O sm osis Typically, the term
diffusion
refers to molecules
or substances such as ions, sugars, fatty acids, gases, and oth-
er substances. The diffusion of one substance—water—has
a special name. The diffusion of water through a selective-
ly permeable membrane is called osmosis (oz-MO-sis).
Osmosis follows the same rules as other types of diffusion;
it requires a membrane permeable to water and a concen-
tration gradient.
What exactly is a concentration gradient for water?
Think of it this way: Pure water is the most concentrated
that you can get. When you dissolve a substance in water,
you are actually diluting the concentration of water in
the solution. So a concentrated salt solution has a lower
water concentration than a dilute salt solution. Here’s
an example of an osmosis problem: You have a dialysis
bag, which is a selectively permeable membrane tied at
both ends. Inside the bag is a concentrated sugar solu-
tion. You place the bag in a beaker with a dilute sugar
solution. To understand what will happen, ask yourself
these questions:
Is the membrane permeable to water? If the answer is
no, the question of osmosis is irrelevant. In this case,
the dialysis tubing is permeable to water.
Which side of the membrane has the higher water
concentration? The side with the lower solute (sugar)
concentration, which is outside the bag.
Which side of the membrane has the lower water
concentration? The side with the higher solute (sugar)
concentration, which is inside the bag.
Which way will water flow? Water will always move from
the area of higher water concentration (lower solute
concentration) to lower water concentration (higher
solute concentration). So water will move from the
outside of the bag (lower sugar concentration) to the
inside of the bag (higher sugar concentration) (
Figure
3.13
) . The bag will swell as water flows into it. However,
as noted in Figure 3.13, other forces are at work to
counter the effects of osmosis, including hydrostatic
pressure (the pressure of the water on the bag).
Osmosis • Figure 3.13
Water moves through the selectively permeable membrane by
osmosis until equilibrium is reached.
G lass tube
Rubber
stopper
Sucrose
molecules
Selectively
permeable
membrane
Water
molecules
a.
At start of experiment
b. At equilibrium
Question
a. At start
b. At equilibrium
Is the m em brane
perm eable to
water?
Yes
Yes
W here is the water
concentration
higher?
Outside
m em brane (No
sucrose)
Outside m em brane
W here is the water
concentration
lower?
Inside m em -
brane (high
sucrose concen-
tration)
Inside m em brane
W hich w ay w ill
there be net m ove-
ment of water?
Outside to inside
(higher water
concentration
to lower water
concentration)
None. W ater flow ing
into the m em brane
by osm osis w ill
build up an outward
pressure (hydrostatic
pressure). Th is pres-
sure forces water
m olecules back out
of the m em brane
despite the concen-
tration gradient.
From the start of the experim ent, water w ill flow through the
m em brane and the level of water in the tube w ill rise.
At equilibrium , water pressure w ill drive som e water m ole-
cules and there w ill be a balance between the force of osm osis
driving water into the m em brane and the force of water pres-
sure inside, w hich d rives water out of the m em brane.
60 CHAPTER 3
Cells and Tissues
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