Human Physiology, 14th edition (2016)

Interactions Between Cells and the Extracellular Environment 135

by diffusion also occurs in the lungs (chapter 16), where the

concentration gradient for oxygen produces net diffusion from

air to blood, and the concentration gradient for carbon dioxide

produces net diffusion from blood to air. In all cases, the direc-

tion of net diffusion is from higher to lower concentration.

Although water is not lipid-soluble, water molecules can

diffuse through the plasma membrane to a limited degree

because of their small size and lack of net charge. In most

membranes, however, the passage of water is greatly aided by

specific water channels (called aquaporins ) that are inserted

into the membrane in response to physiological regulation.

This is the case in the kidneys, where aquaporins aid water

retention by promoting the net diffusion of water out of micro-

scopic tubules into the blood (chapter 17). The net diffusion of

water molecules (the solvent) across the membrane is known

as osmosis. Because osmosis is the simple diffusion of solvent

instead of solute, a unique terminology (discussed shortly) is

used to describe it.

Larger polar molecules, such as glucose, cannot pass

through the double layer of phospholipid molecules and thus

require special carrier proteins in the membrane for transport.

Carrier proteins will be discussed separately in section 6.3.

The phospholipid portion of the membrane is similarly imper-

meable to charged inorganic ions, such as Na^1 and K^1. How-

ever, tiny ion channels through the membrane permit passage

of these ions. The ion channels are provided by some of the

proteins that span the thickness of the membrane ( fig. 6.6 ).

Some ion channels are always open, so that diffusion of

the ion through the plasma membrane is an ongoing process.

diffusion occurs quickly across such small distances. However,
with distances much beyond about 100 m m, the mean diffusion
time becomes too long for effective exchange of molecules
and ions by diffusion, which is why cells in most body organs
are within 100  m m of a blood capillary, and why neurons have
special transport mechanisms to move molecules along axons
(which can be as long as a meter in length).

Diffusion Through the Plasma Membrane

Because the plasma (cell) membrane consists primarily of a
double layer of phospholipids, molecules that are nonpolar, and
thus lipid-soluble, can easily pass from one side of the mem-
brane to the other. The plasma membrane, in other words, does
not present a barrier to the diffusion of nonpolar molecules
such as oxygen gas (O 2 ) or steroid hormones. Small molecules
that have polar covalent bonds, but which are uncharged, such
as CO 2 (as well as ethanol and urea), are also able to penetrate
the phospholipid bilayer. Net diffusion of these molecules can
thus easily occur between the intracellular and extracellular
compartments when concentration gradients exist.
The oxygen concentration is relatively high, for example,
in the extracellular fluid because oxygen is carried from the
lungs to the body tissues by the blood. Because oxygen is
combined with hydrogen to form water in aerobic cell respira-
tion, the oxygen concentration within the cells is lower than
in the extracellular fluid. The concentration gradient for car-
bon dioxide is in the opposite direction because cells produce
CO 2. Gas exchange thus occurs by diffusion between the cells
and their extracellular environments ( fig.  6.5 ). Gas exchange

Figure 6.5 Gas exchange occurs by diffusion. The
colored spheres, which represent oxygen and carbon dioxide
molecules, indicate relative concentrations inside the cell and in the
extracellular environment. Gas exchange between the intracellular
and extracellular compartments thus occurs by diffusion.

O 2

CO 2

Extracellular environment

Tissue cells

Oxygen (O 2 )

Carbon dioxide (CO 2 )

Figure 6.6 Ions pass through membrane

channels. These channels are composed of integral proteins

that span the thickness of the membrane. Although some

channels are always open, many others have structures known

as “gates” that can open or close the channel. This figure

depicts a generalized ion channel; most, however, are relatively

selective—they allow only particular ions to pass.

Cytoplasm Extracellular fluid

Channel

proteins

Channel closed

Channel open

Pore

Gate

Ions