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

• Large, multicellular organisms like humans have relatively small surface areas (SA) in comparison to their volumes (in comparison to single-celled organisms)
• This means that the distance between the surface of the organism to its centre is relatively large
• This is why larger organisms usually have exchange surfaces and transport systems; as diffusion, osmosis and active transport cannot happen sufficiently to meet a larger organism’s needs otherwise

Many cells which are adapted for diffusion have increased surface area in some way – eg root hair cells in plants and cells lining the ileum in animals

The small intestine

The highly folded surface of the small intestine increases its surface area

• Role in the body:
  • Most absorption of digested food molecules into the bloodstream occurs across the wall of the small intestine.

• How it’s adapted to its role:
  • It has a highly folded surface which is lined with specialised intestinal epithelial cells (which themselves have a highly folded cell membrane) – this increases SA
  • 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 lungs (mammals)

The alveolus is the gas exchange surface in humans 

• Role in the body:
  • Gas exchange between air in the alveoli and the blood, to supply cells with oxygen for aerobic respiration and to remove carbon dioxide

• How it’s adapted to its role:
  • Millions of alveoli (singular: alveolus) which collectively provide a huge surface area - this increases SA
  • The wall of each alveolus is one cell thick, with a moist lining and excellent blood supply - this maintains a concentration gradient

The gills (fish)

 Gas exchange in fish gills

• Role in the body:
  • Gas exchange between water flowing through the gills and the blood, to supply cells with oxygen for aerobic respiration and to remove the waste product carbon dioxide

• How it’s adapted to its role:
  • Each gill is made from lots of smaller plates called filaments, which themselves are covered in projections called lamellae – this increases SA
  • Dense capillary network ensures a good blood supply which flows in the opposite direction to water passing through the gills – this maintains a concentration gradient

Roots

The cross-section of a root hair cell

• Role in the plant:
  • To absorb water and mineral ions (such as magnesium and nitrate ions) from the soil and anchor the plant

• How it’s adapted to its role:
  • The root network is highly branched - this increases SA
  • The surface of the roots are covered in root hair cells, which have a specialised structure with root hair projections – this increases SA

Leaves

How photosynthesising cells obtain carbon dioxide

• 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:
  • Stomata are tiny openings that predominantly cover the lower side of the leaf, allowing air to circulate inside the leaf – this decreases diffusion distance for carbon dioxide and oxygen
  • The lower layer of the leaf is made from spongy mesophyll cells which allow air to circulate inside the leaf

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