15'4 Gaseous exchange

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15.4 Gaseous exchange

• Essential features of surfaces over which gas
  exchange takes place. Ficks Law provides an
  effective framework for consideration of how the
  maximum rate of diffusion of respiratory gases is
  achieved
• By the body surface of a protoctistan
• In a fish gill
• In the tracheal system of insects
• In the spongy mesophyll of a leaf

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Ficks Law

• All organisms need to exchange substances such
  as____________________________
• These substances must diffuse between the
  organism and the surroundings. The rate of
  diffusion is calculated as follows
• Rate of 8 Surface area x concn
  difference
• Diffusion distance

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Surface Area and Diffusion

• As organisms get bigger their surface area/volume
  ratio gets smaller.
• Bacteria are all surface with not much inside,
  while whales are all insides without much
  surface.
• So as organisms become bigger it is more
  difficult for them to exchange materials with
  their surroundings.

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Table of ratios
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Surface area to volume ratio

• Organisms also need to exchange heat with their
  surroundings, and here large animals have an
  advantage in having a ____ surface area/volume
  ratio they lose less heat than small animals.
• Large mammals keep warm quite easily and don't
  need much insulation or heat generation. Small
  mammals and birds lose their heat very readily,
  so need a high metabolic rate in order to keep
  generating heat, as well as thick insulation.
• So large mammals can feed once every few days
  while small mammals must feed continuously.

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Comparisons

• Human lungs
• 600 million alveoli with a total area of 100m²
• each alveolus 1 cell thick
• constant ventilation replaces the air
• Fish gills
• feathery filaments with secondary lamellae
• lamellae 2 cells thick
• water pumped over gills in countercurrent to
  blood
• Leaves
• For a tree - SA of leaves 200m²  - SA of spongy
  cells inside leaves 6000m².gases diffuse
  straight into leaf cells
• wind replaces air round leaves

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Respiratory Surface

• Gas exchange takes place at a respiratory surface
  - a boundary between the external environment and
  the interior of the body.
• For unicellular organisms the respiratory
  surface is simply the cell membrane, but for
  large organisms it is part of specialised organs
  like lungs, gills or leaves.
• This name can cause problems - in Biology the
  word "respiration" means cellular respiration
  (ATP generation inside cells), however sometimes
  (such as here) it can also refer to breathing,
  which is what most non-biologists mean by it
  anyway.

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Respiratory Surface

• Gases cross the respiratory surface by diffusion,
  so from Fick's law we can predict that
  respiratory surfaces must have
• a ________ surface area  
• a ______ permeable surface
• a ______ exchange surface
• Many also have a mechanism to maximise the
  diffusion gradient by replenishing the source
  and/or sink.

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Plants

• All plant cells respire all the time, and when
  illuminated plant cells containing chloroplasts
  also photosynthesise, so plants also need to
  exchange gases.
• The main gas exchange surfaces in plants are the
  spongy mesophyll cells in the leaves. Leaves of
  course have a huge surface area, and the
  irregular-shaped, loosely-packed spongy cells
  increase the area for gas exchange still further.
  
• You are expected to know leaf structure in the
  detail shown in the diagram

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