AQA GCSE Biology

Revision Notes

1.3.3 Diffusion in Multicellular Organisms

Diffusion in Multicellular Organisms

  • 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

Cell Adaptations for Diffusion, IGCSE & GCSE Biology revision notes

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

Adaptations for Exchange in Animals

The small intestine

The highly folded surface of the small intestine increases its surface area, IGCSE & GCSE Biology revision notes

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, IGCSE & GCSE Biology revision notes

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)

The Fish Gills, downloadable IGCSE & GCSE Biology revision notes

 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

Adaptations for Exchange in Plants

Roots

Root Hair Cell, IGCSE & GCSE Biology revision notes

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, IGCSE & GCSE Biology revision notes

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

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

Exam Tip

We talk about the ‘wall of the alveoli’ or the ‘wall of the intestines’ when we describe structures that maximise exchange in the lungs and intestines. Remember that we are not talking about a ‘cell wall’ here – both of the walls above are made from animal cells and animal cells do not have cell walls.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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