Do you know that depending upon the size of your body, 55% to 78% is water? Osmosis is one of the most important biological processes in living things for it is the method allowing water to spread around the cells; without water, the cells will die. This Resource Guide on Osmosis will illustrate how important this process is to survival.

Overview of Osmosis

Osmosis is the passage of water from a low solute concentration area through a semi-permeable membrane to a high solute concentration area to equalize the solute concentrations on both sides. A solvent is the base substance, where a solute is being dissolved and the result is the solution. Through osmosis, water is provided to the cells of your body; it plays a significant role in the preservation of life. It is this process that helps plants receive water and is even used in kidney dialysis.

Factors Affecting the Rate of Osmosis

Temperature - The higher the temperature, the faster the water molecules move across the semi permeable membrane.

Surface Area - The larger the surface area, the more space for the molecules to move easily across; the smaller the area, the more restricted the movements of the molecules and the slower the movement. 

Difference in Water Potential – The higher the difference in water potential, the faster the osmosis; for the lesser water molecules are in the region of low concentration, more water molecules from the region of higher concentration can enter faster and easier.

Pressure – The more the pressure, the faster the molecules will move for they are being pushed faster across a low concentration.

Concentration gradient - The movement of osmosis is affected by the concentration gradient; the lower the concentration of the solute within a solvent, the faster osmosis will occur in that solvent.

Light and dark – They are also factors of osmosis; since the brighter the light, the faster osmosis takes place.

Osmotic Pressure

Osmotic pressure is a colligative property. In other words, it depends on the molar concentration of the solute but not on its density. It is the pressure which is applied to a solution to prevent the inward flow of water across a semi permeable membrane, or simply put it is the pressure required to stop osmosis. Osmotic pressure is shown when water molecules that attempt to cross the semi permeable membrane are prevented from doing so.

Reverse Osmosis

Example of osmotic pressure: Place some raisins in a cup of water for a few hours, notice that they will swell and if kept longer will burst. The reason is that as water continues to diffuse into the membranous cover of the raisins; this influx of water builds up an internal pressure and upon reaching its limit the outer skin of the raisin will burst once it can no longer bear the pressure. 

Osmotic Gradient  

The osmotic gradient is the difference between two concentration solutions at either side of a semi permeable membrane that distinguishes the different percentage of a specific particle concentration that is dissolved in a solution. The osmotic gradient acts on solutions having a semi permeable membrane between them; allowing water to diffuse between the two solutions toward the solution with the higher concentration. Eventually, water with higher concentration will be equally diffused to the side of a lesser concentration. It creates equilibrium for water continues to flow equally both ways, resulting in a stabilized solution.


Foward Osmosis 1. Reverse osmosis is a separation process using pressure to force a solvent to pass through a semi permeable membrane that keeps the solute on one side and directs the pure solvent to the other side. In other words, this is the process where osmotic pressure is applied to force a solvent from an area of high solute concentration towards an area of low solute concentration.

Examples: As a solution of water shortage, rain water is purified as drinking water. Big industries use reverse osmosis to remove minerals from their boiler water to be recycled. Reverse osmosis is the technique used in liver dialysis. A dialysis machine mimics the function of the kidneys.

2. Forward osmosis uses osmosis to directly separate water from a feed solution with unwanted solutes. A draw solution uses the feed solution to force water through a semi permeable membrane; resulting in the feed solution becoming concentrated and the draw solution becoming diluted. The draw solution which is now diluted can be used with an ingestible solute like glucose or transmitted to a secondary process from the draw solute.

Examples of forward osmosis are desalination, water purification, and food processing.

Types of Solutions

There are three different types of solutions that are used to describe water movement. The first is the hypertonic solution. The hypertonic solution has a higher solute concentration than the other solution types. When an animal cell is placed in a hypertonic solution, it loses water and shrinks. This is called a flaccid cell. The second type of solution is called a hypotonic solution. This solution is one which has a low solute concentration when compared to the other solutions. When an animal cell is placed in this kind of solution, the cell absorbs all the water and becomes bloated. The final solution type is the isotonic solution type. This solution has no difference in solute concentration across the semi permeable membrane, therefore has no net movement of water across the membrane.

Additional Resources

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