is higher inside the leaf.
- Water vapor leaves the leaf through stomata during transpiration.
- Guard cells open stomata when they take in water and become turgid.
- Guard cells close stomata when they lose water and become flaccid.
- Gas exchange follows diffusion: particles move from higher concentration to lower concentration.
Vocabulary
- Stoma
- A stoma is a small pore in the leaf surface that allows gases and water vapor to move in and out of the plant.
- Guard cells
- Guard cells are paired cells around each stoma that change shape to open or close the pore.
- Transpiration
- Transpiration is the loss of water vapor from plant leaves, mostly through stomata.
- Diffusion
- Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
- Turgor pressure
- Turgor pressure is the pressure of water inside plant cells that helps them stay firm and hold their shape.
Common Mistakes to Avoid
- Saying stomata only let oxygen out is wrong because stomata also let carbon dioxide in and water vapor out.
- Thinking guard cells actively pull the pore open like muscles is wrong because the opening is caused by changes in water uptake and turgor pressure.
- Assuming stomata should always stay open is wrong because open stomata increase photosynthesis but also increase water loss.
- Confusing transpiration with photosynthesis is wrong because transpiration is water vapor loss, while photosynthesis is sugar production using light, carbon dioxide, and water.
Practice Questions
- 1 A leaf has 180 stomata in a microscope field of view covering 0.30 mm2. Calculate the stomatal density in stomata per mm2.
- 2 During one hour, a plant loses 2.4 g of water through transpiration. If 80 percent of this loss is through stomata, how many grams of water are lost through stomata in that hour?
- 3 On a hot, dry afternoon, many plants close their stomata partly. Explain how this helps the plant survive and how it affects photosynthesis.