Human Physiology, 14th edition (2016)

138 Chapter 6

Pure water has an osmotic pressure of zero, and a 360 g/L

glucose solution has twice the osmotic pressure of a 180 g/L

glucose solution ( fig. 6.9 b ).

Molarity and Molality

Glucose is a monosaccharide with a molecular weight of 180

(the sum of its atomic weights). Sucrose is a disaccharide of

glucose and fructose, which have molecular weights of 180

each. When glucose and fructose join together by dehydra-

tion synthesis to form sucrose, a molecule of water (molecular

weight  5 18) is split off. Therefore, sucrose has a molecular

weight of 342 (180  1  180  2  18). Because the molecular weights

of sucrose and glucose are in a ratio of 342/180, it follows that

342 grams of sucrose must contain the same number of mol-

ecules as 180 grams of glucose.

Notice that an amount of any compound equal to its

molecular weight in grams must contain the same number of

molecules as an amount of any other compound equal to its

molecular weight in grams. This unit of weight, a mole, always

contains 6.02  3 10 23 molecules (Avogadro’s number). One

mole of solute dissolved in water to make 1 liter of solution

is described as a one-molar solution (abbreviated 1.0 M ).

Although this unit of measurement is commonly used in chem-

istry, it is not completely desirable in discussions of osmosis

because the exact ratio of solute to water is not specified. For

example, more water is needed to make a 1.0 M NaCl solution

(where a mole of NaCl weighs 58.5 grams) than is needed to

make a 1.0 M glucose solution, since 180 grams of glucose

takes up more volume than 58.5 grams of salt.

Clinical Investigation CLUES

Jessica took a diuretic that promotes urinary water loss,

and had glucose in her urine.

• How would loss of water because of the diuretic
  affect her plasma concentration and osmotic
  pressure?


• How would the presence of an extra solute—
  glucose—in the urine affect the urine’s osmotic
  pressure and tendency to draw water by osmosis?

Figure 6.9 Osmotic pressure. Sacs composed of a semipermeable membrane, permeable to water but not to the solute

(sucrose), are suspended in beakers containing pure water. Each sac is surrounded by a rigid box. ( a ) Water enters each sac by

osmosis, but the 360 g/L sucrose solution draws water in more rapidly than the 180 g/L sucrose solution. ( b ) Each sac expands until

it presses against its surrounding box with enough force to stop further osmosis. The force required to stop osmosis, the osmotic

pressure, is twice as great for the 360 g/L sucrose solution as the 180 g/L solution.

180 g/L

sucrose

H 2 O

H 2 O

H 2 O H^2 O

H 2 O

H 2 O

H 2 O

H 2 O

H 2 O H 2 O

(a)

H 2 O

H 2 O

360 g/L

sucrose

Solution

initially

180 g/L

No osmosis

Solution

initially

360 g/L

No osmosis

(b)

CLINICAL APPLICATION

In contrast to other plasma solutes, plasma proteins cannot

pass freely through the capillary pores into the interstitial

fluid. As a result, the protein concentration of the plasma is

higher than that of interstitial fluid, making plasma proteins

osmotically active; their osmotic pressure difference draws

water from the interstitial tissue into the capillary blood. If

a person has an abnormally low concentration of plasma

proteins, excessive accumulation of fluid in the tissues—

a condition called edema— will result. This may occur, for

example, when a damaged liver (as in cirrhosis) is unable to

produce sufficient albumin, the major protein in the blood

plasma.