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1 Osmosis Osmosis means passage of solvent molecules from lower to higher concentration of the solute through a semipermeable membrane, which is permeable to the solvent and impermeable to the solute.] Osmotic pressure It is the hydrostatic pressure needed to prevent osmosis i.e. it is the pressure that should be exerted on the solution to prevent the passage of solvent separated from it by semipermeable membrane. Factors affecting osmotic pressure 1- Osmotic pressure is directly proportional to the concentration of solute. 2- It is inversely proportional to the volume. 3- It is directly proportional to temperature. 4- It is directly proportional to the number of particles in the solution. Biological significance of osmotic pressure 1- Urine formation Inside the glomerular capillaries, there are 2 opposing forces: A- Filtration force caused by capillary blood pressure. It equals +35 mm mercury (Hg) B- Reabsorptive force caused by the osmotic pressure of plasma proteins (-20 mmHg). The net filtration pressure = +35 - 20 = +15 mm Hg. 2- Formation and reabsorption of interstitial fluid Interstitial fluid is formed by filtration of blood plasma at the arterial end of the blood capillaries and reabsorbed at the venous end of the capillaries. 3- Haemolysis RBCs are isotonic with 0.9% NaCl solution. If the RBCs are put in hypertonic solution, they will loose water, shrink and become crenated. If the RBCs are put in hypotonic solution they will absorb water, swell and rupture i.e. they become haemolyzed. 4- Maintenance of blood volume by the osmotic pressure of plasma proteins Osmosis Solvent Colloidal particles Semipermeable membrane

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Page 1: Osmosis - Mansosp.mans.edu.eg/medbiochem_mi/cources/biochemistry/1st_year... · 1 Osmosis Osmosis means passage of solvent molecules from lower to higher concentration of the solute

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Osmosis Osmosis means passage of solvent molecules from lower to higher concentration of the solute through a semipermeable membrane, which is permeable to the solvent and impermeable to the solute.] Osmotic pressure It is the hydrostatic pressure needed to prevent osmosis i.e. it is the pressure that should be exerted on the solution to prevent the passage of solvent separated from it by semipermeable membrane. Factors affecting osmotic pressure

1- Osmotic pressure is directly proportional to the concentration of solute.

2- It is inversely proportional to the volume. 3- It is directly proportional to temperature. 4- It is directly proportional to the number of particles in the

solution. Biological significance of osmotic pressure 1- Urine formation Inside the glomerular capillaries, there are 2 opposing forces:

A- Filtration force caused by capillary blood pressure. It equals +35 mm mercury (Hg)

B- Reabsorptive force caused by the osmotic pressure of plasma proteins (-20 mmHg).

The net filtration pressure = +35 - 20 = +15 mm Hg.

2- Formation and reabsorption of interstitial fluid Interstitial fluid is formed by filtration of blood plasma at the arterial end of the blood capillaries and reabsorbed at the venous end of the capillaries. 3- Haemolysis RBCs are isotonic with 0.9% NaCl solution. If the RBCs are put in hypertonic solution, they will loose water, shrink and become crenated. If the RBCs are put in hypotonic solution they will absorb water, swell and rupture i.e. they become haemolyzed. 4- Maintenance of blood volume by the osmotic pressure of plasma proteins

Osmosis

Solvent

Colloidal particles

Semipermeable membrane

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Concentration =

5- Magnesium sulphate is used as laxative as it absorbs water by its osmotic pressure forming bulky stools. Measurement of osmotic pressure We can measure osmotic pressure by:

1- Direct means by osmometer.

2- Indirect means by measuring changes in the boiling and freezing points of a solvent after addition of the solution, which we want to measure its osmotic pressure.

Expression Of Concentration The concentration of a solution is defined as the amount of solute dissolved in a given amount of solvent or solution. There are many ways in which the concentration of a solution can be described. 1- The molarity (M) of a solution is defined as the number of

moles of solute in the solution divided by the volume of the solution in liters.

Molar solution is a solution, which contains the molecular weight of the solute in grams dissolved in one liter of the solution. Usually the concentration is expressed as a fraction of the mole e.g. Millimole (mM) = 1/1000 mole Micromole (υM) = 1/1000,000 mole Nanomole (nM) = 1/1000,000,000 mole

Column of Mercury

Solution

Distilled water

Semipermeable membrane

Osmometer

Amount of solute

Amount of

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Molarity is the concentration unit most commonly used by chemists. It has one disadvantage. It tells us how much solute we need to make a solution, and it gives us the volume of the solution produced, but it doesn't tell us how much solvent will be required When it is important to know how much solute and solvent are present in a solution, we use two other concentration units; molality and mole fraction. 2- The molality (m) of a solution is defined as the number of moles of solute in the solution divided by the mass in kilograms of the solvent. Notice that since the molality of a solution is determined by the masses of solute and solvent, it is temperature independent. Molality has an important advantage over molarity. The molarity of an aqueous solution changes with temperature, because the density of water is sensitive to temperature. Because molality is defined in terms of the mass of the solvent, not its volume, the molality of a solution does not change with temperature. Molal solution is a solution, which contains the molecular weight of the solute in grams dissolved in one kilogram of the solvent. 3- Normality (N): the normality of a solution is defined as the number of equivalents of solute per liter of solution. The equivalent equals the molecular weight in grams divided by the valence. Normal solution is a solution, which contains the equivalent weight of the solute in grams dissolved in one liter of the solution.

4- Mass percent is literally the percentage of the total mass of a solution that is due to the solute. A 3.5% solution of hydrochloric acid, for example, has 3.5 grams of HCl in every 100 grams of solution..

5- Volume percent It is also possible to describe the concentration of a solution in terms of the volume percent. This unit is used to describe solutions of one liquid dissolved in another or mixtures of gases. 6- Mole fraction (Χ) The ratio of solute to solvent in a solution can also be described in terms of the mole fraction of the solute or the solvent in a solution. Mole fraction has no units.

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• The mole fraction of a solute (Χ Solute ) in a solution is equal to the number of moles of solute divided by the total number of moles in the solution (moles of solute plus moles of solvent.) If the solution consists of more than one solute, then the denominator is the sum of the moles of all components.

• The mole fraction of a solvent (Χ Solvent) is the number of moles of solvent divided by the total number of moles of solute and solvent.

• Mole percentage is simply mole fraction multiplied by 100.

7- Parts per million (ppm) A unit of concentration often used when measuring levels of pollutants in air, water, body fluids, etc. One ppm is 1 part in 1,000,000. The common unit mg/liter is equal to one ppm. N.B. :

• The weight of a molecule is the sum of the weights of the atoms of which it is made.

• The unit of weight is the dalton (da) which equals one-twelfth (1/12) of the weight of an atom of 12C. Thus the molecular weight (MW) of water is 18 daltons and that of glucose is 180 daltons.

• The mole is the quantity of a substance whose weight in grams is equal to the molecular weight of the substance. Thus 1 mole of glucose weighs 180 g. Furthermore, if you dissolve 1 mole of a substance in enough water to make 1 liter (L) of solution, you have made a 1 molar (1 M) solution.