Features of Exchange Surfaces (in biology)

Significance of transport and exchange surfaces:

  • Transport
    Organisms need to be able to move materials (such as respiratory gases, nutrients, waste products and heat) both into and out of, and within, themselves - that is transport.
    A transport system (in biology) is a means by which materials are moved ('transported') from an exchange surface or exchange surfaces to cells throughout the organism.
  • Exchange Surfaces
    All living things must breathe (perform respiration) so movement of respiratory gases, e.g. accepting oxygen into the organism, is an important part of transport in biology. Although many microorganisms accept oxygen directly through their cell membrane, organisms whose surface-area:volume ratio is larger, e.g. mammals, need specialized exchange surfaces through which to receive oxygen and release carbon dioxide. Such exchange surfaces are often part of organs in specific parts of the organism's body, e.g. within the lungs of birds and mammals and within the gills of fish.

Specialized exchange surfaces allow the efficient transfer of material e.g. respiratory gases, across them via mechanisms such as diffusion or active transport.

See types of movement across membranes to re-cap transport mechanisms simple diffusion, facilitated diffusion, osmosis, active transport and bulk transport.

The following aspects of exchange surfaces enhance the effectiveness with which materials pass across the surface:

  1. Large surface-area to volume ratio
    increases the rate of exchange according to the relationship stated below*.
  2. Very thin surface (membrane) so that the diffusion distance is short, which increases the rate of exchange according to the relationship stated below*.

    * Relationship between factors pertaining to diffusion:
  3. Partially permeable surface (membrane)
    which means that only particles of specific substances ('materials') are able to pass across the exchange surface.
  4. Permits movement of the environmental medium
    (e.g. air, in the case of the exchange surface of mammilian lungs), which is necessary in order to maintain a diffusion gradient such particles from the environment, e.g. molecules of oxygen, diffuse across the exchange surface.
  5. Permits movement of the internal medium
    (e.g. blood, in the case of the exchange surface of mammilian lungs), which is necessary in order to maintain a diffusion gradient such particles from the environment, e.g. molecules of carbon dioxide, diffuse across the exchange surface.

The above list of features of specialised exchange surfaces in biological organisms is similar to a list of factors that affect the rate of diffusion across exchange surfaces. As a cell membrane is an example of an exchange surface (microscopic organisms receive oxygen by diffusion across their cell membranes), this is also similar to a list of factors that affect the rate of diffusion of materials into cells.

List of factors that affect the rate of diffusion across exchange surfaces

List of factors that affect the rate of diffusion of materials into cells

  1. Surface area of exchange surface
  2. Thickness of exchange surface
  3. Permeability of exchange surface to the particular substance - the exchange surface will have different permeabilities to different substances, i.e. the permeability of an exchange surface is specific to a specific substance e.g. oxygen.
  4. The concentration gradient of the specific substance, e.g. oxygen, across the exchange surface.

  5. Temperature
  1. Surface area (of cell)
  2. Thickness of cell membrane
  3. Permeability of cell membrane to the particular substance - the membrane will have different permeabilities to different substances, i.e. the permeability of a membrane is specific to a specific substance e.g. oxygen.
  4. The concentration gradient of the specific substance, e.g. oxygen, between the inside and the outside of the cell.
  5. Temperature

See also comparison of transport systems in mammals vs flowering plants.

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