Diffusion in Multicellular Organisms (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

Diffusion in multicellular organisms

• Multicellular organisms like humans need exchange surfaces and transport systems, as diffusion alone is too slow to meet their needs; this is because

  • they have a relatively small surface area (SA) in relation to their volume (V)

  • the distance between the surface of a multicellular organism to its centre is relatively large

Adaptations for exchange in animals

• Because diffusion alone is too slow to supply the needs of multicellular organisms, specialised structures and systems are needed for them to function

The small intestine

• Role in the body:

  • Absorption of digested food molecules into the bloodstream occurs across the wall of the small intestine

• How it’s adapted to its role:

  • The lining of the small intestine has finger-like projections called villi (singular villus); this increases surface area

  • Only one layer of epithelial cells covers the surface of each villus - this decreases diffusion distance

  • Each villus has a good blood supply - this maintains a concentration gradient

  • The cells of the villi contain many mitochondria, which release energy by aerobic respiration for the active transport of glucose

  • The small intestine is very long, which gives more time for absorption

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Mammalian lungs

• Role in the body:

  • Gas exchange between air in the alveoli and the blood supplies cells with oxygen for aerobic respiration and removes waste carbon dioxide

• How it’s adapted to its role:

  • Millions of alveoli (singular: alveolus) collectively provide a large surface area

  • The walls of the alveoli and the capillaries are each one cell thick - this gives a short diffusion distance

  • Each alveolus is surrounded by a capillary network (good blood supply) - this maintains a steep concentration gradient

  • The lungs are ventilated (breathing moves air in and out) - this also maintains the concentration gradient

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Fish gills

• Role in the body:

  • Gas exchange between water flowing through the gills and the blood

• How it’s adapted to its role:

  • Gills are made of many thin gill filaments, which give a large surface area

  • The walls of the gill filaments and capillaries are one cell thick, so the water is very close to the blood - this gives a short diffusion distance

  • A dense capillary network ensures a good blood supply; this maintains a concentration gradient

  • Water flows constantly over the gills, which maintains the concentration gradient

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Adaptations for exchange in plants

Roots

• Role in the plant:

  • To absorb water and mineral ions (such as magnesium and nitrate ions) from the soil

• How it’s adapted to its role:

  • The root network is highly branched, increasing surface area

  • The surface of the roots are covered in root hair cells, which have a specialised structure with root hair projections; this also increases surface area

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Leaves

• Role in the plant:

  • The leaves contain most of a plant's photosynthetic cells

  • Photosynthesis requires efficient gas exchange between air surrounding the leaf and the photosynthetic cells, whilst minimising water loss

• How it’s adapted to its role:

  • Leaves are thin, decreasing the diffusion distance for gas exchange

  • Spongy mesophyll cells are loosely packed with air spaces between them, giving a large internal surface area for gas exchange

  • Guard cells control the opening and closing of stomata, allowing gas exchange while reducing water loss in dry conditions

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Exchange surfaces

• In summary, multicellular organisms have surfaces and organ systems that maximise the exchange of materials by increasing the efficiency of exchange in a number of ways:

  • Having a large surface area to increase the rate of transport

  • A barrier that is as thin as possible to separate two regions, to provide as short a diffusion path as possible for substances to move across

• In addition, animals have:

  • a large network of blood vessels throughout the body:

    • to reduce distance of exchange of materials between cells and the bloodstream

    • to move substances towards or away from exchange surfaces to maintain concentration gradients

  • gas exchange surfaces that are well ventilated to maintain concentration gradients