Download Osmotic pressure - Lectures For UG-5

Diffusion, Osmosis and Osmotic
Pressure
Lecture 2
27.11.2012
Diffusion
• Molecules are in continuous random motion
(Brownian motion)
• Evident mostly in liquids and gases whose
molecules are free to move
• Greater the concentration of molecules greater
the likelihood of collision and movement to
chamber with low concentration
Factors on which rate of diffusion
depends (Fick’s Law of Diffusion)
• Concentration gradient
• Area of the membrane
• Molecular weight of the substance:
Lighter molecules such as O2 and CO2 bounce
further on collision than do heavier molecules.
Consequently, oxygen and CO2 diffuse rapidly
permitting rapid exchange of these gases across
the respiratory membranes
Factors on which rate of diffusion depends
(Fick’s Law of Diffusion)
• Permeability of the membrane to substance to
be transported
Factors on which rate of diffusion
depends (Fick’s Law of Diffusion)
• The distance through which diffusion must take place:
The greater the diffusion barrier thickness, slower the rate
of diffusion. Biological membranes are thin to allow
faster diffusion of molecules
• Temperature:
Kinetic energy increases with temperature and thus
diffusion rate
Factors on which rate of diffusion
depends (Fick’s Law of Diffusion)
OSMOSIS
• Diffusion of water through the semi permeable
membrane from a solution of lower concentration
towards a solution of higher concentration
Growth in
a shoot tip
Fresh water trout
wilting
Sea water Herring
Turgid plant
Physics of Diffusion & Osmosis
• Particles tend to move from higher chemical
potential to lower chemical potential
• Chemical potential is a form of potential
energy of a system that can be absorbed or
released during a chemical reaction
Physics of Osmosis
• Water tends to flow from where its
chemical potential is higher to where it is
lower
OR
• Movement of water from region where net
hydrostatic pressure is higher to a region
where it is low across a semi permeable
membrane
• Solute decreases chemical potential of water. Reduced
chemical potential causes reduced vapor pressure and
thus higher boiling point of the solution as compared
with pure water
• Movement of molecules from higher chemical
potential to lower chemical potential is accompanied
by a release of free energy.
• Osmosis releases energy and can be made to do work,
as when a growing tree root splits a stone.
Osmotic Pressure and the factors on
which it depends: Van’t Hoffs Equation
• All non penetrable solutes in a solution exerts
osmotic pressure
• According to Van’t Hoff, osmotic pressure (π)
depends on the molar concentration (C) of the
solution and the temperature T
π = R C T where R is the gas constant
Osmotic pressure
• Osmotic pressure is higher when molar
concentration is higher or temperature is higher
and the molecular weight is lower
• Osmotic pressure depends mainly on the molar
concentration or molarity of a solution
• Osmotic pressure is a colligative property,
meaning that the property depends on the
concentration of the solute but not on its identity
Osmotic pressure
• The osmotic pressure of an ionic solution is
π=iRCT
where “i” is the number of ions formed by
dissociation per molecule
• The greater the no of ion/molecule when
dissolved greater the osmotic pressure
Osmolarity/Osmolality
• To describe the total number of osmotically active
particles per litre of solution term osmolarity is used
• Two solutions can have the same molarity but may
have different osmolarities. E.g.
OsM of 1 M glucose solution =1 OsM
OsM of 1 M NaCl solution = 2 OsM
• The higher the osmolarity, the greater the osmotic
pressure of the solution.
Pressures of a solution
• Osmotic pressure (the pulling pressure) of a
solution is the measure of tendency of a solution to
pull water into it by osmosis because of the relative
concentration of non penetrating solute and water
• Hydrostatic pressure of a solution is the pressure
exerted by a stationary fluidic part of the solution on
an object (semi permeable membrane in case of
osmosis)
• Net hydrostatic pressure of a solution = hydrostatic
pressure –osmotic pressure
Example
• Separate pure water
from a sugar solution
with semi permeable
membrane
• Both have same
hydrostatic pressure
• Osmosis take water
from side 1 to side 2
because solution on
side 2 has greater
pulling tendency
Osmosis: due to difference in net
hydrostatic pressure
• The hydrostatic pressure of pure water is higher than that of
solution on right
• Will all water go to side 2?
• No it stops after some time. This is the
equilibrium state
Equilibrium state
• As water moves by osmosis to
side 2.
• Solution on side 2 has two
tendencies now
• Tendency to push water back to
side 1 due to greater hydrostatic
pressure
• Tendency to pull water by
osmosis back to side 2
• Equilibrium is achieved when
tendency to pull water to side 1
and to push water into side 2
balances out
Reference book
• Chapter 3 of Human Physiology: From
Cells to Systems by Lauralee Sherwood