Cell Membranes
Diffusion, Osmosis & Osmotic
Pressure
Dr. Kashif Rahim
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Functions of Membranes
1. Protect cell
2. Control incoming and outgoing substances
3. Maintain ion concentrations of various
substances
4. Selectively permeable - allows some
molecules in, others are kept out
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Phospholipid Bilayer
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Fluid Mosaic Model
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Solutions
• Solutions are made of solute and a
solvent
• Solvent - the liquid into which the solute
is poured and dissolved. We will use
water as our solvent today.
• Solute - substance that is dissolved or put
into the solvent. Salt and sucrose are
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solutes.
Methods of Transport Across
Membranes
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
4. Active Transport
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Methods of Transport Across
Membranes
1. Diffusion -passive transport - no energy
expended
2. Osmosis - Passive transport of water
across membrane
3. Facilitated Diffusion - Use of proteins to
carry polar molecules or ions across
4. Active Transport- requires energy to transport
molecules against a concentration
gradient – energy is in the form of
ATP
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Diffusion
• Particles in a liquid or gas
spread out…
• … from regions of high
concentration…
• … to regions of low
concentration…
• …until the particles are
evenly spread out.
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Dissolving
KMnO4
crystal
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• The difference between the regions of high
concentration and low concentration is called the
concentration gradient
• The steeper the concentration gradient, the faster
diffusion takes place
High concentration gradient
Fast rate of diffusion
Low concentration gradient
Slow rate of diffusion
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• Diffusion occurs because the particles in
gases and liquids are moving.
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Diffusion
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Dissolving substances in water
• The molecules in liquid water are constantly moving
• When water molecules bump into particles of a soluble
substance, they stick to them
Free moving
water molecules
Sugar molecules
in sugar lump
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• When the water molecules move away…
… they carry particles of the solute with them
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• Adding a solute to water reduces the amount of free
water molecules
Free water
molecules
Solute
molecule
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Partially-Permeable Membranes
• A partially-permeable membrane will allow
certain molecules to pass through it, but not
others.
Partially permeable
membrane
• Generally, small
particles can pass
through…
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…but large
particles
cannot
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More free water molecules
on this side of membrane
Partially-permeable
membrane
Water-solute particle
is too large to pass
through membrane
Free water molecules diffuse in this direction
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Osmosis
• Osmosis is the diffusion of free water
molecules…
•… from a region of high concentration of
free water molecules…
• … to a region of low concentration of free
water molecules…
• …across a partially-permeable
membrane…
• …until they are evenly spread out.
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Distilled water separated by a partiallypermeable membrane:
Water molecules are moving from
one side of the membrane to the
other but there is no net osmosis
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If a substance is dissolved in water, the
kinetic energy of the water molecules is
lowered.
This is because some water molecules
aggregate on the surfaces of the other
molecules…
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Tonicity is a relative term
• Hypotonic Solution - One solution has a
lower concentration of solute than
another.
• Hypertonic Solution - one solution has a
higher concentration of solute than
another.
• Isotonic Solution - both solutions have
same concentrations of solute.
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For osmosis we talk about the
potential water molecules have to
move – the OSMOTIC POTENTIAL.
Distilled water has the highest
potential (zero).
When water has another substance
dissolved in it, the water molecules have
less potential to move. The osmotic
potential is NEGATIVE.
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Water molecules always move from
less negative to more negative water
potential.
Net osmosis
= LN MN
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The osmotic potential of a cell
is known as its WATER
POTENTIAL. For animal cells,
the water potential is the
osmotic potential of the
cytoplasm.
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An animal cell with water
potential –50 is
placed in a solution…
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Water potential of
cytoplasm = -50
Osmotic potential of
solution= -20
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If the osmotic
potential of the
solution is less
negative than the
water potential of the
cytoplasm(the
solution is
hypotonic), net
endosmosis will
occur, i.e. water will
move into the cell
from the solution.
The result will be
haemolysis (the cell
will burst)
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Water potential of
cytoplasm= -50
Osmotic potential of
solution = -80
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If the osmotic
potential of the
solution is more
negative than
the water
potential of the
cytoplasm (the
solution is
hypertonic), net
exosmosis will
occur. The result
will be crenation
(the cell will
shrivel up)
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Water potential of
cytoplasm= -50
Osmotic potential of
solution= -50
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If the
osmotic
potential of
the solution
is the same
as the water
potential of
the
cytoplasm
(the solution
is isotonic),
there will be
no net
osmosis.
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What controls osmosis?
• Unequal
distribution of
particles, called a
concentration
gradient, is one
factor that controls
osmosis.
Before
Osmosis
Selectively
permeable
membrane
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After
Osmosis
Water molecule
Sugar molecule
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Osmosis: Diffusion of Water
• Most cells whether in multicellular or unicellular
organisms, are subject to osmosis because they are
surrounded by water solutions.
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Cells in an isotonic solution
• isotonic solution• (= concentrations)
• the concentration of
dissolved substances in
the solution is the same as
the concentration of
dissolved substances
inside the cell.
H2O
H2O
Water Molecule
Dissolved Molecule
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Cells in an isotonic solution
H2O
• water molecules move
into and out of the cell at
the same rate, and cells
retain their normal shape.
H2O
Water
Molecule
Dissolved
Molecule
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Cells in a hypotonic solution
• hypotonic solution: dilute
solution thus low solute
concentration
• In a hypotonic solution,
water enters a cell by
osmosis, causing the cell
to swell.
H2O
H2O
Water
Molecule
Dissolved
Molecule
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Cells in a hypertonic solution
• hypertonic solution:
concentrated solution, thus
a high solute
concentration
In a hypertonic solution,
water leaves a cell by
osmosis, causing the cell
to shrink
H2O
H2O
Water
Molecule
Dissolved
Molecule
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Osmosis
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Semi permeable
Membrane
Theory of Osmosis
Pressure

Fresh
Water
Fresh
Water
H2O
Sea
Water
H2O
Sea
Water
(diluted)
H2O
Fresh
Water
Sea
Water
H2O
Initial Condition
Equilibrium
Reverse Osmosis
The Osmotic Pressure, π, is defined as: π = MRT
For sea water at Dr.
35Kashif
ppt, Rahim
π is about 350 psi.
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Reverse Osmosis is a water treatment process whereby dissolved salts, such as
sodium, chloride, calcium carbonate, and calcium sulfate may be separated from water
by forcing the water through a semi-permeable membrane under high pressure. The
water diffuses through the membrane and the dissolved salts remain behind on the
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surface of the membrane.
Osmotic Pressure
• For the phenomenon of osmosis, a membrane separates
salt/water inside a chamber from pure water in the
container. Water passes through membrane from dilute
to more concentrated. As water rises into tube, it
creates a pressure. Eventually this pressure (osmotic
pressure) prevents further passage of water through the
membrane
• Osmotic pressure is force per area that prevents water
from passing through membrane!
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Diagram of osmotic pressure cell.
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Osmotic Pressure
• Osmotic pressure. A semi permeable membrane
allows water molecules, but not salt ions or large
molecules, to pass by osmosis unless osmotic
pressure is applied. The osmotic pressure cell
pictured here allows water molecules to pass from
the low ion concentration side (left) through the
semi permeable membrane to the higher ion
concentration (right). The pressure gauge shows
the increase in pressure caused by the water
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Osmotic Pressure
•
Methods for the determination of osmotic
pressure are
1. Pfeffer’s method
2. Freezing point determination method.
Decrease in freezing point of the solution
when its osmotic pressure is equal to one
osmole.
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Osmotic Pressure
• Laws of Osmotic Pressure :
1. The Osmotic Pressure is directly
proportional to the concentration of the
solute.
2. The Osmotic Pressure is directly
proportional to the absolute
temperature.
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Osmotic Pressure
•
Importance of osmotic pressure of plasma
proteins:
1. The plasma proteins form a colloidal solution
and are the chief colloid of the plasma.
2. The oncotic pressure of the plasma proteins is
the main force which tends to keep the plasma
water within the blood vessels.
3. If concentration of plasma proteins decreases,
water will leak into tissue spaces and will lead to
development of edema.
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