Gas exchange in single celled organisms and insects

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Gas exchange in single celled organisms and
insects

• 21/11/10

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By the end of this session you should be able to

• Describe how single celled organisms exchange
  gases
• Explain how terrestrial insects balance the need
  to exchange gases with the need to conserve
  water.
• Explain how insects exchange gases

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Single celled organisms

• Because they are so small they have a very large
  surface area to volume ratio. This means that
  diffusion of gases occurs very easily. Because
  these organisms rely on diffusion for gas
  exchange their size is restricted- if they grow
  too big their surface area to volume ratio goes
  down and diffusion does not happen so easily.

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Vocabulary check

• Terrestrial- means on land
• Marine means in the sea
• Aquatic in water

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Insects

• As most insects are terrestrial they face a
  constant battle to stop them selves from drying
  out.
• Water evaporates very easily and an insect could
  become dehydrated in a few hours.

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Problem

• How can insects stop water loss?
• 2 minutes- brainstorm

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• Waterproof coverings all over their body surface-
  reduces water loss.

Layers of a material called Chitin form a
waterproof barrier on the surface of an insect
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• Small surface area to volume ratio minimises area
  on which water is lost.

Insects have very fat bodies which means that
their surface area to volume ratio is low.
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• So if the surfaces of insects bodies are
  waterproof they cannot be used for gas exchange.
• So how do insects get gases in and out of their
  bodies?

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Spiracle
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Have a go at the summary questions on page 179
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Task

• Describe how single celled organisms exchange
  gases in only 20 words
• Draw a diagram to show the problems faced by
  terrestrial insects.
• Explain how insects exchange gases in text
  language