Anatomy and Physiology - Introduction to Biomedical Engineering - page 85

is pumped out through specialized protein channels at a rate equivalent to the rate at which
it leaks in through other channels, it behaves osmotically as if it cannot cross the plasma
membrane. Thus, mammalian cells exist in a steady state, rather than at equilibrium, since
energy in the form of ATP must be used to prevent a net movement of ions across the
plasma membrane.

EXAMPLE PROBLEM 3.4
Consider a simple model cell, such as the one in Figure 3.7, that has the following ion concen-
trations. Is the cell at equilibrium? Explain your answer.

Ion Intracellular

Concentration (mM)

Extracellular
Concentration (mM)

K þ

158 4

Na þ 20 163

Cl 52 167

A

104 —

Solution
Yes. The cell is both electrically and osmotically at equilibrium because the charges within
the inside and outside compartments are equal, and the osmolarity inside the cell equals the
osmolarity outside of the cell.

Inside Outside

Positive 158 þ 20 ¼ 178 mM 4 þ 163 ¼ 167 mM

Negative 52 þ 1.2 * 104 ¼ 177 mM 167 mM

178 mM pos 177 mM neg 167 mM pos ¼ 167 mM neg

158 þ 20 þ 52 þ 104 ¼ 334 mM 4 þ 163 þ 167 ¼ 334 mM

Osmolarity

334 mM inside ¼ 334 mM outside

One of the consequences of the distribution of charged particles in the intracellular
and extracellular fluids is that an electrical potential exists across the plasma membrane.
The value of this electrical potential depends on the intracellular and extracellular con-
centrations of ions that can cross the membrane and will be described more fully in
Chapter 11.
In addition to controlling the cell's volume, the plasma membrane also provides a route
for moving large molecules and other materials into and out of the cell. Substances can be
moved into the cell by means of endocytosis (Figure 3.8a) and out of the cell by means of
exocytosis (Figure 3.8b). In endocytosis, material—for example, a bacterium—outside of
the cell is engulfed by a portion of the plasma membrane that encircles it to form a
vesicle. The vesicle then pinches off from the plasma membrane and moves its contents to
the inside of the cell. In exocytosis, material within the cell is surrounded by a membrane
to form a vesicle. The vesicle then moves to the edge of the cell, where its membrane fuses
with the plasma membrane and its contents are released to the exterior of the cell.