Lab Report

Osmosis is the passage of water from a weak solution to a strong solution through a semi permeable membrane.

A semi-permeable membrane is otherwise known as a selectively permeable membrane. It is a type of barrier that will allow the passage of some molecules but not others. In osmosis, water passes from one side of this membrane to the other. The water will move only in one direction--it will move from the side where the water is in highest concentration to the other side of the membrane. The liquid that the water moves into is a strong solution, so the water is acting to dilute the solution. If one side of the membrane has a solution of sugar and the other side has water then the water will flow into the sugar solution, diluting it. The pore size of the membrane is such that water molecules can readily move through but the larger solute molecules cannot. The water diffuses from one side to another until the concentration is the same on both sides of the membrane. Once the concentration of the solute and the water is the same on both sides the process of osmosis stops. Osmosis will occur only across a diffusion gradient.

Osmosis is a passive process as opposed to an active one. Osmosis requires no energy to drive it. Osmosis generally requires a living cell membrane but it will also happen with suitable non-living membranes such as Visking dialysis tubing.

All solutions have an osmotic potential. The osmotic potential is the amount of net movement that can occur when the solution is compared to pure water. The osmotic pressure of a solution is the pressure that has to be exerted to halt osmosis.

Osmosis occurs in natural systems in cells and in organs. In humans, osmosis occurs in the kidneys to recover the water form waste materials of the body. In plants, osmosis occurs for example at root hairs, allowing the uptake of water from the soil. Individual cells can prevent water rushing into them by osmoregulation. If water were to rush into cells unchecked then the cells would most likely burst. Plant cells are surrounded by a rigid cellulose cell wall. If there is a strong solution external to the cell, water will flow out from the cell into the strong solution. This will result in the cytoplasm and the cell membrane pulling away from the cell wall, a process known as plasmolysis. At this point, the cell is incapable of supporting weight; it is nearly empty of water and it is said to be flaccid. If this situation occurs for too long the cell will dry out and die, but this does not frequently occur in nature. When there is a weaker solution outside the cell water will flow into the cell, allowing the cell to become more supportive. There will be a greater pressure inside the cell and it will become turgid. If water continues to flow into the cell, the cell becomes increasingly turgid. Eventually, as the cell presses against the rigid cell wall, osmosis slows down and eventually stops. This is due to the pressure exerted by the wall of the plant cell. This is called wall pressure and it halts osmosis. A fully turgid cell gives the maximum support for the plant. Generally, in a healthy plant the cells alternate between being flaccid and fully turgid.

Similar reactions can be seen in animal cells although they lack the rigid cell wall. In some simple organisms, such as amoeba, water is actively excreted from the cell in vacuoles. Red blood cells will burst if placed in fresh water and if they are placed in too strong a solution they will shrink. The kidneys regulate the concentration of water in the blood plasma.

If a cell is surrounded by water, the water will flow into the cell. The osmotic pressure of the external solution is lower than that of the internal solution. In this situation, the external solution is said to be hypotonic to the cell. If water flows from the cell into the stronger solution outside, the external solution is said to be hypertonic. If both solutions are of the same concentration and there is no net flow of water then the solutions are said to be isotonic to each other. The flow of water into a cell is endomosis, the flow out is exomosis.


Osmosis

Osmosis is a process by which a solvent (the liquid that dissolves another substance) in solution passes through a barrier. The solvent may pass through the barrier, but the solute (the substance dissolved in the solvent) either does not go through it, or passes through much more slowly than the solvent. The solvent will pass through the barrier until the concentration of solvent is the same on both sides of the barrier. The barrier is a membrane that is either permeable, allowing solvent and solute molecules to pass through, or semipermeable, allowing only solvent molecules to pass through. The pressure of the water passing through the membrane is called osmotic pressure.


The process was first investigated by a French physicist, Abbé Jean Antoine Nollette (1700-1770), in 1748. Nollette covered a glass tube containing sugar water with a piece of paper. He placed the tube, paper end down, into the water. The level of liquid in the tube rose. The pure water passed through the paper faster than the sugar water could. More experiments on osmosis followed. René Dutrochet (1776-1847) investigated the phenomena in the 1820s and 1830s. In the 1840s and 1850s Thomas Graham and Justus von Liebig researched osmosis but could not develop a suitable theory to explain it. Graham did distinguish between those substances that passed through parchment, which he called crystalloids, and those that did not, which he called colloids. Graham's additional research led to the process of dialysis, used today in artificial kidney machines.

The next major advance in the field came in 1877 when Wilhelm Pfeffer (1845-1920), a German botanist, studied osmotic pressure. Again the test subject was sugar water. The sugar solution was placed in a porous clay vessel, which in turn was placed in a container filled with pure water. Using a manometer Pfeffer measured the osmotic pressure and discovered was inversely proportional to the volume of a solution and directly proportional to absolute temperature or PV = kT, where P is pressure, V is volume, and T is absolute temperature. The constant k was later used in defining the universal gas constant by Jacobus Henricus Van't Hoff and was used in other gas laws. It was determined that the osmotic pressure a solute displays is the same pressure it would exert as a gas at the same volume and temperature.

Hugo van Mohl (1805-1872), a German botanist, continued on Pfeffer's path and was the first to describe cell division. He also provided the first lucid explanation for osmosis. Pfeffer's clay pot was a semipermeable membrane, as are the membranes surrounding most animal and vegetable cells. Studies in osmosis led to studies in cell physiology and solution purification.

Reverse osmosis is the process of applying a pressure greater than osmotic pressure on a solution. This reverses the process. In the end, a water-sugar water system would consist of pure water on one side of the barrier and a concentrated solution of sugar water on the other. This method is sometimes used for desalination, the process of removing salt from salt water to make it potable. The method is also used by hikers to remove harmful microorganisms from stream and lake water. Reverse osmosis is sometimes referred to as ultra-or hyper-filtration. The process is used to purify many liquids from milk to polio vaccines. Recently osmosis has been used to dehydrate fruit. Fruits slices are blanched; a sugar solution is pumped over the fruit, which is kept at 140°F (60°C) and the fruit slices lose most of their component water. They are then rinsed and dried briefly. The total process is faster than conventional dehydration, which takes about seven hours.



osmosis
(from Greek, othismos: impulse) Osmosis is a term describing the movement of molecules in SOLUTION; osmotic is the adjectival form. Osmosis is particularly important in regard to the movement of molecules across a SEMIPERMEABLE MEMBRANE (that is, any membrane that is permeable to certain molecules but not others). A small amount of a solute (plain salt, sodium chloride [NaCl] for example) added to a solvent (water in a beaker for example) will, even without stirring, DIFFUSE and come to a more-or less even concentration throughout the solution. This represents osmotic movement. Of more interest is the situation that develops—OSMOTIC PRESSURE—when a semipermeable membrane is present.


If there was a semipermeable membrane dividing the beaker into two equal halves and NaCl was added to only one half, then for a brief period there would be NaCl solution on one side of the semipermeable membrane and only water on the other. Because molecules will always try to move down a CONCENTRATION GRADIENT (that is from a high concentration to a low one) water molecules are drawn across the membrane from the pure water side to the salty water side—that is from a high concentration (only water present) to a lower concentration (water and NaCl molecules present). The molecules of NaCl would make the reverse journey, though again, one that took molecules down a concentration gradient. The movement of solutes across membranes—or more properly, the force that solutes bring to bear on membranes—is known as osmotic pressure. (Note this is not the same thing as HYDROSTATIC PRESSURE: if one had a beaker with a semipermeable partition and poured 100 ml of water into one half and 50 ml into the other, very quickly the volume of water on either side would equalize at 75 ml each side. This is because of hydrostatic pressure: the pressure exerted by water, in this case, by the pull of gravity.)

osmosisThe process whereby a SOLVENT diffuses from a lower concentration solution, through a SEMI-PERMEABLE MEMBRANE to a higher concentration, thereby balancing the concentrations on either side.


The key component of osmosis is the semi-permeable membrane, so called because it will permit the passage of the solvent but not the passage of the substances dissolved in it. Over time the phenomenon of osmosis will (unless influenced by other factors) tend to ensure that the solutions on either side of the membrane are of equal concentrations.