95
CHAPTER THREE
Cells
from a region of lower concentration to one of higher con-
centration.
Sodium ions, for example, can diffuse slowly through
cell membranes. Yet the concentration of these ions typically
remains many times greater outside cells (in the extracellu-
lar fl uid) than inside cells (in the intracellular fl uid). This is
because sodium ions are continually moved through the cell
membrane from regions of lower concentration (inside) to
regions of higher concentration (outside). Movement against
a concentration gradient is called
active transport
(ak
tiv
trans
port) and requires energy derived from cellular metabo-
lism. Up to 40% of a cell’s energy supply may be used for
active transport of particles through its membranes.
PRACTICE
17
What kinds of substances most readily diF
use through a cell
membrane?
18
Explain the diF
erences among diF
usion, facilitated diF
usion, and
osmosis.
19
Distinguish among isotonic, hypertonic, and hypotonic solutions.
20
Explain how ±
ltration occurs in the body.
Active Transport
When molecules or ions pass through cell membranes by
diffusion or facilitated diffusion their net movement is from
regions of higher concentration to regions of lower concen-
tration. Sometimes, however, the net movement of particles
passing through membranes is in the opposite direction,
(a)
(b)
(c)
FIGURE 3.26
In ±
ltration of water and solids, gravity forces water
through ±
lter paper, while tiny openings in the paper retain the solids.
This process is similar to the drip method of preparing coF
ee.
FIGURE 3.25
When red blood cells are placed (
a
) in an isotonic
solution, equal volumes of water enter and leave the cells, and size
and shape remain unchanged. (
b
) In a hypertonic solution, more water
leaves than enters, so cells shrink. (
c
) In a hypotonic solution, more
water enters than leaves, so cells swell and may burst (5,000×).
Gravitational force
Water and
solids
Solids
Filter paper
Water
Capillary wall
Larger molecules
Blood
pressure
Blood
flow
Smaller molecules
Tissue fluid
FIGURE 3.27
In ±
ltration in the body, blood pressure forces smaller
molecules through tiny openings in the capillary wall. The larger
molecules remain inside.
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