AQA biology AS topic: Exchange

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…