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.
Understanding Biology: Stomata and Gas Exchange in Plants
A leaf is built to give gases a large internal exchange surface. Beneath the outer skin are loosely packed spongy mesophyll cells. The gaps between these cells form air spaces that connect to the stomatal pore.
Carbon dioxide reaching a pore can spread through these spaces until it reaches wet cell walls. It then dissolves in a thin film of water before entering cells. This route matters because chloroplasts are inside cells, not exposed directly to the outside air.
A waxy cuticle covers much of the leaf surface and slows water loss. Stomata provide controlled gaps through that protective layer.
Guard cells change shape because they control their water content. When light and suitable conditions trigger ion movement into guard cells, water enters by osmosis. The cells swell, but their inner walls are thicker and less stretchy than their outer walls.
This unequal stretching causes the pair of cells to curve apart, making a gap. When ions leave, water follows and the cells straighten, narrowing the opening. This is not a simple on or off switch.
Each pore can open by a different amount. The plant can therefore adjust gas exchange gradually as conditions change.
Opening stomata has a cost. Evaporation from the moist cell walls pulls more water upward through the xylem from the roots. This upward pull helps move dissolved mineral ions to leaves, but it can become dangerous when soil is dry.
On a warm, windy day, the air removes water vapor quickly. Many plants partly close their stomata at the hottest time of day to reduce water loss.
A plant hormone called abscisic acid helps signal closure during water shortage. Wilting is a visible sign that cells have lost enough water pressure that stems and leaves can no longer stay firm.
Students should remember that plants respire all the time. Their cells use oxygen to release energy from sugars during both day and night. In bright light, photosynthesis usually happens faster than respiration, so the overall movement of gases is easy to observe.
In darkness, photosynthesis stops while respiration continues, so the overall direction changes. Stomatal behaviour also depends on light intensity, temperature, humidity, carbon dioxide level, and water supply. In practical work, a leaf coated with petroleum jelly on one surface loses less water because pores are blocked.
Comparing the upper and lower surfaces can show where most stomata are found. Many land plants have more on the lower surface, where air is often cooler and less drying.
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.