Exchange of substances in leaves

Plant leaves are organs which are the major site of photosynthesis. The diagram below shows the cross section of a leaf.

Diagram showing how a leaf is structured and what happens during photosynthesis

The waxy cuticle on both the top and bottom surface of the leaf reduces uncontrolled water loss. Palisade mesophyll cells are found towards the top of leaves. They are packed with chloroplasts to maximise photosynthesis. Below this layer, are spongy mesophyll cells. These have air spaces between them to allow gases to diffuse more easily.

Stomata

Stomata are tiny pores found mainly in the bottom of leaves. These are surrounded by guard cells which swell to close the stomata or reduce in size to open it. By opening and closing, stomata control the exchange of gases between a leaf and the air surrounding it.

Long, narrow opening flanked by guard cells with chloroplasts, surrounded by epidermal cells. Guard cells have a thick inner wall around stoma and thin outer wall where they border epidermal cells.

Oxygen and carbon dioxide move by diffusion from high to lower concentration through stomata.

Respiration and photosynthesis

It is important to remember that plant cells are respiring all of the time. Plant cells with chloroplasts also photosynthesise in the light.

ProcessDarkLight
RespirationYesYes
PhotosynthesisYesNo

Aerobic respiration requires oxygen and photosynthesis requires carbon dioxide.

ProcessCarbon dioxideOxygen
RespirationProductReactant
PhotosynthesisReactantProduct

The diffusion of gases into and out of leaves depends upon the time of day. If there is enough light during the day, then the rate of photosynthesis is higher than the rate of respiration, so there is an overall release of oxygen and uptake of carbon dioxide. This means oxygen diffuses from the leaves and carbon dioxide diffuses into them, as the guard cells will open the stomata.

In the dark only respiration occurs, so there is an overall intake of oxygen and release of carbon dioxide. This means oxygen tends to diffuse into the leaves and carbon dioxide diffuses out from them. However, in these conditions the guard cells will reduce the size of stomata, to reduce water loss, and so movement of gases into and out of the leaves will be reduced.

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