94
UNIT ONE
Filtration
Molecules move through membranes by diffusion because of
their random movements. In other instances, molecules are
forced through membranes by the process of
f
ltration
(F
l-
tra
shun).
±iltration is commonly used to separate solids from
water. One method is to pour a mixture of solids and water
onto F
lter paper in a funnel
(f
g. 3.26)
. The paper serves
as a porous membrane through which the small water mol-
ecules can pass, leaving the larger solid particles behind.
Hydrostatic pressure, created by the weight of water due
to gravity, forces the water molecules through to the other
side. An example of this is making coffee by the drip
method.
In the body, tissue fl uid forms when water and dissolved
substances are forced out through the thin, porous walls of
blood capillaries, but larger particles such as blood protein
molecules are left inside
(f g. 3.27)
. The force for this move-
ment comes from blood pressure, generated largely by heart
action, which is greater within the vessel than outside it.
However, the impermeant proteins tend to hold water in
blood vessels by osmosis, thus preventing the formation
of excess tissue fl
uid, a condition called edema. (Although
heart action is an active body process, filtration is con-
sidered passive because it can occur due to the pressure
caused by gravity alone.) ±iltration is discussed further in
chapters 15 (p. 578) and 20 (p. 788).
in F gure 3.24 that as osmosis occurs, the level of water on
side
A
rises. This ability of osmosis to generate enough pres-
sure to lift a volume of water is called
osmotic pressure.
Thus
the osmotic movement of water alone achieves equilibrium.
The greater the concentration of impermeant solute par-
ticles (protein in this case) in a solution, the
lower
the water
concentration of that solution and the
greater
the osmotic
pressure. Water always tends to move toward solutions of
greater osmotic pressure.
Cell membranes are generally permeable to water, so
water equilibrates by osmosis throughout the body, and
the concentration of water and solutes everywhere in the
intracellular and extracellular fl uids is essentially the same.
Therefore, the osmotic pressure of the intracellular and extra-
cellular fl
uids is the same. Any solution, such as a 0.9% NaCl
solution (normal saline), that has the same osmotic pressure
as body fl uids is called
isotonic.
Cells will not change size in
this solution.
Solutions that have a higher osmotic pressure than body
fl uids are called
hypertonic.
If cells are put into a hypertonic
solution, there will be a net movement of water by osmosis out
of the cells into the surrounding solution, and the cells shrink.
Conversely, cells put into a
hypotonic
solution, which has a
lower osmotic pressure than body fl uids, gain water by osmo-
sis and swell or possibly even burst (hemolyze). Although cell
membranes are somewhat elastic, the cells may swell so much
that they burst.
Figure 3.25
illustrates the effects of the three
types of solutions on red blood cells.
Region of lower
concentration
Region of higher
concentration
Transported
substance
Protein carrier
molecule
Cell
membrane
FIGURE 3.24
Osmosis. (
1
) A selectively permeable membrane
separates the container into two compartments. At f
rst, compartment
A
contains a higher concentration oF protein (and a lower concentration
oF water) than compartment
B
. Water moves by osmosis From
compartment
B
into compartment
A.
(
2
) The membrane is impermeable
to proteins, so equilibrium can only be reached by movement oF water.
As water accumulates in compartment
A
, the water level on that side oF
the membrane rises.
FIGURE 3.23
±acilitated di²
usion uses carrier molecules to
transport some substances into or out oF cells, From a region oF higher
concentration to one oF lower concentration.
Time
Selectively
permeable
membrane
Protein molecule
Water molecule
A
B
AB
(1)
(2)
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