Applications of Osmosis and Reverse Osmosis

Osmosis is the movement of a solvent, such as water, through a semi-permeable membrane. A semipermeable membrane is a material that allows some materials to flow through it but not others. The reason that semipermeable membranes have this property is that they contain very small holes. Small molecules, such as those of water, can flow easily through the holes. But large molecules, such as those of solutes cannot. Osmosis always moves a solvent in one direction only, from a less concentrated solution to a more concentrated solution. As osmosis proceeds, pressure builds up on the side of the membrane where volume has increased. Ultimately, this pressure prevents more water from entering and osmosis stops. The osmotic pressure of a solution is the pressure needed to prevent osmosis from occurring.

Living cells may be thought of as very small bags of solutions contained within semipermeable membranes. For the cell to survive, the concentration of substances within the cell must stay within a safe range. A cell placed in a solution more concentrated than it (a hypertonic solution) will shrink due to loss of water. It may eventually die of dehydration. We can observe this effect with a carrot placed in salty water. Within a few hours the carrot becomes limp and soft because its cells have shriveled. By contrast, a cell placed in a solution more dilute than it (a hypotonic solution) will expand as water enters it. Under such conditions the cell may burst. In general, plant cells are protected from bursting by the rigid cell wall that surrounds the cell membrane. As water enters the cell, it expands until it pushes up tight against the cell wall. The cell wall pushes back with an equal pressure, so no more water can enter.

Osmosis contributes to the movement of water through plants. Solute concentrations increase going from soil to root cells to leaf cells. The resulting differences of osmotic pressure help to push water upward. Osmosis also controls the evaporation of water from leaves by regulating the size of the openings (stomata) in the leaves' surfaces. Organisms have various other methods for keeping their solute levels within safe range. Some cells live only in surroundings that are isotonic (have the same solute concentration as their own cells). For example, jellyfish that live in salt water have much higher salt-to-water solute concentrations than do freshwater creatures. Other animals continually replace lost water and solutes by drinking and eating. They remove excess water and solutes through excretion of urine.

Applications of osmosis are many and of varied nature. For thousands of years, perishable foods such as fish, olives, and vegetables have been preserved in salt or brine. The high salt concentration is hypertonic to bacteria cells, and kills them by dehydration before they can cause the food to spoil. Preserving fruit in sugar (as in jams or jellies) works on the same principle. People with kidney disease rely upon artificial kidney machines to remove waste products from their blood. Such machines use a process called dialysis, which is similar to osmosis. The difference between osmosis and dialysis is that a dialyzing membrane permits not just water, but also salts and other small molecules dissolved in the blood, to pass through. These materials move out of blood into a surrounding tank of distilled water. Red blood cells are too large to pass through the dialyzing membrane, so they return to the patient's body.

Oceans hold about 97 percent of Earth's water supply, but their high salt content makes them unusable for drinking or agriculture. Salt can be removed by placing seawater in contact with a semipermeable membrane, then subjecting it to great pressure. Under these conditions, reverse osmosis occurs, by which pressure is used to push water from a more concentrated solution to a less concentrated solution. The process is just the reverse of the normal process of osmosis. In desalination, reverse osmosis is used to push water molecules out of seawater into a reservoir of pure water.

The semi permeable membrane allows the passage of water, but not ions (e.g., Na+, Ca2+, Cl-) or larger molecules (e.g., glucose, urea, bacteria). Diffusion and osmosis are thermodynamically favorable and will continue until equilibrium is reached. Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the 'concentrated' side of the membrane. Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. To illustrate it we may imagine a semipermeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side. Reverse osmosis is often used in commercial and residential water filtration. It is also one of the methods used to desalinate seawater. Sometimes reverse osmosis is used to purify liquids in which water is an undesirable impurity (e.g., ethanol).