Exchange and Transport
Organisms need to transfer materials between themselves and their environment to ensure survival. Materials, once absorbed, must be rapidly distributed to the cells that require them and the waste products returned to the exchange surface for removal; this requires a transport system.
The size and metabolic rate of an organism will affect the amount of each material that needs to be exchanges, and it will also influence the type of exchange surface and transport system that has evolved to meet the requirements of each organism.
Exchange between Organisms and their Environment
- Examples of things which need to be interchanged between an organism and its environment include:
- Respiratory gases - oxygen and carbon dioxide.
- Nutrients - amino acids, glucose, fatty acids, vitamins and minerals.
- Excretory products - urea and carbon dioxide.
- This exchange can take place in two ways:
- Passively by diffusion and osmosis.
- Actively by active transport.
Surface Area to Volume Ratio
- Exchange takes place at the surface of an organism, but the materials absorbed are used by the cells the mostly make up its volume.
- For exchange to be effective, the organism must have a large surface area compared to its volume.
- Small organisms have a surface area that is large enough, compared to their volume, to all efficient exchange across their body surface, with no use of specialised transport systems, e.g. single-celled algae.
- As organisms become larger, their volume increases at a faster rate that their surface area; because of this, simple diffusion of materials can only meet the needs of relatevelt inactive organisms.
- Even if the surface could supply enough materials, it would take too long to reach its destination in the middle of the organism.
- To overcome this, organisms have evolved one or more of the following features:
- A flattened shape so that no cell is ever far from the surface and so simple diffusion for the exchange of materials is used, e.g. a flatworm.
- Specialised exchange surfaces with large areas to increase the surface area to volume ratio, e.g. lungs in mammals and gills in fish.
Features of Specialised Exchange Surfaces:
Exchange surfaces have the following characterists to ensure effective transer of materials across them by diffusion or active transport:
- A large surface area to volume ratio to increase the rate of exchange.
- Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly.
- Partially permeable to allow selected materials to cross without obstruction.
- Movement of environmental medium, e.g. air, to maintain a diffusion gradient.
- Movement of internal medium, e.g. blood, to maintain a diffusion gradient.
Diffusion is proportional to surface area x the difference in concentration, divided by the length of the diffusion path.
- Being thing, specialised exchanged surfaces are easily damaged and so are located inside an organism.
- Due to it being inside the organism a means of moving the external medium over the surface is needed, for example, a means of ventilating the lungs in mammals.
Gas Exchange in Single-Celled Organisms