Gas Exchange in the leaf of a Plant

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- All plant cells take in oxygen and produce carbon dioxide during RESPIRATION. Plants are different to animals as they carry out photosynthesis as well as respiration, where they take in CO2 and give out O2.

- There are times when the gases produced in one process can be used in another process, which reduces the need for gas exchange with the external air. This means that the volumes and the types of gases that are exchanged in a leaf constantly change. This depends on the balance between the rates of photosynthesis and respiration.

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Balance of Respiration and Photosynthesis

- When photosynthesis is taking place, although some carbon dioxide comes from respiration of cells, most of it has to be obtained from the external air. Similarly, the oxygen from photosynthesis is used in respiration but most of it DIFFUSES out of the plant.

- When photosynthesis is not occuring, e.g. in the dark, oxygen diffuses in to the leaf because it is constantly being used by cells during respiration. Similarly, carbon dioxide produced during respiration diffuses out.

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Structure of a Plant Leaf and Gas Exchange

Gas exchange in plants is slightly similar to that of insects.
- No living cell is very far away from the externail air, and so is not very far away from a source of carbon dioxide and oxygen.
-Diffusion takes place in the gas phase (AIR), which makes it more rapid than if it were in water.
THEREFORE, there is a very short diffusion pathway. Also, a plant leaf has a very large surface area to volume ratio. For these reasons, there isn't a specialised transport system needed for gases, which just move in and out of the plant by diffusion. Most gaseous exchange occurs in the leaves, which are adapted for rapid diffusion because:
- They are a thin, flat shape, that provides a large surface area
- Many 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?
They are minute pores which occur mainly, but not just, on the leaves especially underneath. Each stoma is surrounded by a special pair of cells called GUARD CELLS. These cells can open and close the stomatal pore. In this way they can control the rate of gaseous exchange. This is important because terrestrial organisms lose water by evaporation. Plants have to balance the conflicting needs of gas exchange and control of water loss. They do this by completely or partly closing the stomata at times when water loss would be excessive.

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