These revision notes provide an in depth summary of this specific chapter of AQA Biology in the A Level Specification -
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Biology revision – Organisms exchange substances with their environment (Unit 6)
Exchange between Organisms and their environment
- To survive, organisms transfer materials between within the organism and outside
- Takes place at exchange surfaces and involves crossing cell plasma membranes
- Environment around cells of multicellular organisms is called tissue fluid
- Majority of cells are too far from exchange surfaces for diffusion alone to supply or
remove their tissue fluid with the various materials needed to keep its composition
relatively constant
- Once absorbed, materials are rapidly distributed to the tissue fluid and the waste
products returned to the exchange surface for removal
- This involves a mass transport system and its this that maintains the diffusion
gradients that bring materials to and from the cell-surface membranes
- Size and metabolic rate of organism affect amount of each material that’s exchanged
- Those with higher metabolic rate exchange more materials and so need a large SA:V
ratio
- Examples of things that are interchanged between organism and environment are:
o Respiratory gases – O2 and CO2
o Nutrients – Glucose, fatty acids, aminos, vitamins and minerals
o Excretory products – Urea and CO2
o Heat
- Except for heat these exchanges happen in two ways
o Passively – No metabolic energy needed, happens by diffusion and osmosis
o Actively – Metabolic energy needed, happens by active transport
SA:V ratio
- Exchange occurs at surface of an organism but materials absorbed are used by the
cells that mostly make up its volume
- For exchange to be effective, exchange surfaces of organism must be large compared
with its volume
- Small organisms have a SA large enough, compared to volume, to allow efficient gas
exchange across their body surface
- As organisms become larger, volume increases at faster rate than SA – simple
diffusion of substances across outer surface only meet needs of relatively inactive
organisms
- If outer surface could supply enough, it would still take a long time to reach middle of
organism if diffusion alone was the method of transport
- Organisms have adapted in the following ways:
o Flattened shape so no cell is far from surface – e.g. a leaf
o Specialise exchange surfaces with large SA to increase SA:V ratio – e.g. lungs
Features of specialise exchange surfaces
- Allow effective transfer of material across specialised exchange surfaces by diffusion
or active transport – show following characteristics:
o Large SA:V ratio increasing the rate of exchange
o Thin so diffusion distance is short – rapid exchange across the surface
o Selectively permeable to allow selected materials to cross
o Movement of environmental medium, e.g. air, to maintain a diffusion gradient
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