Osmosis
Osmosis…
• …is the diffusion of water molecules
• …happens across a semi-permeable
membrane
ONLY WATER
– Water is a small but extremely important
molecule that makes up most of the liquid part of
the cytoplasm in living things.
– Deals ONLY with the diffusion of WATER
– The molecules (in this case, water - not solute
molecules) will tend to move from an area of high
(water) concentration to an area of
low(water)concentration until equilibrium is
reached.
OSMOSIS: FACILITATED DIFFUSION OF WATER ACROSS
A CELL MEMBRANE
Why would water molecules normally have a
hard time getting across the cell membrane?
The inside of a cell’s lipid bilayer
is hydrophobic (water hating)
Click me!
Aquaporins
• Most cells have special
water channel proteins
– Known as – Aquaporins
• Allow H2O to pass right
through them by facilitated
diffusion.
• This EXTREMELY important
process is = OSMOSIS
• By knowing the concentrations of solute and
solvent on the inside and outside of a cell, we
can predict the direction of osmosis and the
result on the cell.
• Solutions on the outside of a cell can be
described based on how they affect the cell
– hypERtonic
– hypOtonic
– isotonic
Solutions on the outside of a cell (in its environment) can be
described based on how they affect the cell:
Hypertonic
Hypotonic
Isotonic
• “Above Strength”
• Cell Shrinks
• The solution
outside the cell
has a [higher] of
solutes than
inside
• Water moves OUT
of cell
• “Below Strength”
• Cell Swells
• The solution
outside the cell
has a [lower] of
solutes than
inside
• Water moves
INTO the cell
• “Same Strength”
• Cell stays the
same
• the solution
outside the cell
has the [SAME] of
solutes than
inside
• Water moves
EQUALLY in/out of
the cell
NOTE: (*tonic = solute. [High] solute means [low] water)
"HYPER" = HIGH; "HYPO" = LOW; "ISO" = equal or same.
VISUALIZE HYPERTONIC
• Cell with 2% solute,
98% solvent
• Beaker with 3%
solute, 97% solvent
What will happen?
• a) net movement of
water _______of cell
• b) cell will ________
H2O
H2O
BEFORE
AFTER
H2O
• c) solution is
hypertonic to the
cell
VISUALIZE HYPOTONIC
• Cell with 2% solute,
98% solvent
• Beaker with 1%
solute, 988% solvent
• b) cell will ________
H2O
H2O
BEFORE
What will happen?
• a) net movement of
water _______of cell
• c) solution is
hypotonic to the cell
AFTER
H2O
VISUALIZE ISOTONIC
• Cell with 2% solute
concentration, 98%
solvent
• Beaker = 2% solute,
98% solvent
What will happen
• a) no net movement
• b) cell won’t change
in size
• c) solution =
isotonic to the cell
Common mistakes when discussing
hyper-, hypo-, and isotonic solutions
• The solutions are named for the
concentrations of the SOLUTES
• The substance that moves to balance the
solute concentration is the WATER
• The solutes to not “pull” or “suck” the water
across the membrane – the water simply
diffuses from where it is in high concentration
to low concentration
Solute and solvent concentrations can
be expressed as percentages of the
entire solution.
• When added together, the solute and solvent
concentrations must equal 100%.
• A solution with a 10 % solute concentration
has a 90% solvent concentration.
Let’s do some math!
• What is the solvent
concentration of a solution
with a 3% concentration of
solute?
• What is the solvent
concentration of a solution
with a 15% concentration of
glucose?
• What is the solute
concentration of a solution
with 98% solvent?
• What is the solute
concentration of a solution
with 75% water?
OSMOTIC PRESSURE
Driven by differences in solute
concentration, the net movement of
water into or out of a cell produces a
force known as osmotic pressure
Almost always hypertonic…
• Because cells contain a variety of
solutes such as:
– sugars, proteins, salts, etc.
– they are almost always hypertonic
• (*the environment = HYPOtonic!) to fresh
water;
• as a result, a typical cell exposed to fresh water
will tend to swell up quickly from the entering
water.
• This may in fact cause an animal cell to
swell like an overinflated balloon.
• Plant cells contain a central vacuole which stores
excess water - shrinking and swelling as water
enters or exits the cell. Plant cells wouldn't
generally burst thanks to their protective cell walls.
• In fact, most cells in large organisms are not in
contact with fresh water on a regular basis - rather,
they tend to be bathed in blood or other isotonic
fluids which have solute concentrations
approximately equal to themselves.
• Cells which are plump and rigid in hypotonic
environments are called turgid; when a cell shrinks
in a hypertonic environment this is called
plasmolysis
Fill in…
Conditions
Water will…
Environment is...
Move in and out in
equal amounts
No net movement
Isotonic to the cell
Cell is isotonic to its
environment
Solute concentration in
the environment is
greater than the cell
Move OUT of the cell
The Cell Shrinks
Hypertonic to the cell
Cell is hypotonic to its
environment
Solute concentration in
the environment is less
than the cell
Move INTO the cell
The Cell Swells
Hypotonic to the cell
Cell is hypertonic to its
environment
Solute concentration in
the environment is equal
to that in the cell
Some more practice!
Show what happens to plant and animal cells
subjected to isotonic, hypertonic, and hypotonic
solutions in the environment:
Conditions
Environment
Solute concentration in
the environment is equal
to that in the cell:
Isotonic solution
Solute concentration in
the environment is
greater than the cell:
Hypertonic Solution
Solute concentration in
the environment is less
than the cell:
Hypotonic Solution
Plant Cell (leaf cell)
Before
After
Animal cell (blood cell)
Before
After
APPLY what you have learned about
osmosis…
• Why do doctor’s use a saline solution in an IV
drip?
APPLY what you have learned about
osmosis…
• Why would salt kill plants?
APPLY what you have learned about
osmosis…
• Why do restaurants put out free salty snacks
such as peanuts, pretzels or chips?