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Osmosis

OSMOSIS

CONCEPT

The term osmosis describes the movement of a solvent through a semipermeable membrane from a less concentrated solution to a more concentrated one. Water is sometimes called "the perfect solvent," and living tissue (for example, a human being's cell walls) is the best example of a semipermeable membrane. Osmosis has a number of life-preserving functions: it assists plants in receiving water, it helps in the preservation of fruit and meat, and is even used in kidney dialysis. In addition, osmosis can be reversed to remove salt and other impurities from water.

HOW IT WORKS

If you were to insert a hollow tube of a certain diameter into a beaker of water, the water would rise inside the tube and reach the same level as the water outside it. But suppose you sealed the bottom end of the tube with a semipermeable membrane, then half-filled the tube with salt water and again inserted it into the beaker. Over a period of time, the relative levels of the salt water in the tube and the regular water in the beaker would change, with the fresh water gradually rising into the beaker.

This is osmosis at work; however, before investigating the process, it is necessary to understand at least three terms. A solvent is a liquid capable of dissolving or dispersing one or more other substances. A solute is the substance that is dissolved, and a solution is the resulting mixture of solvent and solute. Hence, when you mix a packet of sugar into a cup of hot coffee, the coffeewhich is mostly wateracts as a solvent for the sugar, a solute, and the resulting sweetened coffee is a solution. (Indeed, people who need a cup of coffee in the morning might say that it is a "solution" in more ways than one!) The relative amount of solute in the solution determines whether it can be described as more or less concentrated.

Water and Oil: Molecular Differences

In the illustrations involving the beaker and the hollow tube, water plays one of its leading roles, as a solvent. It is possible to use a number of other solvents for osmosis, but most of the ones that will be discussed here are water-based substances. In fact, virtually everything people drink is either made with water as its central component (soft drinks, coffee, tea, beer and spirits), or comes from a water-based plant or animal life form (fruit juices, wine, milk.) Then of course there is water itself, still the world's most popular drink.

By contrast, people are likely to drink an oily product only in extreme circumstances: for instance, to relieve constipation, holistic-health practitioners often recommend a mixture of olive oil and other compounds for this purpose. Oil, unlike water, has a tendency to pass straight through a person's system, without large amounts of it being absorbed through osmosis. In fact, oil and water differ significantly at the molecular level.

Water is the best example of a polar molecule, sometimes called a dipole. As everyone knows, water is a name for the chemical H2O, in which two relatively small hydrogen atoms bond with a large atom of oxygen. You can visualize a water molecule by imagining oxygen as a basketball with hydrogen as two baseballs fused to the basketball's surface. Bonded together as they are, the oxygen tends to pull electrons from the hydrogen atoms, giving it a slight negative charge and the hydrogen a slight positive charge.

As a result, one end of a water molecule has a positive electrical charge, and the other end a negative charge. This in turn causes the positive end of one molecule to attract the negative side of its neighbor, and vice versa. Though the electromagnetic force is weak, even in relative terms, it is enough to bond water molecules tightly to one another.

By contrast, oily substanceswhether the oil is animal-, vegetable-, or petroleum-basedare typically nonpolar, meaning that the positive and negative charges are distributed evenly across the surface of the molecule. Hence, the bond between oil molecules is much less tight than for water molecules. Clean motor oil in a car's crankshaft behaves as though it were made of millions of tiny ball-bearings, each rolling through the engine without sticking. Water, on the other hand, has a tendency to stick to surfaces, since its molecules are so tightly bonded to one another.

This tight bond gives water highly unusual properties compared to other substances close to its molecular weight. Among these are its high boiling point, its surprisingly low density when frozen, and the characteristics that make osmosis possible. Thanks to its intermolecular structure, water is not only an ideal solvent, but its closely packed structure enables easy movement, as, for instance, from an area of low concentration to an area of high concentration.

In the beaker illustration, the "pure" water is almost pure solvent. (Actually, because of its solvent qualities, water seldom appears in a pure state unless one distills it: even water flowing through a "pure" mountain stream carries all sorts of impurities, including microscopic particles of the rocks over which it flows.) In any case, the fact that the water in the beaker is almost pure makes it easy for it to flow through the semipermeable membrane in the bottom of the tube. By contrast, the solute particles in the salt-water solution have a much harder time passing through, and are much more likely to block the openings in the membrane. As a result, the movement is all in one direction: water in the beaker moves through the membrane, and into the tube.

A few points of clarification are in order here. A semipermeable membrane is anything with a structure somewhere between that of, say, plastic on the one hand and cotton on the other. Were the tube in the beaker covered with Saran wrap, for instance, no water would pass through. On the other hand, if one used a piece of cotton in the bottom of the tube, the water would pass straight through without osmosis taking place. In contrast to cotton, Gore-tex is a fabric containing a very thin layer of plastic with billions of tiny pores which let water vapor flow out without allowing liquid water to seep in. This accounts for the popularity of Gore-tex for outdoor gearit keeps a person dry without holding in their sweat. So Gore-tex would work well as a semipermeable membrane.

Also, it is important to consider the possibilities of what can happen during the process of osmosis. If the tube were filled with pure salt, or salt with only a little water in it, osmosis would reach a point and then stop due to osmotic pressure within the substance. Osmotic pressure results when a relatively concentrated substance takes in a solvent, thus increasing its pressure until it reaches a point at which the solution will not allow any more solvent to enter.

REAL-LIFE APPLICATIONS

Cell Behavior and Salt Water

Cells in the human body and in the bodies of all living things behave like microscopic bags of solution housed in a semipermeable membrane. The health and indeed the very survival of a person, animal, or plant depends on the ability of the cells to maintain their concentration of solutes.

Two illustrations involving salt water demonstrate how osmosis can produce disastrous effects in living things. If you put a carrot in salty water, the salt water will "draw" the water from inside the carrotwhich, like the human body and most other forms of life, is mostly made up of water. Within a few hours, the carrot will be limp, its cells shriveled.

Worse still is the process that occurs when a person drinks salt water. The body can handle a little bit, but if you were to consume nothing but salt water for a period of a few days, as in the case of being stranded on the proverbial desert island, the osmotic pressure would begin drawing water from other parts of your body. Since a human body ranges from 60% water (in an adult male) to 85% in a baby, there would be a great deal of water availablebut just as clearly, water is the essential ingredient in the human body. If you continued to ingest salt water, you would eventually experience dehydration and die.

How, then, do fish and other forms of marine life survive in a salt-water environment? In most cases, a creature whose natural habitat is the ocean has a much higher solute concentration in its cells than does a land animal. Hence, for them, salt water is an isotonic solution, or one that has the same concentration of soluteand hence the same osmotic pressureas in their own cells.

Osmosis in Plants

Plants depend on osmosis to move water from their roots to their leaves. The further toward the edge or the top of the plant, the greater the solute concentration, which creates a difference in osmotic pressure. This is known as osmotic potential, which draws water upward. In addition, osmosis protects leaves against losing water through evaporation.

Crucial to the operation of osmosis in plants are "guard cells," specialized cells dispersed along the surface of the leaves. Each pair of guard cells surrounds a stoma, or pore, controlling its ability to open and thus release moisture.

In some situations, external stimuli such as sunlight may cause the guard cells to draw in potassium from other cells. This leads to an increase in osmotic potential: the guard cell becomes like a person who has eaten a dry biscuit, and is now desperate for a drink of water to wash it down. As a result of its increased osmotic potential, the guard cell eventually takes on water through osmosis. The guard cells then swell with water, opening the stomata and increasing the rate of gas exchange through them. The outcome of this action is an increase in the rate of photosynthesis and plant growth.

When there is a water shortage, however, other cells transmit signals to the guard cells that cause them to release their potassium. This decreases their osmotic potential, and water passes out of the guard cells to the thirsty cells around them. At the same time, the resultant shrinkage in the guard cells closes the stomata, decreasing the rate at which water transpires through them and preventing the plant from wilting.

Osmosis and Medicine

Osmosis has several implications where medical care is concerned, particularly in the case of the storage of vitally important red blood cells. These are normally kept in a plasma solution which is isotonic to the cells when it contains specific proportions of salts and proteins. However, if red blood cells are placed in a hypotonic solution, or one with a lower solute concentration than in the cells themselves, this can be highly detrimental.

Hence water, a life-giving and life-preserving substance in most cases, is a killer in this context. If red blood cells were stored in pure water, osmosis would draw the water into the cells, causing them to swell and eventually burst. Similarly, if the cells were placed in a solution with a higher solute concentration, or hypertonic solution, osmosis would draw water out of the cells until they shriveled.

In fact, the plasma solution used by most hospitals for storing red blood cells is slightly hypertonic relative to the cells, to prevent them from drawing in water and bursting. Physicians use a similar solution when injecting a drug intravenously into a patient. The active ingredient of the drug has to be suspended in some kind of medium, but water would be detrimental for the reasons discussed above, so instead the doctor uses a saline solution that is slightly hypertonic to the patient's red blood cells.

One vital process closely linked to osmosis is dialysis, which is critical to the survival of many victims of kidney diseases. Dialysis is the process by which an artificial kidney machine removes waste products from a patients' bloodperforming the role of a healthy, normally functioning kidney. The openings in the dialyzing membrane are such that not only water, but salts and other waste dissolved in the blood, pass through to a surrounding tank of distilled water. The red blood cells, on the other hand, are too large to enter the dialyzing membrane, so they return to the patient's body.

Preserving Fruits and Meats

Osmosis is also used for preserving fruits and meats, though the process is quite different for the two. In the case of fruit, osmosis is used to dehydrate it, whereas in the preservation of meat, osmosis draws salt into it, thus preventing the intrusion of bacteria.

Most fruits are about 75% water, and this makes them highly susceptible to spoilage. To preserve fruit, it must be dehydrated, whichas in the case of the salt in the meatpresents bacteria with a less-than-hospitable environment. Over the years, people have tried a variety of methods for drying fruit, but most of these have a tendency to shrink and harden the fruit. The reason for this is that most drying methods, such as heat from the Sun, are relatively quick and drastic; osmosis, on the other hand, is slower, more moderateand closer to the behavior of nature.

Osmotic dehydration techniques, in fact, result in fruit that can be stored longer than fruit dehydrated by other methods. This in turn makes it possible to provide consumers with a wider variety of fruit throughout the year. Also, the fruit itself tends to maintain more of its flavor and nutritional qualities while keeping out microorganisms.

Because osmosis alone can only remove about 50% of the water in most ripe fruits, however, the dehydration process involves a secondary method as well. First the fruit is blanched, or placed briefly in scalding water to stop enzymatic action. Next it is subjected to osmotic dehydration by dipping it in, or spreading it with, a specially made variety of syrup whose sugar draws out the water in the fruit. After this, air drying or vacuum drying completes the process. The resulting product is ready to eat; can be preserved on a shelf under most climatic conditions; and may even be powdered for making confectionery items.

Whereas osmotic dehydration of fruit is currently used in many parts of the world, the salt-curing of meat in brine is largely a thing of the past, due to the introduction of refrigeration. Many poorer families, even in the industrialized world, however, remained without electricity long after it spread throughout most of Europe and North America. John Steinbeck's Grapes of Wrath (1939) offers a memorable scene in which a contemporary family, the Joads, kill and cure a pig before leaving Oklahoma for California. And a Web site for Walton Feed, an Idaho company specializing in dehydrated foods, offers reminiscences by Canadians whose families were still salt-curing meats in the middle of the twentieth century. Verla Cress of southern Alberta, for instance, offered a recipe from which the following details are drawn.

First a barrel is filled with a solution containing 2 gal (7.57 l) of hot water and 8 oz (.2268 kg) of salt, or 32 parts hot water to one part salt, as well as a small quantity of vinegar. The pig or cow, which would have just been slaughtered, should then be cut up into what Cress called "ham-sized pieces (about 10-15 lb [5-7 kg]) each." The pieces are then soaked in the brine barrel for six days, after which the meat is removed, dried, "and put in flour or gunny sacks to keep the flies away. Then hang it up in a cool dry place to dry. It will keep like this for perhaps six weeks if stored in a cool place during the Summer. Of course, it will keep much longer in the Winter. If it goes bad, you'll know it!"

Cress offered another method, one still used on ham today. Instead of salt, sugar is used in a mixture of 32 oz (.94 l) to 3 gal (11.36 l) of water. After being removed, the meat is smokedthat is, exposed to smoke from a typically aromatic wood such as hickory, in an enclosed barnfor three days. Smoking the meat tends to make it last much longer: four months in the summer, according to Cress.

The Walton Feeds Web page included another brine-curing recipe, this one used by the women of the Stirling, Alberta, Church of the Latter-Day Saints in 1973. Also included were reminiscences by Glenn Adamson (born 1915): "When we butchered a pig, Dad filled a wooden 45-gal (170.34 l) barrel with salt brine. We cut up the pig into maybe eight pieces and put it in the brine barrel. The pork soaked in the barrel for several days, then the meat was taken out, and the water was thrown away. In the hot summer days after they [the pieces of meat] had dried, they were put in the root cellar to keep them cool. The meat was good for eating two or three months this way."

For thousands of years, people used salt to cure and preserve meat: for instance, the sailing ships that first came to the New World carried on board barrels full of cured meat, which fed sailors on the voyage over. Meat was not the only type of food preserved through the use of salt or brine, which is hypertonicand thus lethalto bacteria cells. Among other items packed in brine were fish, olives, and vegetables.

Even today, some types of canned fish come to the consumer still packed in brine, as do olives. Another method that survives is the use of sugarwhich can be just as effective as salt for keeping out bacteriato preserve fruit in jam.

Reverse Osmosis

Given the many ways osmosis is used for preserving food, not to mention its many interactions with water, it should not be surprising to discover that osmosis can also be used for desalination, or turning salt water into drinking water. Actually, it is not osmosis, strictly speaking, but rather reverse osmosis that turns salt water from the ocean97% of Earth's water supplyinto water that can be used for bathing, agriculture, and in some cases even drinking.

When you mix a teaspoon of sugar into a cup of coffee, as mentioned in an earlier illustration, this is a non-reversible process. Short of some highly complicated undertakingfor instance, using ultrasonic sound wavesit would be impossible to separate solute and solvent.

Osmosis, on the other hand, can be reversed. This is done by using a controlled external pressure of approximately 60 atmospheres, an atmosphere being equal to the air pressure at sea level14.7 pounds-per-square-inch (1.013 × 105 Pa.) In reverse osmosis, this pressure is applied to the area of higher solute concentrationin this case, the seawater. As a result, the pressure in the seawater pushes water molecules into a reservoir of pure water.

If performed by someone with a few rudimentary tools and a knowledge of how to provide just the right amount of pressure, it is possible that reverse osmosis could save the life of a shipwreck victim stranded in a location without a fresh water supply. On the other hand, a person in such a situation may be able to absorb sufficient water from fruits and plant life, as Tom Hanks's character did in the 2001 film Cast Away.

Companies such as Reverse Osmosis Systems in Atlanta, Georgia, offer a small unit for home or business use, which actually performs the reverse-osmosis process on a small scale. The unit makes use of a process called crossflow, which continually cleans the semipermeable membrane of impurities that have been removed from the water. A small pump provides the pressure necessary to push the water through the membrane. In addition to an under-the-sink model, a reverse osmosis water cooler is also available.

Not only is reverse osmosis used for making water safe, it is also applied to metals in a variety of capacities, not least of which is its use in treating wastewater from electroplating. But there are other metallurgical methods of reverse osmosis that have little to do with water treatment: metal finishing, as well as recycling of metals and chemicals. These processes are highly complicated, but they involve the same principle of removing impurities that governs reverse osmosis.

WHERE TO LEARN MORE

Francis, Frederick J., editor-in-chief. Encyclopedia of Food Science and Technology. New York: Wiley, 2000.

Gardner, Robert. Science Project Ideas About Kitchen Chemistry. Berkeley, N.J.: Enslow Publishers, 2002.

Laschish, Uri. "Osmosis, Reverse Osmosis, and Osmotic Pressure: What They Are" (Web site). <http://members.tripod.com/~urila/> (February 20, 2001).

"Lesson 5: Osmosis" (Web site). <http://www.biologylessons.sdsu.edu/classes/lab5/semnet/> (February 20, 2001).

Rosenfeld, Sam. Science Experiments with Water. Illustrated by John J. Floherty, Jr. Irvington-on-Hudson, NY: Harvey House, 1965.

"Salt-Curing Meat in Brine." Walton Feed (Web site). <http://waltonfeed.com/old/brine.html> (February 20, 2001).

KEY TERMS

HYPERTONIC:

Of higher osmotic pressure.

HYPOTONIC:

Of lower osmotic pressure.

ISOTONIC:

Of equal osmotic pressure.

OSMOSIS:

The movement of a solvent through a semipermeable membrane from a less concentrated solution to a more concentrated one.

OSMOTIC POTENTIAL:

A difference in osmotic pressure that draws water from an area of less osmotic pressure to an area of greater osmotic pressure.

OSMOTIC PRESSURE:

The pressure that builds in a substance as it experiencesosmosis, and eventually stops that process.

SOLVENT:

A liquid capable of dissolving or dispersing one or more other substances.

SOLUTE:

A substance capable of being dissolved in a solvent.

SOLUTION:

A mixture of solvent and solute.

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Osmosis

Osmosis

Osmosis is the movement of a solvent, such as water, through a semi-permeable membrane. (A solvent is the major component of a solution, the liquid in which something else is dissolved.) 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 (the component being dissolved, for instance sugar), cannot. Figure 1 illustrates this process. Notice that smaller molecules of water are able to pass through the openings in the membrane shown here but larger molecules of sugar are not.

Osmotic pressure

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 (for example, the bag in Figure 1), and osmosis stops. The osmotic pressure of a solution is the pressure needed to prevent osmosis from occurring.

Osmosis in living organisms

Living cells may be thought of as very small bags of solutions contained within semipermeable membranes. For example, Figure 1 might be thought of as a cell surrounded by a watery fluid. 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 itself (a hypertonic solution) will shrink due to loss of water. It may eventually die of dehydration. You 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 shrivelled.

By contrast, a cell placed in a solution more dilute than itself (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 (the ratio of solutes to solvents in a solution) 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.

Words to Know

Concentration: The quantity of solute (for example sugar) dissolved in a given volume of solution (for example water).

Hypertonic solution: A solution with a higher osmotic pressure (solute concentration) than another solution.

Hypotonic solution: A solution with a lower osmotic pressure (solute concentration) than another solution.

Isotonic solutions: Two solutions that have the same concentration of solute particles and therefore the same osmotic pressure.

Osmotic pressure: The pressure which, applied to a solution in contact with pure solvent through a semipermeable membrane, will prevent osmosis from occurring.

Semipermeable membrane: A thin barrier between two solutions that permits only certain components of the solutions, usually the solvent, to pass through.

Solute: A substance dissolved to make a solution, for example sugar in sugar water.

Solution: A mixture of two or more substances that appears to be uniform throughout except on a molecular level.

Solvent: The major component of a solution or the liquid in which some other component is dissolved, for example water in sugar water.

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

Preserving food. 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.

Artificial kidneys. 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.

Desalination by reverse osmosis. 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 mole-cules out of seawater into a reservoir of pure water.

[See also Diffusion; Solution ]

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osmosis

osmosis (ŏzmō´sĬs), transfer of a liquid solvent through a semipermeable membrane that does not allow dissolved solids (solutes) to pass. Osmosis refers only to transfer of solvent; transfer of solute is called dialysis. In either case the direction of transfer is from the area of higher concentration of the material transferred to the area of lower concentration. This spontaneous migration of a material from a region of higher concentration to a region of lower concentration is called diffusion.

Principles of Osmosis

Osmosis will occur if a vessel is separated into two compartments by a semipermeable membrane, both compartments are filled to the same level with a solvent, and solute is added to one side. The level of the liquid on the side containing the solute will rise as the solvent flows from the side of its higher concentration to the side of lower concentration. If an external pressure is exerted on the side containing the solute, the transfer of solvent can be stopped and even reversed (reverse osmosis). Two solutions separated by a semipermeable membrane are said to be isotonic if no osmosis occurs. If osmosis occurs, transfer of solvent is from the hypotonic solution to the hypertonic solution, which has the higher osmotic pressure.

The minimum pressure necessary to stop solvent transfer is called the osmotic pressure. Since the osmotic pressure is related to the concentration of solute particles, there is a mathematical relationship between osmotic pressure, freezing-point depression, and boiling-point elevation. Properties such as osmotic pressure, freezing point, and boiling point, which depend on the number of particles present rather than on their size or chemical nature, are called colligative properties. For dilute solutions the mathematical relationship between the osmotic pressure, temperature, and concentration of solute is much like the relation between pressure, temperature, and volume in an ideal gas (see gas laws). A number of theories explaining osmotic pressure by analogy to gases have been devised, but most have been discarded in favor of thermodynamic interpretations using such concepts as the entropy of dilution.

Biological Importance of Osmosis

Osmosis and dialysis are of prime importance in living organisms, where they influence the distribution of nutrients and the release of metabolic waste products. Living cells of both plants and animals are enclosed by a semipermeable membrane called the cell membrane, which regulates the flow of liquids and of dissolved solids and gases into and out of the cell. The membrane forms a selective barrier between the cell and its environment; not all substances can pass through the membrane with equal facility. Without this selectivity, the substances necessary to the life of the cell would diffuse uniformly into the cell's surroundings, and toxic materials from the surroundings would enter the cell.

If blood cells (or other cells) are placed in contact with an isotonic solution, they will neither shrink nor swell. If the solution is hypertonic, the cells will lose water and shrink (plasmolyze). If the solution is hypotonic (or if pure solvent is used) the cells will swell; the osmotic pressure that is developed may even be great enough to rupture the cell membrane. Saltwater from the ocean is hypertonic to the cells of the human body; the drinking of ocean water dehydrates body tissues instead of quenching thirst.

In plants osmosis is at least partially responsible for the absorption of soil water by root hairs and for the elevation of the liquid to the leaves of the plant. However, plants wilt when watered with saltwater or treated with too much fertilizer, since the soil around their roots then becomes hypertonic.

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osmosis

osmosis A term describing the movement of fluid (usually water) across a semipermeable membrane. The membrane is described as semipermeable because it allows water, but not dissolved substances, to cross it. Water moves across the membrane from where the concentration of dissolved substances is lowest to where it is highest. Thus, water moves down its concentration gradient from high concentration to low concentration. The process continues until the concentration of solutes is the same on both sides of the membrane. The nature of the dissolved substances is unimportant, other than their not being able to penetrate the membrane. The membranes of most living cells are semipermeable, and cells swell if they are placed in a solution containing less dissolved substance than blood (hypotonic), and shrink in more concentrated (hypertonic) solutions. Hydrostatic pressure can be applied to oppose fluid movement; the pressure required to oppose the movement exactly is the ‘osmotic pressure’. Thus cells can act as osmometers, by changing shape when the tonicity of the bathing solution changes. In the hypothalamus are cells which are very sensitive to osmotic changes in the blood. If, for example, blood becomes hypertonic, as in thirst, the cells respond by sending impulses to the posterior pituitary to release antidiuretic hormone, which prevents further fluid loss by the kidneys. Alternatively, if large amounts of fluid (beer, for example) are imbibed, antidiuretic hormone is cut off and diuresis ensues.

Alan W. Cuthbert


See also body fluids; cell.

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osmosis

osmosis The net movement of water molecules from a region where their concentration is high to a region where their concentration is low through a partially permeable membrane. The distribution of water in living organisms is dependent to a large extent on osmosis, water entering the cells through their partially permeable plasma membranes. The pressure required to stop the flow of pure water into a solution across a partially permeable membrane is a characteristic of the solution, and is called the osmotic pressure. Thus water will move from a region of low osmotic pressure to a region of high osmotic pressure (see also oncotic pressure). In terms of water potential, water moves from an area of high (less negative) water potential to an area of low (more negative) water potential. Both water potential and osmotic pressure can be used to explain osmosis but it is now recommended that only water potential be used in plant studies (see also plasmolysis; turgor). Animals have evolved various means to counteract the effects of osmosis (see osmoregulation); in animals solutions are still described in terms of osmotic pressure (see hypertonic solution; hypotonic solution; isotonic).

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"osmosis." A Dictionary of Biology. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

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osmosis

os·mo·sis / äzˈmōsis; äs-/ • n. Biol. & Chem. a process by which molecules of a solvent tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, thus equalizing the concentrations on each side of the membrane. ∎ fig. the process of gradual or unconscious assimilation of ideas, knowledge, etc.: what she knows of the blue-blood set she learned not through birthright, not even through wealth, but through osmosis. DERIVATIVES: os·mot·ic / -mätik/ adj. os·mot·i·cal·ly / -ˈmädik(ə)lē/ adv.

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"osmosis." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

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osmosis

osmosis Diffusion of a solvent (such as water) through a selectively permeable membrane (one which only allows the passage of certain dissolved substances) into a more concentrated solution. Because the more concentrated solution contains a lower concentration of solvent molecules, the solvent flows by diffusion to dilute it until concentrations of solvent are equal on both sides of the membrane. Osmosis is a vital cellular process to distribute water in animals and plants. This happens, for example, when plant roots take up water from the soil. See also turgor pressure

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"osmosis." World Encyclopedia. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

"osmosis." World Encyclopedia. . Encyclopedia.com. (December 12, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/osmosis

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osmosis

osmosis The passage of water through a semi‐permeable membrane, from a region of low concentration of solutes to one of higher concentration. Reverse osmosis is the passage of water from a more concentrated to a less concentrated solution through a semi‐permeable membrane by the application of pressure. Used for desalination of sea water, concentration of fruit juices, and processing of whey. The membranes commonly used are cellulose acetate or polyamide of very small pore size, 10−4−10−3 μm. See also osmotic pressure.

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"osmosis." A Dictionary of Food and Nutrition. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

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osmosis

osmosis (oz-moh-sis) n. the passage of a solvent from a less concentrated to a more concentrated solution through a semipermeable membrane. In living organisms, the process of osmosis plays an important role in controlling the distribution of water.
osmotic (oz-mot-ik) adj.

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"osmosis." A Dictionary of Nursing. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

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osmosis

osmosis The movement of water or of another solvent from a region of low solute concentration to one of higher concentration through a semi-permeable membrane. It is an important mechanism in the uptake of water by plants.

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"osmosis." A Dictionary of Earth Sciences. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

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osmosis

osmosis The movement of water or of another solvent from a region of low solute concentration to one of higher concentration through a semi-permeable membrane. See also HYPEROSMOTIC; HYPOSMOTIC.

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"osmosis." A Dictionary of Zoology. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

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osmosis

osmosis The net movement of water or of another solvent from a region of low solute concentration to one of higher concentration through a semi-permeable membrane.

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"osmosis." A Dictionary of Ecology. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

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osmosis

osmosis The net movement of water or of another solvent from a region of low solute concentration to one of higher concentration through a semi-permeable membrane.

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"osmosis." A Dictionary of Plant Sciences. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

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osmosis

osmosisglacis, Onassis •abscess •anaphylaxis, axis, praxis, taxis •Chalcis • Jancis • synapsis • catharsis •Frances, Francis •thesis • Alexis • amanuensis •prolepsis, sepsis, syllepsis •basis, oasis, stasis •amniocentesis, anamnesis, ascesis, catechesis, exegesis, mimesis, prosthesis, psychokinesis, telekinesis •ellipsis, paralipsis •Lachesis •analysis, catalysis, dialysis, paralysis, psychoanalysis •electrolysis • nemesis •genesis, parthenogenesis, pathogenesis •diaeresis (US dieresis) • metathesis •parenthesis •photosynthesis, synthesis •hypothesis, prothesis •crisis, Isis •proboscis • synopsis •apotheosis, chlorosis, cirrhosis, diagnosis, halitosis, hypnosis, kenosis, meiosis, metempsychosis, misdiagnosis, mononucleosis, myxomatosis, necrosis, neurosis, osmosis, osteoporosis, prognosis, psittacosis, psychosis, sclerosis, symbiosis, thrombosis, toxoplasmosis, trichinosis, tuberculosis •archdiocese, diocese, elephantiasis, psoriasis •anabasis • apodosis •emphasis, underemphasis •anamorphosis, metamorphosis •periphrasis • entasis • protasis •hypostasis, iconostasis

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"osmosis." Oxford Dictionary of Rhymes. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

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