Solutions
 Water
is considered the solvent.
 The substance(s) dissolved in water is /
are the solute(s).
 Together, solvent + solute  solution.
 Concentration is the number of solutes
in each volume of solvent
Osmosis
 The
diffusion of water across
a semipermeable membrane
is called osmosis.
 In a solution, there are water
molecules and dissolved
particles (the solute).
 The more dissolved particles
there are, the lower the
concentration of water
molecules.
ANIMATION
Comparing Solutions
 A solution
may be desribed as isotonic,
hypertonic or hypotonic relative to
another solution
 These are comparisons; they require a
point of reference (ie, my hair is shorter…
…than it was last year).
 The comparison in biology is usually to the
inside of a cell.
Isotonic

A solution is isotonic to a
cell if it has the same
concentration of dissolved
particles as the cell.
 This means the water
concentration is also the
same.
 Water molecules move into
and out of the cell at an
equal rate in an isotonic
solution.
 The cell size remains the
same.
Hypertonic

A hypertonic solution has
a higher concentration of
dissolved particles than a
cell.
 This means the water
concentration is lower than
that of the cell.
 Thus, water flows out of
the cell – so, the cell will
shrivel and eventually die.
ANIMATION
Hypotonic

A hypotonic solution has a
lower concentration of
dissolved particles than a
cell.
 Therefore the water
concentration is higher
than that of the cell.
 Thus, water diffuses into
the cell – causing the cell
to expand and potentially
burst.
Impact on Cells

In an isotonic solution (center), water enters / exits
red blood cells at equal rates.
 In a hypertonic solution (like salt water – right),
water rushes out and the cell shrivels.
 In a hypotonic solution (like distilled water – left),
water rushes in and the cell swells / bursts (lysis).
Video clips:
RBC in isotonic solution
RBC in hypertonic solution
RBC in hypotonic solution
Adaptations - Plants
 Plant
cells use the cell wall to prevent bursting.
At center, the plant cell is in an isotonic solution. Water
moves in / out at equal rates (no NET movement)
At left, the plant
cell is in a
hypotonic
solution.
At right, the
plant cell is in
a hypertonic
solution.
Water rushes
in, filling the
vacuole.
Water rushes
out of the
cell, draining
the vacuole.
This cell is
turgid / has
high turgor
pressure.
Video: Elodea in isotonic / hyper / hypo
This is called
plasmolysis.
Adaptations - Protists




Paramecia live in
freshwater
This makes paramecia
hypertonic to their
surroundings
Water is constantly
rushing into the
paramecium
So the paramecium uses
a contractile vacuole to
pump the water back out
(and prevent bursting)
Video: The contractile vacuole in action