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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Diffusion in Gas Exchange
Plants need to exchange gases with their environment just like animals do, but they do it in a completely different way! Instead of having lungs, plants exchange gases through tiny pores in their leaves called stomata. This process is vital for photosynthesis and respiration.
Key Definitions:
Plants need carbon dioxide for photosynthesis and produce oxygen as a waste product. They also need oxygen for respiration and produce carbon dioxide as a waste product. Without efficient gas exchange, plants couldn't make their food or release energy from it!
Diffusion is the movement of particles from an area of high concentration to an area of low concentration. It happens naturally without using energy. In plants, carbon dioxide diffuses into leaves and oxygen diffuses out through the stomata.
Diffusion is the key physical process that drives gas exchange in plants. It's a passive process, which means it doesn't require energy from the plant.
Inside a leaf, there are air spaces between the cells. When stomata are open, gases can diffuse in and out of these spaces. The concentration of carbon dioxide is usually higher in the air than inside the leaf (especially during the day when photosynthesis is using up CO₂), so carbon dioxide diffuses into the leaf. The concentration of oxygen is usually higher inside the leaf than in the air (because photosynthesis produces oxygen), so oxygen diffuses out of the leaf.
The difference in concentration between two areas. Gases always diffuse down their concentration gradient - from high to low concentration.
The steeper the concentration gradient, the faster diffusion occurs. This is why plants with high photosynthesis rates can take in CO₂ quickly.
Leaves have a large surface area to maximise gas exchange. The flattened shape provides a huge surface for gas diffusion to occur.
Stomata are tiny pores found mainly on the underside of leaves. Each stoma is surrounded by a pair of guard cells that control whether the pore is open or closed.
Guard cells are bean-shaped cells that change shape to open or close the stomata. When they take in water, they become turgid (swollen) and curve away from each other, opening the stoma. When they lose water, they become flaccid and the stoma closes.
Most plants open their stomata during the day to allow carbon dioxide in for photosynthesis. They close them at night when photosynthesis stops, which helps to conserve water.
A single square millimetre of a leaf can contain between 50-500 stomata! That's a lot of tiny doors for gases to move through. Some desert plants have stomata that are sunken into pits to reduce water loss while still allowing some gas exchange.
Several factors can affect how quickly gases diffuse in and out of plant leaves:
Plants have evolved to respond to their environment by controlling their stomata. This helps them balance the need for gas exchange with the need to conserve water.
Bright light usually causes stomata to open, allowing more CO₂ in for increased photosynthesis. In darkness, stomata typically close.
Higher temperatures increase the rate of diffusion as particles move faster. However, very high temperatures can cause stomata to close to prevent water loss.
When water is scarce, plants close their stomata to reduce water loss, even though this also reduces gas exchange and photosynthesis.
Leaves have several adaptations that make them excellent structures for gas exchange:
Different plants have adapted their gas exchange systems to suit their environments:
Crassulacean Acid Metabolism (CAM) plants like cacti and pineapples have a special adaptation for gas exchange in hot, dry environments. They open their stomata at night when it's cooler and close them during the day to conserve water. At night, they take in CO₂ and store it as an acid. During the day, they release the CO₂ internally for photosynthesis while keeping their stomata closed. This clever adaptation allows them to survive in deserts where water conservation is crucial.
Plants face a constant dilemma: they need to open their stomata for gas exchange, but this also leads to water loss through transpiration. Different plants have evolved different strategies to deal with this challenge.
When stomata open to let carbon dioxide in for photosynthesis, water vapour inevitably escapes through the same openings. This water loss is called transpiration. Plants must balance their need for carbon dioxide with their need to conserve water.
Diffusion is the fundamental process that allows plants to exchange gases with their environment. Without it, photosynthesis and respiration couldn't happen efficiently. Plants have evolved sophisticated structures and mechanisms to maximise gas exchange while minimising water loss, allowing them to thrive in diverse environments around the world.
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