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Fig. 3.3
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Distilled
water
1 When a salt crystal (green) is placed
into a beaker of water, a concentration
gradient exists between the salt from
the salt crystal and the water that
surrounds it.
2 Salt ions (green) move down their
concentration gradient into the water.
3 Salt ions and water molecules are
distributed evenly throughout the
solution. Even though the salt ions
and water molecules continue to
move randomly, an equilibrium exists,
and no net movement occurs because
no concentration gradient exists.
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
Fig. 3.6
* Because the tube contains salt ions
(green and pink spheres) as well
as water molecules (blue spheres),
there is proportionately less
water in the tube than in the beaker,
which contains only water. The
water molecules diffuse with their
concentration gradient into the
tube (blue arrows). Because the
salt ions cannot leave the
tube, the total fluid volume inside
the tube increases, and fluid moves
up the glass tube (black arrow) as a
result of osmosis.
3% salt solution
Selectively
permeable
membrane
Salt solution
rising
Weight
of water
column
The solution
stops rising when
the weight of the
water column
prevents further
movement of
water into the
tube by osmosis.
Distilled
water
Water
1
The end of a tube
containing a 3% salt
solution (green) is closed
at one end with a
selectively permeable
membrane, which allows
water molecules to pass
through but retains the
salt ions within the tube.
2
The tube is immersed in
distilled water. Water
moves into the tube by
osmosis (see inset above*).
The concentration of salt in
the tube decreases as
water rises in the tube
(lighter green color).
Osmosis
3 Water moves by osmosis into
the tube until the weight of
the column of water in the
tube (hydrostatic pressure)
prevents further movement
of water into the tube. The
hydrostatic pressure that
prevents net movement of
water into the tube is equal
to the osmotic pressure of
the solution in the tube.
Fig. 3.7
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
Red blood cell
H2O
Hypotonic solution
(a) When a red blood cell is
placed in a hypotonic
solution (one having a low
solute concentration),
water enters the cell by
osmosis (black arrows),
causing the cell to swell or
even burst (lyse; puff of red
in lower part of cell).
Isotonic solution
(b) When a red blood cell is placed
in an isotonic solution (one
having a concentration of
solutes equal to that inside the
cell), water moves into and out
of the cell at the same rate
(black arrows). No net water
movement occurs, and the cell
shape remains normal.
a-c: ©David M. Phillips/ Science Source
Hypertonic solution
(c) When a red blood cell is
placed in a hypertonic
solution (one having a high
solute concentration), water
moves by osmosis out of the
cell and into the solution
(black arrows), resulting in
shrinkage (crenation).