Human Physiology, 14th edition (2016)

Interactions Between Cells and the Extracellular Environment 149

6.4 The Membrane Potential

As a result of the permeability properties of the plasma

membrane, the presence of nondiffusible negatively

charged molecules inside the cell, and the action of the

Na^1 /K^1 pumps, there is an unequal distribution of charges

across the membrane. As a result, the inside of the cell

is negatively charged compared to the outside. This dif-

ference in charge, or potential difference, is known as the

membrane potential.

The process of endocytosis resembles exocytosis in
reverse. In receptor-mediated endocytosis (see fig. 3.4), spe-
cific molecules, such as protein-bound cholesterol, can be
taken into the cell because of the interaction between the
cholesterol transport protein and a protein receptor on the
plasma membrane. Cholesterol is removed from the blood
by the liver and by the walls of blood vessels through this
mechanism.
Exocytosis and endocytosis together provide bulk
transport out of and into the cell, respectively. (The term bulk
is used because many molecules are moved at the same time.)
It should be noted that molecules taken into a cell by endocy-
tosis are still separated from the cytoplasm by the membrane
of the endocytotic vesicle. Some of these molecules, such as
membrane receptors, will be moved back to the plasma mem-
brane, while the rest will end up in lysosomes.
Reference to figure  6.21 reveals that there is a definite
direction, or polarity, to transport in epithelial cells. This fig-
ure illustrates the polarization of membrane transport processes
involved in absorption and reabsorption across the epithelium
lining the small intestine or kidney tubules. There is also a polar-
ization of organelles involved in exocytosis (see fig. 3.12) and
endocytosis. For example, exocytotic vesicles that bud from the
Golgi complex fuse with the plasma membrane at its apical, or
top, surface, while the nucleus and endoplasmic reticulum are
located more toward the bottom of the cell (nearer to the base-
ment membrane). Because of the polarity of transport processes
across the plasma membrane and polarity of intracellular organ-
elles, scientists often distinguish between the apical surface of
epithelial cells and their basolateral surface (see fig. 6.21 ).

Figure 6.23 Endocytosis and exocytosis. Endocytosis
and exocytosis are responsible for the bulk transport of
molecules into and out of a cell.

Endocytosis

Exocytosis

Secretion of

cellular

product

Formation

of vesicle

Invagination Formation
of pouch
Extracellular
fluid

Cytoplasm

Joining of

vesicle

with plasma

membrane

Extracellular

substances

now within

vesicle

Secretion

now in

extracellular

fluid

| CHECKPOINTS

7a. List the three characteristics of facilitated diffusion

that distinguish it from simple diffusion.

7b. Draw a figure that illustrates two of the

characteristics of carrier-mediated transport and

explain how this type of movement differs from

simple diffusion.

7c. Describe active transport, including primary and

secondary active transport in your description.

Explain how active transport differs from facilitated

diffusion.

8. Discuss the physiological significance of the Na^1 /K^1
   pumps.

LEARNING OUTCOMES

After studying this section, you should be able to:

9. Describe the equilibrium potentials for Na^1 and K^1.


10. Describe the membrane potential and explain how it
    is produced.

If you understand how the membrane potential is produced,

and how it is affected by the permeability of the plasma mem-

brane to specific ions, you will be prepared to learn how

neurons and muscles (including the heart muscle) produce

impulses and function. Thus, this section serves as a basis for

the discussion of nerve impulses that follows in chapter 7, and

for discussions of muscle (chapter 12) and heart (chapter 13)

function.

In section 6.3, the action of the Na^1 /K^1 pumps was dis-

cussed in conjunction with the topic of active transport, and

it was noted that these pumps move Na^1 and K^1 against their

concentration gradients. This action alone would create and

amplify a difference in the concentration of these ions across

the plasma membrane. There is, however, another reason why

the concentration of Na^1 and K^1 would be unequal across the

membrane.