What do plants take in and produce during the day?
Take in carbon dioxide, produce oxygen
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Why is gas exchange efficient?
There is a short, fast diffusion pathway and a large surface area
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Why is gas exchange more rapid than if it were in water?
Because diffusion takes place in the gas phase
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Name 3 plant adaptations that allow rapid exchange
A thin, flat shape that provides a large surface area, small pores called stomata, mostly in the lower epidermis, numerous interconnecting air-spaces that occur throughout the mesophyll
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What are stomata?
Minute pores which mainly occur on the leaves
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Where are stomata mostly found?
On the underside of the leaf
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What surrounds the stomata?
2 guard cells
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What do guard cells do?
Control the opening and closure of the stomatal pore
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What is the function of opening and closing the stomata?
To control the diffusion of carbon dioxide into the leaf and water vapour out of it
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Why is it important to control the loss of water?
Terrestrial organisms lose water by evaporation
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How do plants balance the conflicting needs of gas exchange and control of water loss?
Completely or partially closing stomata at times when water loss would be excessive
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Name one strategy plants have to reduce water loss
The upper surface of the leaf is covered in a waterproof cuticle made of lipids secreted by the upper epidermal cells
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Name a second strategy plants have to reduce water loss
The sub-stomatal air space remains moist to reduce the water concentration gradient so less water evaporates from the spongy cells
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Name a third strategy plants have to reduce water loss
The guard cells can close the stomata to stop water loss when conditions are very dry. This also prevents gas exchange, stopping photosynthesis and respiration. So plants can't close their stomata for very long.
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Name one reason why plants don't need a ventilation mechanism?
Because their leaves are highly exposed, so the air surrounding them is constantly being replaced in all but the stillest days
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Name a second reason why plants don't need a ventilation mechanism
During the hours of daylight, photosynthesis increases the oxygen concentration in the sub-stomatal air space, and decreases the carbon dioxide concentration
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What is the function of palisade mesophyll cells?
Photosynthesis
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How are palisade mesopyhll cells adapted for photosynthesis?
They have a thin cytoplasm densely packed with chloroplasts, which can move around the cell to regions of greatest light intensity
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What is the advantage of densely packed palisade cells?
They maximise light collection
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What is the function of spongy mesophyll cells?
Gas exchange
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How are spongy mesophyll cells adapted for gas exchange?
They are loosely-packed with unusually large inter cellular air spaces where gases can collect and mix. They have fewer chloroplasts so do less photosynthesis.
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