70
UNIT 1
Organization of the Body
3
the membrane. If the solute concentration on the two sides of the
membrane differs, water concentration differs as well (as solute
concentration increases, water concentration decreases).
Te extent to which solutes decrease water’s concentration
depends on the
number
—not the
type
—of solute particles, be-
cause one molecule or one ion of solute (theoretically) displaces
one water molecule. Te total concentration of all solute par-
ticles in a solution is referred to as the solution’s
osmolarity
(oz
0
mo-lar
9
ĭ-te). When equal volumes of aqueous solutions of
different osmolarity are separated by a membrane that is
perme-
able to all molecules
in the system, net diffusion of both solute
Water also moves freely and reversibly through water-
specific channels constructed by transmembrane proteins
called
aquaporins (AQPs)
, which allow single-file diffusion
of water molecules. Although water-filled aquaporin channels
are believed to be present in all cell types, they are particularly
abundant in red blood cells and in cells involved in water bal-
ance such as kidney tubule cells.
Osmosis occurs whenever the water concentration differs on
the two sides of a membrane. If distilled water is present on both
sides of a selectively permeable membrane, no
net
osmosis occurs,
even though water molecules move in both directions through
(a)
Membrane permeable to both solutes and water
(b)
Membrane permeable to water, impermeable to solutes
Solute and water molecules move down their concentration gradients
in opposite directions. Fluid volume remains the same in both compartments.
Left
compartment:
Solution with
lower osmolarity
Right
compartment:
Solution with
greater osmolarity
Freely
permeable
membrane
H
2
O
Solute
Solute
molecules
(sugar)
Both solutions have the
same osmolarity: volume
unchanged
Both solutions have identical
osmolarity, but volume of the
solution on the right is greater
because only water is
free to move
Solute molecules are prevented from moving but water moves by osmosis.
Volume increases in the compartment with the higher osmolarity.
Left
compartment
Right
compartment
Selectively
permeable
membrane
Solute
molecules
(sugar)
H
2
O
Figure 3.8
Influence of membrane permeability on diffusion and osmosis.
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