AQA biology AS topic: Exchange



Exchange between organisms and their environments

  • External environment is different from internal environment within organism and within cells 
  • To survive, organisms transfer materials between the two environments 
  • Transfer takes place at exchange surfaces = involves crossing cell plasma membranes 
  • Environment around cells of multicellular organisms = tissue fluid 
  • Majority of cells are too far from exchange surfaces for diffusion alone to supply or remove their tissue fluid with various materials needed to keep its composition constant 
  • Once absorbed, materials are rapidly distributed to tissue fluid snd waste products returned to exchange surface for removal 
  • Involves mass transport system = maintains diffusion gradients that bring materials to and from cell-surface membranes 
  • Size and metabolic rate of organism will affect amount of each material that is  exchanged 
  • Organisms with high metabolic rate exchange more materials and so require a larger surface area to volume ratio
  • Reflected in type of exchange surface and transport system that evolved to meet the requirements of each organism
  • Things that need to interchanged between organism and its environment 
    • Respiratory gases = oxygen and carbon dioxide
    • Nutrients = glucose, fatty acids, amino acids, vitamins, minerals
    • Excretory products = urea and carbon dioxide 
    • Heat 
  • Except for heat, exchanges can take place in two ways: 
    • Passively = no metabolic energy required, by diffusion and osmosis 
    • Actively = metabolic energy required by active transport 

Surface area to Volume ratio

  • Exchange takes place at surface of an organism, but materials absorbed are used by the cells that mostly make up its volume. 
  • For exchange to be effective, the exchange surface of the organism must be large compared with its volume 
  • Small organism have surface area that is large enough compared with their volume = allows efficient exchange across their body surface
  • As organisms become larger, their volume increases at a faster rate then their surface area
  • Because of this diffusion of substances across the outer surface can only meet the needs of relatively inactive organisms. 
  • Even if the outer surface could supply enough of a substance it would take too long for it to reach the middle of the organism if diffusion was method of transport
  • Organism have evolved one or more of the following features: 
    • Flattened shape so that no cell is ever far from the surface = flatworm or leaf 
    • Specialised exchange surfaces with large areas to increase surface area to volume ratio = lungs in mammals, gills in fish

Features of specialised exchange surfaces

  • For effective transfer of materials across specialised exchange surfaces by diffusion or active transport, surfaces show following characteristics:
    • Large surface area relative to the volume of the organism which increases the rate of exchange
    • Very thin = diffusion distance is short = materials cross the exchange surface rapidly
    • Selectively permeable = allow selected material to cross
    • Movement of the environmental medium = e.g. air to maintain diffusion gradient
    • A transport system = ensure the movement of internal medium = e.g. blood in order to maintain a diffusion gradient
    • diffusion = surface area x difference


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