General Principles for Efficient Gas Exchange

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General Principles for Efficient Gas Exchange

Different organisms have different mechanisms for obtaining the gases they require.

Diffusion is required to supply all organisms with oxygen.

The efficiency of diffusion is increased if there is:

  1. A large surface area over which exchange can take place.
  2. A concentration gradient without which nothing will diffuse.
  3. A thin surface across which gases diffuse.

There are other factors involved, e.g. where appropriate, a good blood supply to carry the oxygen away to the tissues and to maintain a steep concentration gradient.

The process of diffusion will be faster in warm conditions as molecules have more energy and therefore move faster. So temperature will also affect the efficiency of the diffusion.

Fick's law is used to measure the rate of diffusion.

Fick's Law states that:

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(The symbol a means 'proportional to')

The larger the area and difference in concentration and the thinner the surface, the quicker the rate.

This means that water, a small molecule at a higher concentration inside the body than outside, will continually diffuse out of the animal and evaporate away. This makes the diffusion surface moist.

Unicellular Organisms do not have specialised gas exchange surfaces. Instead gases diffuse in through the cell membrane.

The smaller something is, the smaller the surface area is but, more importantly, the bigger the surface area is compared to its volume.

In other words, unicellular organisms have a large surface area to volume ratio. They are therefore efficient when it comes to exchanging gases through their membrane.

Also, all parts of the organism are supplied with oxygen because the diffusion path is short.

Multicellular Organisms are bigger than Unicellular organisms. This makes efficient diffusion of gases more difficult.

However, if they are small, or large but very thin (like the flatworms, Platyhelminths), the outer surface of the body is sufficient as an exchange surface because the surface area to volume ratio is still high.

Multicellular organisms

Larger organisms need specialised exchange surfaces e.g. gills or lungs.

Larger organisms

Land-living organisms usually have internal gas exchange surfaces to prevent too much water being lost from the body.

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