Topic 6 - Exchange Surfaces

• The environment around the cells of a multicellular organism is called tissue fluid.
• Once absorbed, materials are rapidly distributed to the tissue fluid and the waste products are returned to the exchange system for removal. This involves is the mass transport system that maintains the diffusion gradient that brings materials to and from the cell-surface membranes.
• The size and metabolic rate of an organism will affect the amount of each material that is exchanged.
• Examples of exchange substances between organisms and the environment include respiratory gases, nutrients, excretory products and heat.
• Substance exchange (apart from heat) can occur through active and passive transport.
• Passive exchange, such as diffusion and osmosis, occurs without the need for metabolic (ATP) energy.
• Active exchange, such as active transport, depends on metabolic (ATP) energy.
• Small organisms have a surface area that is large enough, compared with their volume to allow efficient exchange across their entire body. surface.
• As organisms become larger, their volume increases at a faster rate than their surface area. Therefore, simple diffusion of substances across their outer surface can only meet the needs of relatively inactive organisms.
• Relatively active organisms have adapted to possess certain features to enable efficient exchange surfaces. These can include a flattened shape, so that all cells are close to an exchange surface (e.g. a flatworm), or having specialised exchange surfaces to increase SA:V ratio (e.g. lungs in mammals).
• Specialised exchange surfaces may have the following features: large surface area to volume ratio (to increase rate of exchange), thin (to provide short diffusion distance), selectively permeable (to only permit certain materials to cross), and movement of an environmental medium (e.g. air, to maintain a concentration gradient).