Exchange
- Created by: parmar123
- Created on: 09-04-17 10:23
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- Exchange
- surface area to volume ratio
- surface area to volume ratio gets smaller as an object gets larger
- organisms have evolved one or more of these features:
- a flattened shape so that no cell is ever far from the surface
- specialised exchange surfaces with large areas to increase the surface area to volume ratio
- calculating the surface area to volume ratio of cells with different shapes
- surface area of a sphere: 4x PI x r (4PIr). the volume is calculated using: 4/3 PI r3 (four thirds x PI x radius cubed. surface area to volume ration = surface area divided by volume
- features of specialised exchange surfaces
- A large surface area relative to the volume of the organism which increases the rate of exchange
- very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly
- selectively permeable to allow selected materials to cross
- movement of the environmental medium e.g air to maintain a diffusion gradient
- a transport system to ensure the movement of the internal medium, eg blood, in order to maintain a diffususion gradient
- the relationship between these factors can be expressed as : diffusion is equal to surface area X difference in concentration divided by length of diffusion pathway
- gas exchange in insects
- insects have evolved an internal network of trachea. the trachea are supported by strengthening rings to prevent them from collapsing. the trachea divide into smaller dead- end tubes - tracheoles. the tracheoles extend throughout the body tissues of the insect. in this way, atmospheric air, with the oxygen it contains, it brought directly to the respiring tissues, as there is a a short diffusion pathway
- respititory gases move in and out of the tracheal system in 3 ways
- along a diffusion gradient: when cells are respiring, oxygen is used up and so its concerntration towards the ends of the trachioles falls. this creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the tracheae and tracheoles to the cells. carbon dioxide is produced by cells during repiration, this creates a diffusion radient in the opposite direction. this causes carbon dioxide to diffuse along the diffusion gradient to the atmosphere
- mass transport: the contraction of muscles in insects can squeeze the trahea enabling mass movements of air in and out. this further speeds up the exchange of respiratory gases
- the ends of the tracheoles are filled with water: during periods of major activity, the muscle cells around the tracheole respire anaerobically. this produces lactae, which is soluble and lowers the water potential of the muscle cells. water therefore moves into the cells by osmosis. the water in the ends of the tracheoles decreases in volume and so draws air further into them. this means the final diffusion pathway is in a gas rather than liquid phase, so diffusion is more rapid
- gases enter and leave trachea through tiny pores, spiracles. the spiracles may be opened and closed by a valve. When the spiracles are open, water vapour can evaporate from the insect
- limitations: the tracheal system relies mostly on diffusion to exchange gases between the environment and the cells. for diffusion to be effective, the diffusion pathway needs to be short which is why insects are small. as a result the length of thw diffusion pathway limits the size that insects can be.
- surface area to volume ratio
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