Download Colligative properties of biological liquids

Colligative properties of biological
liquids
Colligative properties are properties of solutions that depend on
the number of molecules in a given volume of solvent and not on
the properties (e.g. size or mass) of the molecules.
Colligative properties include: lowering of vapor pressure;
elevation of boiling point; depression of freezing point and osmotic
pressure. Measurements of these properties for a dilute aqueous
solution of a non-ionized solute such as urea or glucose can lead to
accurate determinations of relative molecular masses. Alternatively,
measurements for ionized solutes can lead to an estimation of the
percentage of ionization taking place. Colligative properties are
mostly studied for dilute solutions.
Vapor pressure
The relationship between the lowering of vapor pressure
and concentration is given by Raoult's law, which states that:
The vapor pressure of an ideal solution is dependent on the
vapor pressure of each chemical component and the mole
fraction of the component present in the solution.
The individual vapor pressure p for each component is
p = p0 x ,
where p is the partial pressure of the component in
mixture, p0 is the vapor pressure of the pure component,
x is the mole fraction of the component in solution.
Boiling point elevation
Boiling point is achieved in the establishment of equilibrium
between liquid and gas phase. At the boiling point, the number of
gas molecules condensing to liquid equals the number of liquid
molecules evaporating to gas. Adding any solute effectively
dilutes the concentration of the liquid molecules, slowing the
liquid to gas portion of this equilibrium. To compensate for this,
boiling point is achieved at higher temperature.
ΔTb = molality * Kb * i,
(Kb = ebullioscopic constant, which is 0.51°C kg/mol for the boiling
point of water; i = Van 't Hoff factor)
Van't Hoff factor is the actual number of particles in solution
after dissociation : i=1+α(n-1)
Freezing point depression
Freezing point, or the equilibrium between a liquid and
solid phase is generally lowered in the presence of a solute
compared to a pure solvent.
ΔTf = molality * Kf * i,
(Kf = cryoscopic constant, which is 1.86°C kg/mol for the
freezing point of water,; i = Van 't Hoff factor)
Osmosis
Osmosis is the movement of water across a partially permeable
membrane from an area of low solute concentration to an area of
high solute concentration.
A semipermeable membrane
is a thin layer of material that
contains various sized holes,
or pores. Smaller solutes and
fluid pass through the
membrane, but the membrane
blocks the passage of larger
substances (for example, red
blood cells, large proteins)
Osmotic pressure
Osmotic pressure is the pressure applied by a solution to prevent
the inward flow of water across a semipermeable membrane. The
phenomenon of osmotic pressure arises from the tendency of a pure
solvent to move through a semi-permeable membrane. This process
is of vital importance in biology as the cell's membrane is selective
towards many of the solutes found in living organisms.
Osmoregulation is the homeostasis mechanism of an organism to
reach balance in osmotic pressure.
Morse equation:
πV = nRTi,
where: π = osmotic pressure; V is the volume; T is absolute
temperature; n is the number of moles of solute; R = 8.3145 J K-1 mol-1,
the molar gas constant; i = Van 't Hoff factor.
Osmoregulation in mammalian cells
Ion
Concentration in
cytosol (millimolar)
Concentration in
blood (millimolar)
Potassium
139
4
Sodium
12
145
Cloride
4
116
Bicarbonate
12
29
Amino acids in
proteins
138
9
Magnesium
0.8
1.5
<0.0002
1.8
Calcium
The loss of sodium and chloride ions compensates for the osmotic
effect of the higher concentration of organic molecules inside the cell.
Balance in osmotic pressure
Hypertonic solution causes cells to shrink.
Hypotonic solution causes cells to swell.
Isotonic solution produces no change in cell volume.
Dialysis
In medicine, dialysis is primarily used to provide an artificial
replacement for lost kidney function in people with renal failure.
Substances in water tend to
move from an area of high
concentration to an area of low
concentration. Hemodialysis
remove wastes and excess water
from the blood.
A hemodialysis machine