Organization of the Body
processes, whether a molecule can pass through the mem-
brane depends chiefly on its size or its solubility in lipid, not
on its structure. Facilitated diffusion, on the other hand, is
ofen highly selective. Te carrier ±or glucose, ±or example,
combines specifically with glucose, in much the same way an
enzyme binds to its specific substrate and ion channels allow
only selected ions to pass.
Homeostatic Imbalance
Hypertonic solutions are sometimes in±used intravenously into
the bloodstream o± patients who are edematous (swollen be-
cause their tissues retain water). Tis is done to draw excess
water out o± the extracellular space and move it into the blood-
stream so the kidneys can eliminate it. Hypotonic solutions
may be used (with care) to rehydrate the tissues o± extremely
dehydrated patients. In mild cases o± dehydration, drinking
hypotonic fluids (such as apple juice and sports drinks) usually
does the trick.
Check Your Understanding
What is the energy source for all types of diffusion?
What determines the direction of any diffusion process?
What are the two types of facilitated diffusion and how do
they differ?
For answers, see Appendix H.
Active Processes
Differentiate between primary and secondary active transport.
Compare and contrast endocytosis and exocytosis in terms
of function and direction.
Compare and contrast pinocytosis, phagocytosis, and
receptor-mediated endocytosis.
Hypotonic solutions
are more dilute (contain a lower con-
centration o± nonpenetrating solutes) than cells. Cells placed
in a hypotonic solution plump up rapidly as water rushes into
them (Figure 3.9c). Distilled water represents the most extreme
example o± hypotonicity. Because it contains
solutes, water
continues to enter cells until they finally burst, or
Notice that osmolarity and tonicity are not the same. A solu-
tion’s osmolarity is based solely on its total solute concentra-
tion. In contrast, its tonicity is based on how the solution affects
cell volume, which depends on (1) solute concentration and
(2) solute permeability o± the plasma membrane. Osmolarity is
expressed as osmoles per liter (osmol/L) where 1 osmol is equal
to 1 mole o± nonionizing molecules.* A 0.3 osmol/L solution
o± NaCl is isotonic because sodium ions are usually prevented
±rom diffusing through the plasma membrane. But i± the cell is
immersed in a 0.3 osmol/L solution o± a penetrating solute, the
solute will enter the cell and water will ±ollow. Te cell will swell
and burst, just as i± it had been placed in pure water.
Osmosis is extremely important in determining distribution
o± water in the various fluid-containing compartments o± the
body (cells, blood, and so on). In general, osmosis continues
until osmotic and hydrostatic pressures acting at the membrane
are equal. For example, the hydrostatic pressure o± blood against
the capillary wall ±orces water out o± capillary blood, but the sol-
utes in blood that are too large to cross the capillary membrane
draw water back into the bloodstream. As a result, very little net
loss o± plasma fluid occurs.
Simple diffusion and osmosis occurring directly through
the plasma membrane are not selective processes. In those
*Osmolarity (Osm) is determined by multiplying molarity (moles per liter, or
by the number o± particles resulting ±rom ionization. For example, since NaCl
ionizes to Na
, a 1
solution o± NaCl is a 2 Osm solution. For substances
that do not ionize (e.g., glucose), molarity and osmolarity are the same. More
precisely, the term
is used, which is equal to the number o± particles
mixed into a kilogram o± water.
Table 3.1
Passive Membrane Transport Processes
Simple diffusion
Kinetic energy
Net movement of molecules from an area
of their higher concentration to an area
of their lower concentration, that is, along
their concentration gradient
Fats, oxygen, carbon dioxide move
through the lipid bilayer of the membrane
Facilitated diffusion
Kinetic energy
Same as simple diffusion, but the diffusing
substance is attached to a lipid-soluble
membrane carrier protein (carrier-
mediated facilitated diffusion) or moves
through a membrane channel (channel-
mediated facilitated diffusion)
Glucose and some ions move into cells
Kinetic energy
Diffusion of water through a selectively
permeable membrane
Movement of water into and out of cells
directly through the lipid bilayer of the
membrane or via membrane channels
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