1. Definition: Osmolarity approximately equals the sum of the molarities of the particles in solution. The more precise definition is based on the force exerted to induce water flow. Examples:
a. a 0.1 M glucose has an approximate osmolarity of 0.1 OsM.
b. a 0.3 M KCl has an approximate osmolarity of 0.6 OsM because the KCl will dissociate into 0.3 M K+ and 0.3 M Cl-.
c. a 0.4 M MgCl2 has an approximate osmolarity of 1.2 OsM because the MgCl2 will dissociate into 0.4 M Mg++ and 2*0.4 M Cl -.
d. a solution containing 0.1 M glucose, 0.3 M KCl, and 0.4 M MgCl2 has an approximate osmolarity of 1.9 OsM (0.1 + 0.6 + 1.2).
2. Experimentally determined rule: Water flows from a low osmolarity to a high osmolarity at a rate directly proportional to the difference (gradient) in osmolarity:
PH2O is the water permeability coefficient across the membrane between compartment A and compartment B;
A is the surface area of the membrane.
Not that the rate becomes zero when the gradient becomes zero (that is, the osmolarities on each side of the membrane are equal). Obviously systems tend toward such an equilibrium.
The phenomenon of water osmotic flow is well determined (by the equation above), but the reasons that osmosis occurs is poorly understood (something to do with the mucking and stability of hydrogen bonds between water molecules).
Examples:
a. If a 10 µL cell is losing water at the rate of 0.2 µL per minute and the cell has an osmolarity of 0.8 OsM and it is suspended in a 90 µL solution of 0.7 M NaCl, then what is the rate of water flow if the cell is 0.4 OsM?
Initially the cell is 0.8 OsM and the surrounding environment is 1.4 OsM (the NaCl disassociates into two particles) thus the initial gradient is 1.4 - 0.8 OsM = 0.7 OsM.
If the cell is 0.4 OsM then the gradient becomes 1.4 - 0.4 = 1.0 OsM. The gradient is now 1.0/0.7 = 1.43 times larger thus the rate should be 1.43 times larger. Thus the new rate is 1.43 * 0.2 µL per minute = 0.286 µL per minute
b. then what will be the equilibrium concentrations when the water flow stops.
The total volume of the system is 90 µL + 10 µL = 100 µL and can't change.
The total moles of solute can't change either and is equal to
0.8 Osmoles/Liter * 10 µL + 1.4 Osmoles/Liter * 90 µL =
8 µOsmoles + 126 µOsmoles = 134 µOsmoles.
Thus the equilibrium concentration is when this number of Osmoles are equally distributed through out the entire volume =
134 µOsmoles / 100 µL = 1.34 µOsM.
3. Experimentally determined rule: Cell membranes are typically very permeable to water. The exceptions are rare but very important (cells impermeable to water can be found in kidneys and salt glands).
4. Experimentally determined rule: Cell membranes are typically mostly impermeable to ions (although that impermeability is not zero and the are some important consequences because it is not zero). Ions primarily get across membranes by facilitated diffusion or active transport.
5. Definitions:
isoosmotic = the osmolarity of two solutions are equal.
hyporosmotic = the solution that is hyporosmotic has a lower osmolarity than the solution to which it is being compared. A hypoosmotic solution has a tendency to gain water.
hyperosmotic = the solution that is hyperosmotic has a higher osmolarity than the solution to which it is being compared. A hyperosmotic solution has a tendency to lose water.
Two solutions that are isoosmotic may still have a subsequent flow of water from one to the other. For example consider a 1 M solution of glucose separated by a membrane permeable only to glucose from a 1 M solution of urea. Even though the two solutions are initially isoosmotic, there will be a subsequent flow of water from the glucose solution into the urea solution because the glucose will move down its concentration gradient via diffusion to the urea side thus increasing the osmolarity on the urea causing water to flow in that direction. Thus, our two original solutions would be regarded as isoosmotic but the glucose solution would be hypertonic to the urea solution. Tonicity looks at the future while osmolarity looks at the present condition.
isotonic = two solutions where there is no subsequent flow of water between them.
Notice that osmolarity only depends upon the osmolarity of two solutions while tonicity additionally depends upon the kind of chemicals that make up the solutions and the nature of the separating membrane.
6. Definitions:
osmoconformer = the osmolarity of the animal changes with the osmolarity of the surrounding environment.
osmoregulator = the osmolarity of the animal is held at a constant level independent of the surrounding osmolarity.
7. Experimentally determined rule: The rate of diffusion of any molecule is directly proportional to the concentration gradient of that molecule:
where Px is the permeability coefficient of X to the membrane.
The gradient (subtract the two concentrations) for the situation on the right is three times the gradient on the left, therefore the rate of flow will be three times higher for the situation on the right.
8. Experimentally determined 'general rule of thumb': Measured osmolaritites of plasmas of animals are usually around 250 to 350 mOsM.
Is this the best osmolarity for living reactions?
mOsM Na+ (mM) Cl- (mM) Ocean 1000 450 540 Fresh water 1 0.08 0.05 Goldfish 293 142 107 Flounder 337 180 160 Alligator 278 140 111 Snake 330 160 140 Dove 372 176 136 Mammal 370 269 258 Shark 1075 269 258 The cytoplasm of cells is absolutely equal in osmolarity to that of plasma, but the concentration of Na+ inside the cells is much lower and the concentration of K+ is much higher.