Osmosis
The passive transport of water across a selectively permeable
membrane is called osmosis.
In order to fully understand this concept, you must know
the difference between solute and solvent and the
difference between “free” and “bound” water molecules.
When salt is added to
water, it readily
dissolves forming a
solution of saltwater.
salt
Any substance that dissolves
in another is called a solute.
Any substance that does the
dissolving is the solvent.
In this case, salt is the solute
and water is the solvent.
water
salt
Table salt is known chemically as sodium
chloride. Let’s look at what makes up a
crystal of table salt.
A crystal of salt is
composed of many
sodium and chlorine
ions.
Ions are atoms that
have a charge.
The sodium ions have a
positive (+) charge and
the chlorine ions have a
negative (–) charge.
Sodium
Chlorine
Na Cl Na Cl Na Cl
Cl Na Cl Na Cl Na
Na Cl Na Cl Na Cl
Cl Na Cl Na Cl Na
Let’s look at what happens
to a crystal of salt as it
dissolves in water.
salt crystal
When a crystal of salt enters
water, the (-) ions are
attracted to the positive
poles of water molecules.
water molecules
Let’s look at what happens
to a crystal of salt as it
dissolves in water.
When a crystal of salt enters
water, the (-) ions are
attracted to the positive
poles of water molecules.
The (-) ion will then
dissociate itself from the
salt crystal and become
surrounded by other
water molecules.
salt crystal
Conversely, the (+) ions
are attracted to the
negative poles of water
molecules.
salt crystal
Conversely, the (+) ions
are attracted to the
negative poles of water
molecules.
The (+) ion will also
dissociate and become
surrounded by water
molecules.
salt crystal
Conversely, the (+) ions
are attracted to the
negative poles of water
molecules.
salt crystal
The (+) ion will also
dissociate and become
surrounded by water
molecules.
As long as there is
sufficient water, this
process will continue
until all of the ions
are dissolved.
Dissolved ions
When water molecules
are clumped around
ions, they are
considered “bound.”
When water molecules
are not bound, they
are called “free.”
Bound water molecules
Ions
Free water molecules
Bound water
molecules cannot
pass through a
selectively permeable
membrane.
Free water molecules
however are permeable.
Selectively permeable
membrane
In osmosis, water flows
from the side of the
membrane that has the
greatest number of free
water molecules to the side
with the fewest number.
Net flow stops when the
free water concentration in
both areas is equal.
Selectively permeable
membrane
Equalibrium
Which side of the
membrane shown has
the greatest number of
free water molecules?
Answer:
The right side has 10
free water molecules.
The left side only has 4
free water molecules.
Selectively permeable
membrane
In which direction will
the free water molecules
flow through the
membrane?
Answer:
Flow will go from the
right side (the most
concentrated in free
water) to the left side
(the least concentrated).
Selectively permeable
membrane
When will net flow stop?
Answer:
When equilibrium is
reached (7 free water
molecules on each side).
Selectively permeable
membrane
Will free water molecules
continue to flow back
and forth across the
membrane?
Answer:
Yes, but at equal rates.
Selectively permeable
membrane
Salt
In this example of osmosis,
an equal amount of water
and salt are placed in a Utube apparatus.
In the middle of the Utube are two barriers
which prevent the two
salt solutions from mixing.
In addition, there is a
membrane that only allows
the flow of free water
molecules.
Barriers
Water
Salt Water
Membrane
What do you hypothesize will
happen to the fluid level on each
side of the membrane when the
barriers are removed?
Answer
The fluid levels on both sides
stay the same. This is
because both volumes of
water have an equal
concentration of free water
molecules, so no net
movement is observed.
Salt Water
Membrane
This time we will double
the amount of salt on the
right side of the U-tube.
The diluted water on the left
side is said to be hypotonic
(have less dissolved solutes)
compared to the concentrated
water on the right.
The concentrated water on the
right is said to be hypertonic
(have more dissolved solutes)
compared to the diluted water
on the left.
Dilute
Saltwater
hypotonic
Concentrated
Saltwater
Membrane
hypertonic
Now hypothesize what
will happen to the water
levels on each side of the
membrane when the
barriers are removed.
Hint: Adding salt results
in an increase of bound
water molecules and a
decrease in free water
molecules.
Dilute
Saltwater
hypotonic
Concentrated
Saltwater
Membrane
Free
Bound
hypertonic
Answer:
The left side has a lower
concentration of salt but
a higher concentration of
free water molecules.
Thus the water will flow
through the membrane
from left to right.
Dilute
Saltwater
Concentrated
Saltwater
Membrane
Let’s run the same
experiment again but this
time place a pressure
gage on the right side.
The pressure gage will give
an indication of the amount
of pressure the water on
the left side exerts. We’ll
now remove the barriers.
Now watch the red gage
needle…
Dilute
Saltwater
Concentrated
Saltwater
Membrane
The red needle indicated an
increase in water pressure.
Notice the water level did
not change. This is
because the gage prevents
the water from moving.
Dilute
Saltwater
Concentrated
Saltwater
Membrane
The blinking red arrow does
not represent water flow,
but the direction of water
pressure instead.
Pressure as a result of
osmosis is called
osmotic pressure.
Dilute
Saltwater
Concentrated
Saltwater
Membrane
Now let’s see what effect
osmosis has on living cells.
In this beaker of pond water is a
microscopic one-celled organism
called a paramecium.
Paramecia are so small that
you need a microscope to see
them.
Pond
water
Notice how the two
contractile vacuoles
inside the paramecium
expand and contract.
Make a hypothesis as to
why they are doing this.
Answer:
Water constantly enters
the paramecium by
osmosis. The contractile
vacuoles expel the water
from the cell.
Pond
water
What will happen to the
paramecium if the
contractile vacuoles fail
to function?
Answer:
The paramecium will
probably expand to the
point that it lysis (breaks
open).
Pond
water
Let’s add some salt to
the pond water.
salt
The salty pond water is now
hypertonic compared to the
inside of the paramecium.
Will water tend to flow out of
the paramecium or into it?
Salty
Pond
water
Answer:
Water will tend to flow
out of the paramecium.
H 2O
H 2O
How will the added salt affect the
rate at which the paramecium
contracts its vacuoles?
Answer:
To conserve water, the
paramecium will slow
the rate at which its
vacuoles contract.
Before salt
was added
Compare
the rates
Salty
Pond
water
After salt
was added
Assume we now place the
paramecium in distilled water.
Distilled water is pure water. It does
not have any impurities or salts
dissolved in it. Distilled water has
nothing but free water molecules.
Distilled
water
Salty
Pond
water
Now with the paramecium in distilled
water, hypothesize how the rate of
vacuole contractions will change.
Answer:
Too much water is entering
into the paramecium. The
vacuoles must contract faster
to expel the water.
In pond
water
Compare
the rates
Distilled
water
In distilled
water
Osmoregulation is the ability of an organism,
like this paramecium, to regulate its internal
environment via osmosis.
Cheek cells are placed in three different solutions below.
Based on what happens to the cells, determine if the
solution is hypotonic, hypertonic or isotonic to the cell’s
internal environment. Refer to the definitions below.
H 2O
H 2O
H 2O
H 2O
Isotonic: Having a solute concentration equal to that of another solution.
Hypertonic: Having a higher concentration of solute than another solution.
Hypotonic: Having a lower concentration of solute than another solution.
Answer:
Hypertonic
Isontonic
Hypotonic
Unlike paramecia, cheek cells lack contractile vacuoles to
regulate osmosis. This makes cheek cells more susceptible
to death by shriveling in hypertonic solutions and swelling in
hypotonic solutions.
Sea water is hypertonic to fresh water.
Ocean fish like tuna cannot survive in fresh water.
Sea water
Fresh water
Fresh water is hypotonic to sea water.
Fresh water fish like brown trout cannot survive in sea water.
The cell below represents a typical plant cell with the
following components:
Nucleus
Cell wall
Chloroplast
Central
vacuole
(acqueous)
Mitochondria
Golgi body
Cytosol
H 2O
H2 O
Water moves into the cell… and out of the cell by via osmosis.
Let’s place plant cells in isotonic, hypertonic and hypotonic
solutions.
Isotonic
H 2O
Hypertonic
H 2O
H 2O
H 2O
In an isotonic solution,
water moves in and
out at equal rates.
Hypotonic
In an hypertonic
solution, water
moves out of the cell.
In an hypotonic
solution, water
moves into the cell.
In a hypertonic solution, plant cells
are plasmolyzed, which is a condition
where the plasma membrane pulls
away from the cell wall.
Isotonic
In a hypotonic solution, plant
cells become turgid, or swollen.
The osmotic pressure that
causes this is called turgor.
Hypertonic
Hypotonic
Plasmolyzed
cell
Turgid
cell
Match each plant’s state below to the plant cell’s condition
in each solution.
Answer:
Isotonic
A
Hypertonic
Hypotonic
Plasmolyzed
cell
Turgid
cell
B
C
Isotonic
B
Hypertonic
Hypotonic
Plasmolyzed
cell
Turgid
cell
C
A
Isotonic
Isotonic solutions
make plants slightly
wilted (flaccid).
B
Hypertonic
Hypotonic
Plasmolyzed
cell
Turgid
cell
Hypertonic
solutions make
plants very flaccid.
C
Hypotonic
solutions help
keep plants firm
and upright.
A