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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Stomata Function
Plants need to exchange gases with their environment, just like we do when we breathe. However, plants don't have lungs! Instead, they have tiny pores called stomata (singular: stoma) on the surfaces of their leaves. These microscopic openings are crucial for plant survival as they allow carbon dioxide in for photosynthesis and let oxygen and water vapour out.
Key Definitions:
Each stoma is surrounded by two guard cells that are bean-shaped. Unlike regular plant cells, guard cells have an unusual thickened inner wall. When these cells fill with water, they curve outward, creating a gap (the stoma) between them. When they lose water, they straighten and close the gap. This simple but effective mechanism controls gas exchange in plants.
Most plants have more stomata on the lower surface of their leaves than the upper surface. This is an adaptation that reduces water loss, as the lower surface is generally cooler and less exposed to direct sunlight. Some plants that grow in very dry conditions may have stomata in pits or grooves to further reduce water loss.
Stomata are like the gatekeepers of plants, controlling what goes in and out. They need to balance two crucial but competing needs: letting in carbon dioxide for photosynthesis while preventing excessive water loss through transpiration.
The opening and closing of stomata is controlled by changes in the turgidity (water content) of the guard cells. This clever mechanism works as follows:
When light is available for photosynthesis, guard cells actively pump potassium ions (K+) into themselves. This creates a lower water potential inside the cells, causing water to move in by osmosis. As guard cells fill with water, they become turgid and curve outward, opening the stoma.
When water is scarce or photosynthesis isn't occurring (like at night), potassium ions leave the guard cells. This increases their water potential, causing water to move out by osmosis. The guard cells become flaccid (floppy), straighten out and close the stoma.
The unique structure of guard cells, with their thickened inner walls, means they can only bulge outwards when turgid. This creates the pore opening. It's a bit like how a balloon can only expand in certain directions when inflated.
Several environmental factors influence whether stomata open or close. Plants have evolved to respond to these factors to maximise photosynthesis while minimising water loss.
Light is perhaps the most important factor. Stomata typically open in the light and close in the dark. This makes sense because photosynthesis only occurs in the light, so that's when plants need carbon dioxide. Blue light in particular triggers stomatal opening.
When water is scarce, plants can't afford to lose it through transpiration. In drought conditions, a plant hormone called abscisic acid (ABA) signals guard cells to close stomata, helping the plant conserve water even if it means reducing photosynthesis.
Very high temperatures can cause stomata to close to prevent excessive water loss. However, this creates a dilemma for plants in hot conditions: they need to cool down (which transpiration helps with) but also need to conserve water.
When CO₂ levels inside the leaf drop (because it's being used up in photosynthesis), stomata tend to open wider to let more in. Conversely, if CO₂ levels rise in the atmosphere, plants can partially close their stomata while maintaining the same rate of photosynthesis.
Stomata may be tiny, but they play several crucial roles in plant survival and in global ecological processes:
Plants face a fundamental dilemma: they need to open stomata to get CO₂ for photosynthesis, but this inevitably leads to water loss through the same openings. This is sometimes called the "CO₂-H₂O trade-off" and represents one of the most important compromises in plant biology.
Stomata allow carbon dioxide to enter the leaf, which is essential for photosynthesis. Without this gas exchange, plants couldn't make their own food and would die.
The water vapour that exits through stomata creates a "transpiration stream" that pulls water up from the roots. This helps transport minerals throughout the plant and cools the leaf on hot days.
Collectively, plant stomata release enormous amounts of water vapour into the atmosphere, influencing local and global weather patterns. They also play a key role in the carbon cycle by absorbing CO₂.
Plants have evolved different stomatal adaptations based on their environments:
Understanding stomatal function has important applications in agriculture and environmental science:
Scientists are developing crop varieties with stomata that respond more efficiently to environmental conditions. For example, rice varieties with stomata that close more quickly in drought conditions can survive with less water, helping farmers in regions affected by climate change.
As CO₂ levels rise in our atmosphere due to human activities, researchers are studying how this affects stomatal function. Many plants are showing reduced stomatal density (fewer stomata per leaf area) in response to higher CO₂, which could affect global water cycles.
Stomata are a perfect example of how a seemingly simple structure can perform complex and vital functions. These microscopic pores allow plants to:
The next time you look at a plant, remember that its leaves are covered with thousands of these tiny but sophisticated structures, constantly adjusting to help the plant survive and thrive in its environment.
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