Organization of the Body
. Te constant random and high-speed
motion of molecules and ions (a result of their intrinsic kinetic
energy) results in collisions. With each collision, the particles
ricochet oﬀ one another and change direction. Te overall eﬀect
of this erratic movement is to scatter or disperse the particles
throughout the environment
. Te greater the dif-
ference in concentration of the diﬀusing molecules and ions
between the two areas, the more collisions occur and the faster
the net diﬀusion of the particles.
Because the driving force for diﬀusion is the kinetic energy
of the molecules themselves, the speed of diﬀusion is inﬂuenced
(the smaller, the faster) and by
(the warmer, the faster). In a closed container, diﬀusion eventu-
ally produces a uniform mixture of molecules. In other words,
the system reaches equilibrium, with molecules moving equally
in all directions (no
Diﬀusion is immensely important in physiological systems
and it occurs rapidly because the distances molecules are mov-
ing are very short, perhaps 1/1000 (or less) the thickness of this
page! Examples include the movement of ions across cell mem-
branes and the movement of neurotransmitters between two
Te plasma membrane is a physical barrier to free diﬀusion
because of its hydrophobic core. However, a molecule or ion
diﬀuse through the membrane if the molecule is (1) lipid
soluble, (2) small enough to pass through membrane channels,
or (3) assisted by a carrier molecule.
Te unassisted diﬀusion of lipid-soluble or very small par-
ticles is called
. Assisted diﬀusion is known as
A special name,
, is given to the dif-
fusion of a solvent (usually water) through a membrane.
, nonpolar and lipid-
soluble substances diﬀuse directly through the lipid bilayer
. Such substances include oxygen, carbon diox-
ide, and fat-soluble vitamins. Because oxygen concentration is
always higher in the blood than in tissue cells, oxygen continu-
ously diﬀuses from the blood into the cells. Carbon dioxide, on
the other hand, is in higher concentration within the cells, so it
diﬀuses from tissue cells into the blood.
Certain molecules, notably glucose
and other sugars, some amino acids, and ions are transported
Our cells are bathed in an extracellular ﬂuid called
al) that is derived from the blood. Like a rich,
nutritious “soup,” interstitial ﬂuid contains thousands of in-
gredients, including amino acids, sugars, fatty acids, vitamins,
regulatory substances such as hormones and neurotransmitters,
salts, and waste products. ±o remain healthy, each cell must ex-
tract from this mix the exact amounts of the substances it needs
at speciﬁc times.
Although there is continuous traﬃc across the plasma mem-
brane, it is a
It allows some substances to pass while excluding others. It al-
lows nutrients to enter the cell, but keeps many undesirable sub-
stances out. At the same time, it keeps valuable cell proteins and
other necessary substances in the cell, but allows wastes to exit.
Substances move through the plasma membrane in essen-
tially two ways—passively or actively. In
stances cross the membrane without any energy input from the
, the cell provides the metabolic energy
(usually A±P) needed to move substances across the membrane.
±able 3.1 on p. 72 summarizes passive transport processes, and
±able 3.2 on p. 78 summarizes active transport.
Selective permeability is a characteristic of healthy, intact cells.
When a cell (or its plasma membrane) is severely damaged, the
membrane becomes permeable to virtually everything, and
substances ﬂow into and out of the cell freely. Tis phenomenon
is evident when someone has been severely burned. Precious
ﬂuids, proteins, and ions “weep” from the damaged cells.
Te two main types of passive transport are
. Diﬀusion is an important means of passive mem-
brane transport for every cell of the body. Because ﬁltration gen-
erally occurs only across capillary walls, we will discuss it later in
conjunction with capillary transport.
is the tendency of molecules or ions to move from an
area where they are in higher concentration to an area where
they are in lower concentration, that is, down or along their
Molecules in solution move continuously and collide constantly with other
molecules, causing them to move away from areas of their highest concentration and become
evenly distributed. From left to right, molecules from a dye pellet diffuse into the surrounding water
down their concentration gradient.