Human Physiology, 14th edition (2016)

Interactions Between Cells and the Extracellular Environment 137

Osmotic Pressure

Osmosis could be prevented by an opposing force. Imagine

two beakers of pure water, each with a semipermeable mem-

brane sac; one sac contains a 180 g/L glucose solution, the

other a 360 g/L glucose solution. Each sac is surrounded by

a rigid box ( fig.  6.9 a ). As water enters each sac by osmosis,

the sac expands until it presses against the surrounding box.

As each sac presses tightly against the box, the box exerts a

pressure against the sac that can prevent the further osmosis

of water into the sac ( fig.  6.9 b ). The pressure needed to just

stop osmosis is the osmotic pressure of the solution. Plant

cells have such rigid boxes, cell walls composed of cellulose,

around them; animal (including human) cells lack cell walls,

and so animal cells would burst if placed in pure water.

Because osmotic pressure is a measure of the force

required to stop osmosis, it indicates how strongly a solu-

tion “draws” water by osmosis. Thus, the greater the solute

concentration of a solution, the greater its osmotic pressure.

360 g/L solution (from the higher to the lower water concen-
tration), the former solution becomes more concentrated while
the latter becomes more dilute. This is accompanied by volume
changes (assuming the sac can expand freely), as illustrated
in figure  6.8. The net movement of water (osmosis) ceases
when the concentrations become equal on both sides of the
membrane.
Plasma membranes are permeable to water and so behave
in a similar manner. Specific proteins present in the plasma
membranes serve as water channels, known as aquaporins,
which permit osmosis. In some cells, the plasma membrane
always has aquaporin channels; in others, the aquaporin chan-
nels are inserted into the plasma membrane in response to
regulatory molecules. Such regulation is especially important
in the functioning of the kidneys (chapter 17, section 17.3),
which are the major organs regulating total body water bal-
ance. Other organs notable for aquaporin channels in the
plasma membrane of particular cells include the lungs, eyes,
salivary glands, and brain.

Figure 6.7 A model of osmosis. The diagram illustrates
the net movement of water from the solution of lesser solute
concentration (higher water concentration) to the solution of
greater solute concentration (lower water concentration).

More dilute More concentrated

Solute

Water Figure 6.8 The effects of osmosis. A membranous

sac composed of a semipermeable membrane that is permeable

to water but not to the solute (sucrose) is immersed in a beaker.

The solution in the sac contains twice the solute concentration

as the solution surrounding it in the beaker. Because sucrose

cannot diffuse through the membrane, water moves by osmosis

into the sac. If the bag is able to expand without resistance, it

will continue to take in water until both solutions have the same

concentration (270 g/L sucrose).

H 2 O

H 2 O

H 2 O

H 2 O

Semipermeable

membrane sac

expanding

Sucrose

360 g/L

sucrose

180 g/L

sucrose