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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Oxygen and Carbon Dioxide Exchange
Plants need to exchange gases with their environment just like animals do, but they do it in a completely different way! Instead of lungs, plants use tiny pores called stomata found mainly on the undersides of their leaves. These amazing structures allow plants to take in carbon dioxide for photosynthesis and release oxygen as a waste product.
Key Definitions:
Plants need to exchange two main gases:
While exchanging gases, plants face a tricky problem: the same openings that let gases in and out also let water vapour escape! This is called transpiration. Plants need to balance getting enough CO₂ while not losing too much water, especially on hot, dry days.
Stomata are the stars of plant gas exchange. Each stoma (singular of stomata) is made up of two kidney-shaped guard cells that can change shape to open or close the pore between them.
Guard cells are special because:
Opening: When guard cells take in water, they swell and bend outward (because of their thicker inner walls), creating an opening.
Closing: When guard cells lose water, they become flaccid and straighten, closing the pore between them.
Carbon dioxide is the raw material plants need for photosynthesis. Without it, plants couldn't make glucose, grow, or survive!
Carbon dioxide follows a specific path to reach the cells where photosynthesis happens:
The spongy mesophyll layer is particularly important for gas exchange because it has lots of air spaces that create a large surface area for gases to diffuse across.
A single square millimetre of a leaf can contain 50-100 stomata! That means a typical leaf might have millions of these tiny pores working to exchange gases.
Oxygen is produced during photosynthesis and most of it diffuses out of the leaf into the atmosphere. This is one of the most important processes on Earth as it provides the oxygen that humans and other animals need to breathe!
Oxygen follows the reverse path of carbon dioxide:
Remember that plants also use some oxygen themselves for cellular respiration, especially at night when photosynthesis isn't happening.
During daylight, photosynthesis occurs at a faster rate than respiration. Plants produce more oxygen than they use, so there is a net release of O₂ and uptake of CO₂.
At night, photosynthesis stops but respiration continues. Plants take in O₂ and release CO₂, just like animals do. However, the amounts are much smaller than during daytime photosynthesis.
Overall, plants produce far more oxygen than they consume. A mature tree can produce enough oxygen to support 2-4 people's breathing needs!
Several environmental factors affect how quickly gases move in and out of plants:
More light generally means more photosynthesis, which increases CO₂ uptake and O₂ production. Light also directly triggers stomata to open.
Warmer temperatures speed up photosynthesis (up to an optimum point), increasing gas exchange. However, very high temperatures can cause stomata to close to prevent water loss.
Gentle wind helps remove the layer of humid air around leaves, making it easier for water vapour to exit and fresh CO₂ to enter. Strong winds can cause stomata to close to prevent excessive water loss.
When water is scarce, plants close their stomata to reduce water loss. This also reduces CO₂ intake and slows photosynthesis - a necessary trade-off to prevent dehydration.
Leaves have evolved several features that make them brilliant gas exchange organs:
Plants in different environments have adapted their gas exchange systems to suit their conditions:
Plant gas exchange doesn't just matter for plants - it's vital for all life on Earth! Through photosynthesis and gas exchange, plants:
This is why protecting forests and plant life is so important for our planet's health. Every leaf is like a tiny gas exchange factory working to keep our atmosphere balanced!
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