The diffusion of water
across a differentially permeable membrane is called osmosis (GK. Osmos = a
pushing). Osmosis constantly occurs in living organisms. For example due to
osmosis (a) water is absorbed from the human large intestine (b) water is
retained by the kidneys, and is taken by the blood.
Tonicity refers to the
solution in relationship to osmosis. In isotonic solution, the solute
concentration is the same on both sides of the membrane, and therefore there is
no net gain or loss of water. The prefix also means the same as and the term
tonicity refers to the strength of the solution. A solution of 0.09% of the
salt or sodium chloride is known to be isotonic to red blood cells, because the
cells neither shrink nor swell, when placed in this solution.
Solutions that cause
cells to swell, or even burst, due to intake of water are said to be hypotonic
solution. The prefix hypo means less than and refers to a solution with a lower
power percentage of solute (more water) than the cell.
Solutions that cause
cells to shrink or shrivel due to the loss of water are said to be hypertonic
solutions. The prefix hyper means more than and refers to a solution with a
higher percentage of solute (less water) than the cell.
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