136 Chapter 6
is about 20 times more permeable to potassium (K^1 ) than to
sodium (Na^1 ); consequently, K^1 diffuses much more rapidly
than does Na^1. Changes in the protein structure of the mem-
brane channels, however, can change the permeability of
the membrane. This occurs during the production of a nerve
impulse (chapter 7, section 7.2), when specific stimulation
opens Na^1 channels temporarily and allows a faster diffusion
rate for Na^1 than for K^1 .
In areas of the body that are specialized for rapid diffusion,
the surface area of the plasma membranes may be increased by
numerous folds. The rapid passage of the products of diges-
tion across the epithelial membranes in the small intestine, for
example, is aided by tiny fingerlike projections called micro-
villi (chapter 3, section 3.1). Similar microvilli are found in the
kidney tubule epithelium, which must reabsorb various mol-
ecules that are filtered out of the blood.Osmosis
Osmosis is the net diffusion of water (the solvent) across
the membrane. For osmosis to occur, the membrane must
be selectively permeable; that is, it must be more perme-
able to water molecules than to at least one species of sol-
ute. There are thus two requirements for osmosis: (1) there
must be a difference in the concentration of a solute on the
two sides of a selectively permeable membrane; and (2) the
membrane must be relatively impermeable to the solute.
Solutes that cannot freely pass through the membrane can
promote the osmotic movement of water and are said to be
osmotically active.
Like the diffusion of solute molecules, the diffusion of
water occurs when the water is more concentrated on one
side of the membrane than on the other side; that is, when
one solution is more dilute than the other ( fig. 6.7 ). The more
dilute solution has a higher concentration of water molecules
and a lower concentration of solute. Although the terminol-
ogy associated with osmosis can be awkward (because we are
describing water instead of solute), the principles of osmosis
are the same as those governing the diffusion of solute mol-
ecules through a membrane. Remember that, during osmo-
sis, there is a net movement of water molecules from the
side of higher water concentration to the side of lower water
concentration.
Imagine that a semipermeable membrane is formed into
a spherical sac containing a 360 g/L (grams per liter) glucose
solution, and that this sac is inserted into a beaker containing
a 180 g/L glucose solution ( fig. 6.8 ). One solution initially
contains 180 g/L of the glucose solution and the other solu-
tion contains 360 g/L of glucose. If the membrane is perme-
able to glucose, glucose will diffuse from the 360 g/L solution
to the 180 g/L solution until both solutions contain 270 g/L
of glucose. If the membrane is not permeable to glucose but
is permeable to water, the same result (270 g/L solutions on
both sides of the membrane) will be achieved by the diffusion
of water. As water diffuses from the 180 g/L solution to theCLINICAL APPLICATION
Cystic fibrosis occurs most commonly among people of
Northern European ancestry, at a frequency of about once
in every 2,500 births. As a result of a genetic defect, abnor-
mal NaCl and water movement occurs across the epithe-
lial membranes of sweat glands, pancreatic ductules, and
small respiratory airways. This produces an excessively
salty sweat, and in the pancreas and lungs can result in
dense, viscous mucus that promotes pancreatic and pul-
monary disorders. The genetic defect involves a particular
glycoprotein that forms chloride (Cl^2 ) channels in the api-
cal membrane of the epithelial cells. This protein, known as
CFTR (for cystic fibrosis transmembrane conductance reg-
ulator ), is formed in the endoplasmic reticulum, but does
not move into the Golgi complex for processing, and as
a result does not get correctly inserted into vesicles that
would introduce it into the plasma membrane (chapter 3).
There are different mutations in the CFTR gene that can
cause cystic fibrosis with differing levels of severity. This
disease cannot currently be cured, but relatively effective
treatments are available.Many ion channels, however, are gated —they have structures
(“gates”) that can open or close the channel ( fig. 6.6 ). In this
way, particular physiological stimuli (such as binding of the
channel to a specific chemical regulator) can open an other-
wise closed channel. In the production of nerve and muscle
impulses, specific channels for Na^1 and others for K^1 open
and close in response to changes in membrane voltage (dis-
cussed in chapter 7, section 7.2).
Rate of Diffusion
The rate of diffusion through a membrane, measured by the
number of diffusing molecules passing through the membrane
per unit time, depends on
- the magnitude of the concentration difference across the
membrane (the “steepness” of the concentration gradient), - the permeability of the membrane to the diffusing
substances, - the temperature of the solution, and
- the surface area of the membrane through which the sub-
stances are diffusing.
The magnitude of the concentration difference across a
membrane serves as the driving force for diffusion. Regard-
less of this concentration difference, however, the diffusion of
a substance across a membrane will not occur if the membrane
is not permeable to that substance. With a given concentration
difference, the speed at which a substance diffuses through a
membrane will depend on how permeable the membrane is
to it. In a resting neuron, for example, the plasma membrane