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Osmosis is the diffusion of small molecules through a semi-permeable membrane. Such a membrane is one in which small molecules such as water and small ion such as Sodium and Potassium ions may pass in either direction through the membrane. A example of osmosis is what happens when you place a membrane over the top of a thistle tube. Such a tube is a long capillary tube that is flared open at one end. The membrane is stretched over the top of the flared end. A solution of is placed inside the tube and it is immersed into a container of pure water. Such a solution could be anything but a sugar solution will serve as our example. If we mark the level of the solution inside the thistle tube with a black marking pen and wait for several hours, we will discover that the liquid level inside the thistle tube has risen. After a certain amount of time the liquid level ceases to rise and levels off. What is happening here? From a macroscopic view the liquid inside the tube has increased and the resulting osmotic pressure has pushed the liquid farher up in the tube. However, why does the level suddenly level off?

The solution is composed of sugar molecules, C₁₂H₂₂O₁₁, which are relatively large when compared to the smaller water molecules, H₂O. The water molecules can pass through the pores in the semi-permeable membrane, but the large sugar molecules cannot pass through. As a result water molecules pass in both directions. But why does the levels rise? There is a higher population of water molecules in the pure water in the container than there is in the sugar solution. I draw an analogy of an elevator which has 5 people inside. As the elevator stops at a floor, were it not for the reasoned manners of the people on the outside of the elevator trying to get in, none of the people on the inside would be able to get off because there are many more tring to get inside the elevator. Water molecules cannot reason. They simply abide by the brute physical laws. More water molecules can get inside the tube then can get outside. The solution id diluted and the entropy (measure of the disorder of a system) is increased which is the natural order of things according to the second law of thermodynamics. The direction of osmosis is always from the more concentrated side to the more dilute side of the membrane.

Water molecules will cause the liquid levels to rise, but why does the liquid level stabilize with time? As the sugar solution inside the tube becomes diluted, the water molecule population increases so that the osmosis toward the outside of the tube increases. Eventually just as many water molecules pass into the tube as pass out of it, and we have reached a state of dynamic equilibrium where the liquid level is in a steady state condition.

Osmosis displays itself in most biological systems that are cellular. The cell membrane is a semi-permeable membrane. So is the cell nuclear membrane which keeps DNA molecules inside the nucleus while allowing the transfer RNA molecules to pass through in and out of the nucleus. Nutrients,oxygen, water, and waste gases can pass in or out of the cell through the semi-permeable cell membrane. Cells in which water passes out of the cell faster than can get in are said to shrink and undergo crenation. This happens when cells are placed in an extracellular fluid of more than .9% salt solution. The extracellular solution is concentrated enough to allow water molecules to osmotically pass to the outside of the cell faster than water can pass into the cell. As a result the cells dehydrate and shrink. This is what happens if a person dying of thirst on a raft in the middle of the ocean decides to drink sea water. The sea water increases the salinity of the extracellular fluids, and a condition arises where more water osmotically leaves the cells than can get out. The cells dehydrate, and ironically, the person suffers from cellular dehydration which becomes more severe as the person takes in more sea water. Such an extracellular solution is called a hypertonic solution.

On the other hand if cells are placed in a salt solution of less than .9% then the water on the outside of the cell in the extracellular fluid can pass into the cell faster than water can get out. The result is that the cells swell with the excess water and eventually burst open. When red blood cells do this it is referred to as hemolysis. The extracellular fluid itself is said to be hypotonic. A condition known as water intoxication results in too much water being ingested where the Sodium/Potassium ion pump can't control the water intake into the cells and the cells rupture due to this intoxication.

Pysiological saline (salt) solution is 0.9% and results in an equilibrium between osmosis into the cell and out of the cell. This kind of solution is the extracellular fluid that our cells respond most positively to. The solution is said to be isotonic. This intricate balance in osmosis in and out of the cells needs to be maintained at all times. The controller of this process is the kidneys themselves which filters out waste products and recycles needed nutrients while maintaining this isotonic condition in the extracellular fluids.

Another example of osmosis is found in the plant kingdom. Plants through their leaves, stems, stalks and root systems take in needed CO₂ and water during their photosynthetic process and takes in Oxygen during the respiration cycle of the plant. It is important that water be allowed to enter and leave in a balanced way just as in the animal kingdom. If the roots of plants are placed in soil that is too saline (salty), then a condition of hypertonicity occurs, and the cells in the roots will shrink and dehydrate. That is why plants shrivel up and die in a soil that is too salty. Have you ever wondered how large tall trees can receive the necessary nutrients that only come from the soil? The total osmotic pressure of a large tree with an extensive network of roots can generate an osmotic pressure of several atmospheres enough to overcome the gravity pull downward.

Osmotic pressure is the pressure necessary to reverse osmosis and return to the initial condition. It would be the difference between the liquid levels at equilibrium and the initial liquid level in our thistle tube example above. Normally osmotic pressure can be measured using an instrument know as an osmometer which measures osmotic pressure in suitable pressure units. The Osmotic Pressure at a given temperature depends upon the molar concentration. The mathematical relationship is as follows:

Calculate the osmotic pressure of a sugar solution that contained 100 grams of sucrose, C₁₂H₂₂O₁₁, dissolved in enough water to make 1 liter of solution at 25 degrees Celsius. R = 0.0821 liter-atm/mole kelvin

Calculate the Osmotic pressure at 50 degrees Celsius of a glucose solution C₆H₁₂O₆ that has 60 grams of glucose dissolved in enough water to make 1500 ml.

When you have completed the problem you may check your work with the correct answer

R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.

All textual content copyrighted (c) 1997
R.H. Logan, Instructor of Chemistry, DCCCD
All Rights reserved

Calculate the Osmotic pressure at 50 degrees Celsius of a glucose solution C₆H₁₂O₆ that has 60 grams of glucose dissolved in enough water to make 1500 ml.

Osmosis and Osmotic Pressure

A Molecular View Of Osmosis

Biological Applications Of Osmosis

Osmotic Pressure And Its Measurement

Acknowledgements:

Solution To Osmotic Pressure Problem

Solution To The Problem

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