93
CHAPTER THREE
Cells
Facilitated diffusion is similar to simple diffusion in that
it can move molecules only from regions of higher concentra-
tion toward regions of lower concentration. However, unlike
simple diffusion, the number of carrier molecules in the cell
membrane limits the rate of facilitated diffusion
(f g. 3.23)
.
Osmosis
Osmosis
(oz-mo
sis) is the movement of water across a selec-
tively permeable membrane into a compartment contain-
ing solute that cannot cross the same membrane. What you
may have heard is essentially true—“water follows salt”—
although any impermeant solute will draw water by osmosis.
The mechanism of osmosis is complex and beyond the scope
of this discussion, but in part involves diffusion of water.
Therefore, it might help to think of the entire process as diffu-
sion of water down its concentration gradient. In the following
example, assume that the membrane is permeable to water
(the solvent) and impermeable to protein (the solute).
In solutions, a higher concentration of solute means a
lower concentration of water; a lower concentration of solute
means a higher concentration of water. This is because the
solute molecules take up space that water molecules would
otherwise occupy.
Like molecules of other substances, molecules of water
will move from areas of higher concentration to areas of
lower concentration. In
f gure 3.24
, the greater concentra-
tion of protein in compartment
A
means that the water con-
centration there is less than the concentration of pure water
in compartment
B.
Therefore, water moves from compart-
ment
B
across the selectively permeable membrane and into
compartment
A.
In other words, water moves from com-
partment
B
into compartment
A
by osmosis. Protein, on the
other hand, cannot move out of compartment
A
because the
selectively permeable membrane is impermeable to it. Note
gradient but uses membrane proteins as “carriers,” so it is
termed
facilitated diffusion
(fah-sil
ı˘-ta
ˉt
ed dı˘-fu
zhun).
Facilitated diffusion is very important not only for ions,
but for larger water-soluble molecules, such as glucose and
amino acids.
Most sugars and amino acids are insoluble in lipids,
and they are too large to pass through cell membrane pores.
Facilitated diffusion includes not only protein channels, but
also certain proteins that function as carriers to bring such
molecules across the cell membrane. In the facilitated diffu-
sion of glucose, for example, glucose combines with a pro-
tein carrier molecule at the surface of the cell membrane.
This union of glucose and carrier molecule changes the shape
of the carrier in a way that moves glucose to the inner sur-
face of the membrane. The glucose portion is released, and
the carrier molecule returns to its original shape to pick up
another glucose molecule. The hormone
insulin,
discussed
in chapter 13 (p. 510), promotes facilitated diffusion of glu-
cose through the membranes of certain cells.
Time
Solute molecule
Water molecule
AB
AB
(2)
(3)
Permeable
membrane
AB
(1)
FIGURE 3.21
Dif
usion is a passive movement oF molecules.
(
1
) A membrane permeable to water and solute molecules separates a container into
two compartments. Compartment
A
contains both types oF molecules, while compartment
B
contains only water molecules. (
2
) As a result oF molecular
motions, solute molecules tend to dif
use From compartment
A
into compartment
B;
water molecules tend to dif
use From compartment
B
into
compartment
A
. (
3
) Eventually, equilibrium is reached.
Low CO
2
High CO
2
High O
2
Low O
2
FIGURE 3.22
Oxygen enters cells and carbon dioxide leaves cells by
dif
usion.
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