Osmosis, Osmotic pressure and Molecular processes

Introduction All molecules have random motion because of their kinetic energy. Gases have a large amount of random motion, liquids and solids have less random motion because of the large attractive force between their molecules. The random motion of molecules in a liquid is often called brownian movement because Scottish botanist Robert Brown first described this phenomenon.

Diffusion DIFFUSION: the process by which molecules intermingle as a result of their random motion or the movement of molecules from an area of high concentration to an area of low concentration. Since diffusion is the result of molecular motion, anything which increases molecular motion will increase diffusion, ie: heating and stirring. Example: oxygen and carbon dioxide in the lungs.

Solvents and Solutes When describing the characteristics of solutions, the following terms are used: A. Solvent: a liquid in which another substance is dissolved. B. Solute: a substance that is dissolved in a liquid. 0p4 0p4

Solvents There are two important parts of a solution. The liquid used to make the solution is called the solvent. The best known solvent of all is water. Because of its unique properties, water dissolves a remarkable number of other substances – gases, liquids and solids. Solutions that use water as a solvent are known as aqueous solutions. There are many other liquids that can be solvents, however. These are called non-aqueous solvents and include methylated spirits, nail-polish remover (mainly propanone) and paint stripper. Many glues, paints and varnishes have non-aqueous solvents which have a strong smell.

Solutes The substance that is dissolved in a solvent is called the solute The solute is commonly a solid, but it can also be a gas or a liquid. Substances that dissolve in a particular solvent are designated as soluble, while substances that do not dissolve are insoluble, in that particular solvent. For example, salt is soluble in water but insoluble in petrol, while wax is insoluble in water but soluble in petrol. Even when a substance is soluble in a solvent, only a certain amount will dissolve in a fixed quantity of that solvent. When the point is reached where no more solute will dissolve, the solution is saturated.

Solution Solution is a homogeneous mixture. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. The solvent does the dissolving. The solution more or less takes on the characteristics of the solvent including its phase, and the solvent is commonly the major fraction of the mixture. The concentration of a solute in a solution is a measure of how much of that solute is dissolved in the solvent.

Dissolving Gases and Liquids Examples of dissolved gases are fizzy drinks, swimming pool water and tap water. Swimming pool water and tap water have small quantities of chlorine gas, to help kill germs. Fizzy drinks have carbon dioxide dissolved in them, and are put under pressure to make as much as possible dissolve. When the cap is taken off a carbonated drinks bottle, the pressure is released and bubbles are formed in the drink – which are bubbles of carbons dioxide coming out of solution. Liquids too, can be dissolved in other liquids. For example, adding detergent to water creates a solution of the detergent. Liquids which dissolve like this are called miscible liquids, liquids which do not mix, like oil and water are called immiscible.

Semi-permeable Membrane a membrane which allows the passage of solvent but not solute molecules. type of biological membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion” The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size, solubility, properties, or chemistry. How the membrane is constructed to be selective in its permeability will determine the rate and the permeability.

Semi-permeable Membrane An example of a semi-permeable membrane is the lipid bilayer, on which is based the plasma membrane that surrounds all biological cells. A group of phospholipids arranged into a double-layer, the phospholipid bilayer is a semipermeable membrane that is very specific in its permeability.

Osmosis The diffusion of the molecules of a solvent across a semi-permeable membrane while the molecules of the solute are left behind. It is important to remember the solvent molecules are diffusing from high to low concentration, the solute molecules cannot diffuse through the membrane.

Osmotic Pressure The pressure required to stop movement of pure solvent across the membrane. Example: The osmotic pressure of 1% NaCl would be that pressure which would stop diffusion of pure water through a membrane to a solution of 1% NaCl. The greater the osmotic pressure, the faster water will dilute the solution. Ex: Plasma

Osmotic Pressure ISOTONIC: an isotonic solution has the same osmotic pressure as the reference solution, ie: 0.9% NaCl or normal saline is isotonic compared to tissue, so normal saline would not cause water to move into or out of tissue cells via osmosis. HYPERTONIC: a hypertonic solution has a greater osmotic pressure than the reference solution, ie: 3% NaCl is hypertonic compared to tissue, so 3% NaCl would cause water to move out of tissue cells via osmosis.

Osmotic Pressure HYPOTONIC: a hypotonic solution has a smaller osmotic pressure than the reference solution, ie: 0.45% NaCl or 1/2 normal saline is hypotonic compared to tissue, so 0.45% NaCl would cause water to move into the tissue cells via osmosis.

Diffusion of solvents A solvent diffuses through a semi-permeable membrane toward the: 1. the hypertonic solution. 2. the solution with the greatest osmotic pressure 3. the solution with the greatest solute concentration.

Diffusion of solvents A solvent diffuses through a semi-permeable away from the: 1. the hypotonic solution. 2. the solution with the lowest osmotic pressure. 3. the solution with the lowest solute concentration.

Diffusion of solvents When two solutions are isotonic there is no net movement of solvent -- that means, an equal number of solvent molecules cross the membrane in both directions, so the concentration of the solute does not change.

Hypertonic Solution or Hypertonicicty When the osmotic pressure of the solution outside the blood cells in higher than the osmotic pressure inside the red blood cells, the solution is hypertonic. The water inside the blood cells exits the cells in an attempt to equalize the osmotic pressure, causing the cells to shrink. Isotonic Solution or Isotonicity When the osmotic pressure outside the red blood cells is the same as the pressure inside the cells, the solution is isotonic with respect to the cytoplasm. This is the usual condition of red blood cells in plasma. The cells are normal. Hypotonic Solution or Hypotonicity When the solution outside of the red blood cells has a lower osmotic pressure than the cytoplasm of the red blood cells, the solution is hypotonic with respect to the cells. The cells take in water in an attempt to equalize the osmotic pressure, causing them to swell and potentially burst.

Colloid A colloid is a substance microscopically dispersed throughout another substance A colloid is a solution that has particles ranging between 1 and 1000 nanometers in diameter, yet are still able to remain evenly distributed throughout the solution. These are also known as colloidal dispersions because the substances remain dispersed and do not settle to the bottom of the container. In colloids, one substance is evenly dispersed in another. The substance being dispersed is referred to as being in the dispersed phase, while the substance in which it is dispersed is in the continuous phase.

Albumin (biological colloid) Serum albumin is produced by the liver, occurs dissolved in blood plasma and is the most abundant blood protein in mammals. Albumin is essential for maintaining the oncotic pressure needed for proper distribution of body fluids between blood vessels and body tissues; without albumin, the high pressure in the blood vessels would force more fluids out into the tissues. It also acts as a plasma carrier by non- specifically binding several hydrophobic steroid hormones and as a transport protein for hemin and fatty acids. Too much or too little circulating serum albumin may be harmful.