Diffusion • Figure 3.11
In response to concentration
gradients, molecules move
by diffusion. Diffusion occurs
freely in solution, as shown in
the graduated cylinders on the
left, or across semi-permeable
membranes, as in the beakers
on the right.
of dye
The concentration of dye molecules is
highest at the bottom of the cylinder
and on the left side of the selectively
permeable membrane (permeable to
only a few substances). The lowest
concentration is at the top of the
cylinder and on the right side of the
membrane. Note - the membrane
MUST allow dye molecules to move
through it (i.e., be permeable to
The dye molecules spread or
diffuse from the areas of higher
concentration (bottom of the
cylinder, left side of the selectively
permeable membrane) to areas of
lower concentration (top of cylinder,
right side of the selectively
permeable membrane). Some dye
molecules randomly move back to
areas of higher concentration, but
the net movement remains from
higher to lower.
The concentrations of dye molecules
are equal throughout the cylinder
and on both sides of the selectively
permeable membrane. Dye
molecules move randomly about, but
there is no net diffusion in the
cylinder or across the membrane.
the process by
which solutes move from an area of high concentration to
areas of low concentration (Figure 3.11). You encounter
diffusion when you walk into a house and smell something
cooking in the kitchen, such as a pot of soup. The soup mol-
ecules are at the highest concentration in the pot in the
kitchen and the lowest concentration in the room where
you are. The soup molecules (solute) diffuse or spread out
through the air (solvent) from the area of high concentra-
tion to areas of low concentration, which is why you can
smell them upon entering the house. The soup molecules
in the air will continue to diffuse until the concentrations
are equal (equilibrium)—that is, until the smell is equally
strong everywhere in the house.
Substances always diffuse from an area of high concen-
tration to areas of low concentration; this is often referred
to as movement down a concentration gradient. Diffusion
itself requires no added energy. The solute molecules are
in constant motion. They have internal kinetic energy (en-
ergy of motion). This internal kinetic energy drives dif-
fusion. Because diffusion requires no added energy, it is
often referred to as a
passive process.
Diffusion occurs across membranes such as the plas-
ma membrane under two conditions:
The membrane must allow the particular substance to
move across it (that is, it must be permeable to that
There must be a concentration gradient of the particular
substance across the membrane. Physiologists refer to
the concentration gradient as the “driving force” for
Cells and Tissues
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