Osmosis 2876
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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

Figure 1. Osmosis is the movement of a solvent through a semipermeable membrane. (Reproduced by permission of The Gale Group.)
Figure 1. Osmosis is the movement of a solvent through a semipermeable membrane. (Reproduced by permission of
The Gale Group

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 ]

Also read article about Osmosis from Wikipedia

User Contributions:

i am looking for one example of a semi-permeable membrane in living organisms. i am wounding if u can help
Every cell membrane in the human is an example of a semi-permiable as that is how substances get into the cell. In plants the root cell is a good example from the sol to the root cell to the phloem
Is osmosis also applied in salted fish? I understand that the flow of solvent is from lower concentration to higher concentration but why at the end of soaking, the fish became saltier. In fact, salt concent of fish was lesser than the brine solution you have. THe process was just soaking. Or is this possible? Thanks alot. I was just curious if what's in my mind is right... Thanks alot. Cheryll
i am searching for an example of osmotic pressure but ii already know that water moves from high concentrated solutions to less concentrated ones & that water is transported through roots to other parys of the plant are examples of osmotic pressure &i want another example. can you help me please?
thanks for the clear definitions! I think I finally understand what I need to know haha
Please correct the def. for osmosis. It applies to H20 only.
Thanks guys,this article is good for us as learners.this article expand our knowledge.
This article contain all the basic knowledge through which student can eaisly understand the osmotic pressure...etc
how may osmotic pressure be used to remove salt from sea water?
Thank you for explaining it in such an easy and clear way to understand :) I've been learning this topic for the whole year and I didn't understand anything. I then come to this site and read an explanation and understand almost entirely within 5 minutes.
What are the two components of a solution in osmosis?
William P. McKinley
As a professor of science I found this source truly reliable... Well done!
i'm searching of when a bacterium finds itself in strong hyper tonic solution of sugar,which substance will leave the cell and which will move into cell?what processes are involved? i am wounding if u can help
i'm searching of when a bacterium finds itself in strong hyper tonic solution of sugar,which substance will leave the cell and which will move into cell?what processes are involved? i am wounding if u can help
I need help! I have to write a story about my life as a salt molecule inside a animal cell. I have to be specific about the process that allows me to move.. I really dont get this assignment! Please help!
How dose this relate to Osmosis helps how to make jam
i wonder when the strip of plant had immersed in hypertonic solution in 20 min than it immersed in hypotonic solution?does it become to original length?? please help me
Kate Brown RSBA
Osmosis is a special type of diffusion. It means that WATER moves from an area of HIGH WATER CONCENTRATION to an area of LOW WATER CONCENTRATION across a SELECTIVELY PERMEABLE MEMBRANE. Like diffusion it requires no energy and like diffusion is a form of PASSIVE TRANSPORT..
jaffrey barasu
You should know that transport deals with size of the particle to be transported. water in the root hair is lost into the cortical cell by osmosis
example of osmosis occurring in plants and animals
i need hep how does it make maple syrup.i wonder when the strip of plant had immersed in hypertonic solution in 20 min than it immersed in hypotonic solution?does it become to original length?? please help me
How does the solute end up in the semi permeable membrane? Doesn't say that only water particles can go in and out the membrane?
Arminder Kaur
what else could you include for a revision poster?
Narayan Jha
I am very interested in this thing that you have written here. Thanks so much for your help and support for the next day and I am going to be a good idea for a while.You have given me a little bit of a good idea to get the best of my friends and family
how does digestion and filtration deals with osmosis?
How does a pickle stay fresh? I'm in living environment and need to know the type of solution that does this (hypertonic, hypotonic, isotonic)
cedar johnsons
i need help to write a method of how osmosis works?
If a bacterium is in a concentrated solution of sugar, which substance will leave the cell and which will move into the cell? What processes are involved?

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