Organization of the Body
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
—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
ĭ-te). When equal volumes of aqueous solutions of
diﬀerent osmolarity are separated by a membrane that is
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
, 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
even though water molecules move in both directions through
Membrane permeable to both solutes and water
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.
Both solutions have the
same osmolarity: volume
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.
Inﬂuence of membrane permeability on diffusion and osmosis.