Lecture 2

27.11.2012

Diffusion, Osmosis and Osmotic Pressure

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

Fresh water trout Sea water Herring

wilting Turgid plant

Growth in a shoot tip

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 = i R C T

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

• 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

Equilibrium state

Reference book

• Chapter 3 of Human Physiology: From Cells to Systems by Lauralee Sherwood