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Unlock This CourseJust like animals, plants need to exchange gases with their environment to survive. They take in carbon dioxide for photosynthesis and oxygen for respiration, whilst getting rid of waste gases. Unlike animals though, plants don't have lungs or a circulatory system - instead they rely on a simple but brilliant process called diffusion.
Plants are constantly juggling two important processes: photosynthesis (which needs CO₂ and produces O₂) and respiration (which needs O₂ and produces CO₂). The balance between these determines which gases are moving in or out of the plant at any given time.
Key Definitions:
Most gas exchange in plants happens through the leaves, but roots also exchange gases with the soil. The leaf is perfectly designed for this job - it's thin, has a large surface area and is packed with tiny air spaces inside.
Diffusion is the driving force behind gas exchange in plants. It's like when you spray perfume in one corner of a room - eventually the smell spreads everywhere because the perfume particles move from where there's lots of them to where there's fewer.
In plants, gases move along concentration gradients. When a plant is photosynthesising during the day, it uses up CO₂ inside its leaves. This creates a low concentration of CO₂ inside compared to outside, so CO₂ diffuses in through the stomata. At the same time, photosynthesis produces O₂, creating a high concentration inside, so O₂ diffuses out.
CO₂ diffuses IN (for photosynthesis). O₂ diffuses OUT (waste from photosynthesis). Water vapour also diffuses out.
Only respiration occurs. O₂ diffuses IN (for respiration). CO₂ diffuses OUT (waste from respiration).
Less photosynthesis means less CO₂ needed and less O₂ produced. Gas exchange rates are lower.
Leaves are like biological masterpieces when it comes to gas exchange. Every part of their structure helps gases move efficiently in and out of the plant.
The leaf has several layers, each with a specific job in gas exchange. The waxy cuticle on top prevents water loss but also blocks gas exchange, which is why stomata are so important.
A single leaf can have up to 100,000 stomata! Most are found on the bottom surface of the leaf (the underside) because this reduces water loss whilst still allowing gas exchange. The spongy mesophyll layer inside the leaf is full of air spaces - these act like tiny corridors allowing gases to move around inside the leaf.
Stomata are tiny pores that can open and close like microscopic doors. They're controlled by guard cells that change shape depending on how much water they contain. When guard cells take in water, they swell up and curve, opening the stoma. When they lose water, they become less curved and the stoma closes.
Several environmental factors can speed up or slow down gas exchange in plants. Understanding these helps explain why plants behave differently in different conditions.
More light means more photosynthesis, which increases the demand for CO₂. Stomata open wider and gas exchange speeds up. In dim light, photosynthesis slows down and stomata may partially close.
Higher temperatures make particles move faster, speeding up diffusion. However, very high temperatures can cause stomata to close to prevent too much water loss, which actually reduces gas exchange.
When water is scarce, plants close their stomata to prevent water loss through transpiration. This is a survival mechanism, but it also reduces gas exchange. It's like the plant is holding its breath to conserve water!
Desert plants like cacti have adapted to dry conditions by opening their stomata only at night when it's cooler and less water will be lost. They store the CO₂ they collect at night and use it for photosynthesis during the day when their stomata are closed. This clever adaptation is called CAM photosynthesis (Crassulacean Acid Metabolism).
Whilst leaves do most of the gas exchange, other plant parts also need to breathe.
Roots need oxygen for respiration, just like any living tissue. They get this oxygen from air spaces in the soil. In waterlogged soil, roots can't get enough oxygen and may die - this is why overwatering can kill plants.
Young green stems can photosynthesise and exchange gases through their surface. Older stems develop lenticels - small pores that allow gas exchange through the bark.
Scientists can measure how fast plants exchange gases using special equipment. This helps us understand how plants respond to different conditions.
One common method uses a potometer to measure how much water a plant takes up - since most water is lost through stomata along with gas exchange, this gives us an idea of how active gas exchange is. Another method uses sensors to measure oxygen and carbon dioxide levels around leaves.
These experiments show us that gas exchange rates change throughout the day, vary between different plant species and respond to environmental changes like light, temperature and humidity.
Understanding gas exchange helps us grow healthier plants and crops. Farmers and gardeners use this knowledge to create the best conditions for their plants.
Greenhouse growers often add extra CO₂ to the air to boost photosynthesis and plant growth. They also control temperature and humidity to optimise gas exchange. Some even use fans to move air around, ensuring fresh supplies of CO₂ reach all the plants.
Gas exchange in plants is a beautiful example of how simple physical processes like diffusion can support complex life. By understanding how plants breathe, we can better appreciate the intricate balance of life on Earth and how plants contribute to the oxygen we breathe and help remove CO₂ from our atmosphere.