Human Physiology, 14th edition (2016)

Interactions Between Cells and the Extracellular Environment 139

solvent, not on the chemical nature of the solute molecules.

The expression for the total molality of a solution is osmolality

(Osm). Thus, the solution of 1.0 m glucose plus 1.0 m fructose

has a total molality, or osmolality, of 2.0 osmol/L (abbreviated

2.0 Osm). This osmolality is the same as that of the 360 g/L

glucose solution, which has a concentration of 2.0 m and

2.0 Osm ( fig. 6.11 ).

Unlike glucose, fructose, and sucrose, electrolytes such

as NaCl ionize when they dissolve in water. One molecule of

NaCl dissolved in water yields two ions (Na^1 and Cl^2 ); 1 mole

of NaCl ionizes to form 1 mole of Na^1 and 1 mole of Cl^2.

Thus, a 1.0 m NaCl solution has a total concentration of

2.0 Osm. The effect of this ionization on osmosis is illustrated

in fig. 6.12.

Measurement of Osmolality

Plasma and other biological fluids contain many organic

molecules and electrolytes. The osmolality of such complex

solutions can only be estimated by calculations. Fortunately,

however, there is a relatively simple method for measuring

osmolality. This method is based on the fact that the freezing

point of a solution, like its osmotic pressure, is affected by the

total concentration of the solution and not by the chemical

nature of the solute.

Because the ratio of solute to water molecules is of criti-
cal importance in osmosis, a more desirable measurement of
concentration is molality. In a 1-molal solution (abbreviated
1.0 m ), 1 mole of solute (180 grams of glucose, for example) is
dissolved in 1 kilogram of water (equal to 1 liter at 4 8 C). A 1.0 m
NaCl solution and a 1.0 m glucose solution therefore both con-
tain a mole of solute dissolved in exactly the same amount of
water ( fig. 6.10 ).

Osmolality

If 180 grams of glucose and 180 grams of fructose were dis-
solved in the same kilogram of water, the osmotic pressure of
the solution would be the same as that of a 360 g/L glucose
solution. Osmotic pressure depends on the ratio of solute to

Figure 6.10 Molar and molal solutions. The diagrams
illustrate the difference between ( a ) a one-molar (1.0 M ) and ( b ) a
one-molal (1.0 m ) glucose solution.
See the Test Your Quantitative Ability section of the Review
Activities at the end of this chapter.

180 g

1 mole of

glucose (180 g)

Scale

H 2 O

1.0 M
glucose
(a)

1.0 mole per liter

solution — one molar

180 g

1 mole of

glucose (180 g)

Scale

1.0 Kg

1.0 Kg of

H 2 O (1 liter)

1.0 m

glucose

1.0 mole per kilogram

water — one molal

(b)

1.0-liter mark

on flask

1.0-liter mark

on flask

Scale

Figure 6.11 The osmolality of a solution. The osmolality

(Osm) is equal to the sum of the molalities of each solute in the

solution. If a selectively permeable membrane separates two

solutions with equal osmolalities, no osmosis will occur.

Glucose

Glucose

Isotonic: no osmosis

Fructose

2.0 m glucose

2.0 Osm

1 m glucose

1 m fructose

2.0 Osm