Movement of solutes across cell membranes

The plasma membrane constitutes a selective barrier
to the movement of molecules and ions between the
extracellular and intracellular fl uid compartments.
Although fat-soluble substances, water and small uncharged
polar solutes can simply diffuse through the
membrane, ions and water-soluble molecules having
fi ve or more carbon atoms cannot do so. Most biologically
important substances (e.g. glucose, essential
22 Vitamins: their role in the human body
amino acids, water-soluble vitamins and certain inorganic
ions) are translocated across the plasma membrane
by means of transporting membrane proteins
at rates that are suffi ciently rapid to sustain essential
metabolic processes.
The movement of substances across cell membranes
can take place either passively, without the expenditure
of metabolic energy or by active transport,
which involves ‘uphill’ movement of substances from
a region of lower concentration to one of higher concentration
with the expenditure of metabolic energy.
We will encounter two types of passive movement
– simple diffusion and facilitated diffusion – and two
types of active transport – primary and secondary
active transport. We will also encounter receptormediated
endocytosis, which is an energy-dependent
mechanism by which macromolecules can enter cells
without actually crossing the plasma membrane.
Simple diffusion
The most straightforward mechanism for moving
substances across cell membranes or within the cell
itself is simple diffusion. Diffusion is the random
movement of substances that ultimately results in
their equal distribution. All molecules and ions in liquids
or in gases undergo continual random jumping
movements, driven by their inherent thermal energies,
and are continually colliding with one another. These
movements (collectively known as Brownian motion)
and the impact of the collisions between individual
molecules causes the displacement of molecules from
one location to another. If two solutions of a particular
solute at different concentrations are separated by
a permeable membrane, solute molecules will migrate
unaided along the concentration gradient from
the region of higher concentration to the region of
lower concentration. Eventually, the molecules will be
equally distributed throughout the area that encloses
them – a state of equilibrium. At equilibrium, individual
molecules will continue to migrate randomly
back and forth across the membrane, but there will be
no net migration of molecules across the membrane.
The net rate of diffusion of a solute across a cell
membrane depends on the permeability of the membrane
and the pressure difference across the membrane.
In the case of an uncharged solute, the sole
driving force is the difference in solute concentration
between the two sides of the membrane. (Note that
an uncharged solute refers to a molecule that bears
no net electrical charge; this includes solutes that have
an equal number of positive and negative charges.) In
the case of permeating ions, the driving force is the
electrochemical gradient, which is a combination of
the concentration difference and the membrane potential.
Because the inside of the plasma membrane
is negative with respect to the outside, the membrane
potential favours the entry of cations into the cell, but
opposes the entry of anions.
Lipid-soluble molecules readily pass through the
plasma membrane by dissolving in the lipid matrix
and diffusing through the lipid bilayer. Water and
small uncharged water-soluble molecules, which have
little affi nity for the lipid matrix, can pass unhindered
through some membrane protein molecules via narrow
aqueous channels of no more than 0.5–1.0 nm
diameter. Charged molecules, because of their shell of
water molecules, are insoluble in the lipid bilayer and
too big to pass through the narrow aqueous channels.
However, specifi c inorganic ions can diffuse through
membranes via ion channels, as previously discussed.
Facilitated diffusion
Facilitated diffusion refers to the carrier-mediated
diffusion of molecules across the plasma membrane.
Substances translocated in this way cannot usually
pass through the membrane without the aid of a specifi
c carrier protein. As in simple diffusion, the driving
force is the concentration gradient between the inside
and the outside of the cell. When the concentrations
of solutes on the two sides of the membrane are equal,
the carrier-mediated fl ow in both directions will also
be equal. That is, net transport will cease when the
solute distribution is equilibrated. Facilitated diffusion
differs from simple diffusion in that it exhibits
the characteristic properties of carrier-mediated
processes, i.e. saturation kinetics, susceptibility to
competitive inhibition and solute specifi city.
The classic example of facilitated diffusion is glucose
transport across the plasma membrane of erythrocytes.
The glucose concentrations in erythrocytes
are much lower than those in the extracellular fl uid
because the sugar is rapidly metabolized by these cells
after gaining entry. The glucose transporter (a uniporter)
contains a single glucose-binding site and alternates
between two conformational states, one facing
the exoplasmic (outside) surface of the membrane
and the other facing the cytoplasmic (inside) surface.
Only the thermal energy of the system is required for
Background physiology and functional anatomy 23
the conformational change to take place. A molecule
of extracellular glucose binds to the outward-facing
site, which then reorientates to face the inside of the
cell. After release of the glucose into the cell interior,
the transporter (without a bound glucose molecule)
undergoes the reverse conformational change to recreate
the outward-facing binding site.
The facilitated diffusion of glucose in erythrocytes
occurs at a rate that is at least a hundred times faster
than that predicted for simple diffusion. Specifi city
is high: for example, the Km for the transport of the
non-biological L-isomer of glucose is >3000 mM (cf.
1.5 mM for D-glucose).
Primary active transport
Primary active transport is a transport process that
is driven directly by metabolic energy. Such processes
are carried out exclusively by ion pumps, such as the
calcium pumps, the sodium pump and the proton
pumps. Ion pumps are ATPases, which utilize the
energy released by the hydrolysis of ATP.
Ca2+-ATPases (calcium pumps)
A calcium pump in the basolateral membrane of enterocytes
plays a major role in the vitamin D-regulated
intestinal absorption of calcium (see Chapter 8).
The Na+–K+-ATPase (sodium pump)
In most cells the concentrations of K+ and Na+ are respectively
higher and lower than their concentrations
in the extracellular fl uid. A high cytosolic concentration
of K+ is essential to maintain the membrane
potential, and the sodium concentration gradient is
required for the active membrane transport of sugars,
amino acids and certain water-soluble vitamins. The
unequal distribution of K + and Na+ is maintained
by the sodium pump. The sodium pump is also required
to maintain osmotic balance and stabilize cell
volumes: without its function most cells of the body
would swell until they burst.
The sodium pump operates as an antiporter, actively
pumping Na+ out of the cell against its steep
electrochemical gradient and pumping K+ in. The infl
ux of K+ helps to balance the negative charges carried
by organic anions that are confi ned within the cell.
Three sodium ions are moved for every two potassium
ions. The net outward movement of positively
charged ions constitutes an electrical current, creating
a potential difference across the membrane, with the
inside negative to the outside. This electrogenic effect
of the pump, however, seldom contributes more than
10% to the membrane potential. The remaining 90%
depends on the pump only indirectly, as previously
discussed. In epithelial cells such as the intestinal
absorptive cell, sodium–potassium pumping activity
is confi ned to the basolateral domain of the plasma
membrane; there is no such activity on the apical
domain. The drug ouabain competes with K+ for the
same site on the exoplasmic surface of the sodium
pump and specifi cally inhibits its action.
H+-ATPases (proton pumps)
Proton pumps have been described in the membranes
of various intracellular compartments concerned
with endocytosis and potocytosis, such as clathrincoated
vesicles, plasmalemmal vesicles derived from
caveolae, endosomes and lysosomes, where their
function is to acidify the lumen (Mellman et al., 1986;
Anderson & Orci, 1988). The pumping of protons
across a vacuolar membrane from one compartment
to another will generate an electrical potential across
the membrane, and this will oppose further movement
of protons. For proton movement to continue,
there is an accompanying movement of an equal
number of chloride anions in the same direction. A
chloride transporter necessary for the maintenance
of proton pump activity in clathrin-coated vesicles
has been characterized (Xie et al., 1989). Bafi lomycin
A1, a macrolide antibiotic isolated from Streptomyces
sp., specifi cally inhibits the V-type H+-ATPase at nanomolar
concentration (Bowman et al., 1988).
Secondary active transport
Whereas primary active transport is driven directly
by metabolic energy, secondary active transport is
indirectly linked to metabolic energy through a coupling
of the solute to the movement of an inorganic
ion (usually Na+).
Secondary active transport provides the means
whereby physiological amounts of monosaccharides,
amino acids and several water-soluble vitamins in the
intestinal lumen cross the epithelium of the small intestine
to gain access to the blood vessels; that is, the
absorption of these substances. When sodium ions
are transported out of enterocytes by the action of
the ATP-dependent sodium pump at the basolateral
membrane, an inward downhill concentration gradient
of sodium develops across the cell. This gradient
24 Vitamins: their role in the human body
represents a storehouse of energy because the excess
sodium outside the cell is always attempting to diffuse
back into the cell. The sodium concentration gradient
drives the coupled transport of sodium and an accompanying
substance from the intestinal lumen into
the cell via the apical membrane. The coupled transport
system is mediated by a carrier protein that has
sites for both the sodium ion and the accompanying
substance. If the accompanying substance is an anion
(negative charge) and one anion is co-transported
with one sodium ion (positive charge), the loaded
carrier bears no net charge and responds solely to
the sodium concentration gradient. If, however, the
loaded carrier bears a net positive charge (e.g. a 2:1
ratio of Na+ to anion–), it is responsive to the negative
membrane potential as well as the sodium concentration
gradient; together these constitute the electrochemical
gradient for sodium.
In co-transport (also called symport) the sodium
moves down its concentration gradient or electrochemical
gradient and the accompanying substance
moves in the same direction. The absorption of glucose
(Section 3.2.6) is an example of co-transport. In
countertransport (also called antiport) the sodium
and accompanying substance move in opposite directions.
A typical antiporter is the sodium-calcium
exchanger, which exchanges one calcium ion for every
three sodium ions, and is important for extruding calcium
ions from the cytosol of cardiac muscle cells. It
may also play a minor role in the intestinal absorption
of calcium by helping to transport cytosolic calcium
across the basolateral membrane of the enterocyte.