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HOW SUBSTANCES GET INTO AND OUT OF CELLS
Molecules of gases, liquids and substances that are dissolved in liquids (e.g. sugars
dissolved in water) are free to move around. As a result of this movement, the molecules
spread themselves evenly to fill all the available space.
This process is called DIFFUSION. The effect of
diffusion is that the molecules will move from a region where they are concentrated to a
region where they are less concentrated, until the concentration is equal in both regions.
The effect of the
concentration inside and outside of a cell (as in the first image) is to make the molecules
diffuse into the cell until the concentration is equal (as in the second image). Whether this
happens or not depends on whether the cell membrane will allow the molecules.
When a cell uses up oxygen for respiration, the oxygen level in the cell falls, and more oxygen
diffuses into the cell. Carbon Dioxide is released in respiration and so the carbon dioxide
levels in the cell increase. Then diffusion takes place and the carbon dioxide diffuses out of
the cell until the levels are equal both inside and out of the cell.
Rates of Diffusion
The speed with which a substance diffuses through a cell wall will depend on temperature,
pressure and many other conditions including the distance it has to diffuse, its concentration
inside and outside the cell and the size of its molecules or ions.
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Plant cell walls vary in their thickness and permeability but generally the thicker the cell wall
the slower the rate of diffusion.
The bigger the difference in concentration of a substance on either side of a membrane, the
faster it will tend to diffuse. The difference is called a DIFFUSION GRADIENT or
CONCENTRATION GRADIENT. If a substance on one side of the membrane is steadily
removed then the concentration gradient is maintained.
In general the larger the molecules or ions the slower they diffuse.…read more