Unit 3 - Exchange Surface and Diffusion
Getting essential materials to where they are needed in an organism requires specialised tissues and organs. In animals, the transport is taken care of by blood. In plants it is the sap, which is a mixture of water and nutrients. But when the materials arrive at the place they are required, how do they move out of the blood or the sap? The answer is by diffusion through exchange surfaces.
An exchange surface is the place that substances are moved across membranes in an organism, for example the lungs of birds or mammals and the gills of fish. An example of exchange of materials is the gaseous exchange of oxygen and carbon dioxide between the organism and the air. Other exchange surfaces include the villi in the small intestine, root hair cells and leaves of plants.
In plants, the main exchange surfaces are the leaves and roots. Leaves have evolved to allow the efficient gaseous exchange of oxygen and carbon dioxide with the air during photosynthesis. Molecules of carbon dioxide enter the leaves through tiny pores called stomata. Once inside the leaf, these carbon dioxide molecules can diffuse through the cell membrane and into the cell. Oxygen is the waste product from photosynthesis and it is eliminated from the plant in the opposite direction to the carbon dioxide. A plant also needs water and mineral ions. These are absorbed from the soil by osmosis (water) and active transport (mineral ions) via specialised cells called root hair cells.
In animals, the common examples of exchange surfaces are in the lungs (where gaseous exchange takes place across the alveoli) and in the small intestine (where nutrients are absorbed into the bloodstream via the villi). As well as the factors above that make these two exchange surfaces efficient, they are supplied with very dense networks of capillaries. The capillaries have very thin walls that allow substances to diffuse easily in both directions to ensure that the material are efficiently exchanged with the blood.
For the GCSE Biology exam, you need to know the appearance of an alveolus and a villus and to be able to say how these are adapted to their function. You also need to know that an efficient exchange surface must be thin, have a steep concentration gradient and have a very large surface area. That applies to both plants and animals.
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