The Chemistry of Life
(e.g., a plasma membrane) from an area of high
-concentration of water molecules (e.g., pure water) to an
area of low concentration of water molecules (e.g., water to
which a solute such as salt or sugar has been added).
Osmosis can be demonstrated fairly simply by sep-
arating pure distilled water with a selectively perme-able
membrane (a barrier that will allow only water to pass
through it but not solutes such as salt) and adding a 3% salt
solution to the water on the other side of the membrane
(Figure 2-13). The water level on the sol-ute side will rise,
and the water on the pure water side will drop. The rise in
water on the open-ended flask tube opposes atmosphere
pressure and gravity and will eventually stop rising. At this
equilibrium level, the number of water molecules entering
the solute area equals the number of water molecules
leaving the solute area. The amount of pressure required to
stop
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osmosis is a measure of osmotic pressure. The solution
stops rising when the weight of the column equals the
osmotic pressure.
The mechanism of osmosis is simple. The salt in the
column of water in solution cannot pass through the
selectively permeable membrane. Salt is in higher
concentration in the solution. Water is in lower con-
centration in the column because salt has been added to the
water. However, the water in the beaker is pure distilled
water; there are no solutes in it. The water, which can move
through the selectively permeable membrane, causes the
observed increased height of the water column in the flask.
The water “tries” to equalize its concentration in both the
beaker and the flask. Thus, the water moves from an area of
high con-centration in the beaker through the selectively
perme-able membrane to an area of low concentration (the
salt solution in the flask).3% Salt solution Solution stops rising (^)
when weight of column (^)
equals osmotic pressure^
Selectively permeable
membrane^
Salt solution
rising
Distilled
water
Water
Figure 2- 13 A simple experiment to illustrate osmosis.
(^) ®
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