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The exchange of gases between the environment and cells occurs via the
process of diffusion. Diffusion depends on:
· The amount of surface area available for diffusion. The larger the
surface area the greater the rate of diffusion.
· The concentration gradient. An organism which respires very quickly
will have a much lower concentration of oxygen in the cells and a
higher than normal concentration of Carbon Dioxide. So the greater the
concentration gradient across the respiratory surface the quicker the
rate of diffusion.
· The thickness or length of the diffusion path. The greater the
thickness of the path the slower the rate of diffusion because the
gases will have to travel a larger distance. So the respiratory
surface must be as thin as possible.
The relation of the three points above can be summed up by Fick’s Law:
However, the amount of gas which an organism needs to exchange is
greatly proportional to its volume, nevertheless, the amount of
gaseous exchange which takes place is proportional to the surface area
over which the exchange happens.
For single celled/unicellular organisms the surface area to volume
ratio is large, however, for larger organisms the surface area to
volume ratio decreases. The larger the object gets the more
complicated it gets.
The smaller the object the larger the
surface area to volume ratio. This is where single celled organism
have an advantage. Organisms like Amoeba can exchange gases with
environment quickly and easily. However exchange surfaces need to
attain certain properties to maximise gaseous exchange. They are:
· Need to have a good blood supply- high concentration gradient
· Moist- Dissolve gases
· Large Surface Area
· Thin walls
As single celled organisms are small so they do not have to have
specialised systems like much larger organisms like mammals. Single
celled organisms like amoeba do not have to have transport systems or
specialised systems because they are so small so [IMAGE]substances do
not have to travel far to get to their desired destination.
The contractile vacuole in this amoeba is used for water regulation.
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"Exchange and Transport in Protozoa." 123HelpMe.com. 15 Jul 2019
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The excess water, which is collected in the cell, is expelled to the
outside environment that is very costly in terms of energy.
The food vacuole contains ingested food. Pseudopods are flowing
projections of cytoplasm allow movement, and enable food to be
engulfed by phagocytes.
Most single celled organisms are similar to amoeba in the sense that
they are found in moist areas. They also have similar structures which
consist of a or two nuclei, food vacuoles and contractile vacuoles.