Principles o f osm osis ap p lied to red b lo o d cells • Figure 3.1 4
The arrows indicate the direction and degree of water movement into and out of cells.
The scanning electron micrographs of the cells have magnifications of 15,000X.
Question
a. Isotonic
b. Hypotonic
c. Hypertonic
Is the m em brane per-
m eable to water?
Yes
Yes
Yes
W here is the solute
concentration higher?
Equal on both sides of cell
Inside the cell
Outside the cell
W here is the solute
concentration lower?
Equal on both sides of cell
Outside the cell
Inside the cell
W here is the water
concentration higher?
Equal on both sides of cell
Outside the cell
Inside the cell
W here is the water
concentration lower?
Equal on both sides of cell
Inside the cell
Outside the cell
W hich w ay w ill there
be net m ovem ent of
water?
None
Outside to inside
Inside to outside
W hat w ill happen to
Stay the sam e
Sw ell (Cell m ay burst open)
Sh rin k
Why is osmosis so important to your health? As a result of flu-
id intake, transfusions, injuries, and diseases, the salt and wa-
ter concentrations of various fluid compartments within your
body change. So, cells within those compartments, like red
blood cells traveling through blood vessels, may find them-
selves in environments with different solute and water con-
centrations (Figure 3.14). There are specific names for such
environments; it is important to note that these terms refer
to the
concentrations of solutes,
not the concentration of water: •
Isotonic
(l'-so-TON-ik)—The
solute
concentration
outside the cell is the same as that inside the cell.
Therefore, water concentration is also the same on both
sides of the cell, and the net movement of water is zero.
Hypotonic (hi'-po-TON-ik;
hypo-
= less)—The solute
concentration
outside
the
cell
is
less
than
the
concentration inside the cell. Therefore, the water
concentration outside is greater than that inside, and
water flows into the cell.
Hypertonic
(hi'-per-TON-ik;
hyper-
=
more)—The
solute concentration outside the cell is greater than
the concentration inside the cell. Therefore, the water
concentration is greater inside the cell than outside,
and water flows out of the cell.
The balance of water concentration across the cell mem-
brane will change under these various conditions, and the
cell shape and volume may remain the same, swell, or
shrink (Figure 3.14). Dramatic changes in cell shape and
volume may lead to cell death.
So far, we have only talked about substances moving
down a concentration gradient. However, it is possible for
substances to be transported against a concentration gra-
dient via a process called
active transport.
Cells Carry Out Many Processes 61
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