Biology Unit 2 Chapter 13

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  • Biology Unit 2 Exchange and Transport
    • 1. Exchange between living organisms and their environment
      • For exchange to be efficient the surface area of the organism must be large compared to it's volume
        • Small Organisms have a surface area that is large enough compared to their volume to exchange materials over their surface
          • Larger organisms have to make adaptations to survive as volume increase at a faster rate than surface area. These adaptations include; flattening of the organism's shape so no cell is ever far from the surface and specialised exchange surfaces with large areas to increase SA:V.
      • The size and metabolic rate of an rganism affects the amount of material it needs to exchange.
      • Specialised exchange surfaces have features such as: a large SA:V to increase the rate of exchange, very thin so that the diffusion distance is short and exchange is rapid, partially permeable to allow selected materials across without obstruction,, movement of environmental medium to maintain diffusion gradient, movement of internal medium to maintain diffusion gradient.
        • Diffusion is proportional to the surface area x difference in concentration over length of diffusion path
    • 2. Gas exchange in insects
      • Most insects are terrestrial so their main problem is water evaporating from them and becoming dehydrated. Therefore they have to make adaptations to overcome this water loss. However these adaptations conflict with gas exchange. The insect therefor has to balance these two needs,
        • To reduce water loss insects and other organisms exhibit these two features: Water proof coverings over their body surface. Small SA:V to minimise the area over which water is lost. These features mean an insect cannot use its body surface to diffuse gases.
      • Insects respire with an internal network of tubes called tracheae. The tracheae are strengthened byr rings to stop them collapsing. The tracheae divide into smaller tubes called tracheoles. The tracheoles extend throughout all the body tissues and exchange all the gas within the insect.
      • Respiratory gases move in and out of the tracheal system in two ways: Along a diffusion gradient because when cells repsire they convvert oxygen into carbon dioxide so the carbon dioxide diffuses out and oxygen diffuses in, and ventilation as the movement of muscles in insects speeds up the movements of air in and out the tracheae.
      • Gas enter and leave the tracheae through tiny pores called spiracles. The spircales are opened and closed by a valve. when the spiracles open, water evaporates from the insect therefore the spiracles spend most of the time closed.
      • The tracheal system is an efficient method of gas exchange however it has limitations such as relying on diffusion. For diffusion the pathway must be short so this limits the size insects can attain not that this hindered them in becoming a successful group of organisms.


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