Osmosis: Nature's Water Filter

Osmosis is simply the movement of water through a filter (semi-permeable membrane) to equalize the concentration of a dissolved substance in the water on both sides of the filter. In other words, the membrane allows the water to pass through from the side that has a higher concentration of water, to the side that has a lower concentration of water. (Or looking at it from the opposite perspective: the water passes from the side of lower concentration of dissolved substance to the side with the higher concentration of dissolved substance). Osmosis will increase the pressure on the side of the filter that originally had more of the dissolved substance (for example: salt).

Osmosis IllustrationJust how does osmosis happen? To find out, let's look at plant and animal cells. They have a semi-permeable membrane, which is really just a filter that is capable of allowing the small water molecules through, but preventing larger molecules of a solution (such as salt-water) through. You can imagine the cell-walls of a plant with tightly bonded molecules, but spaced just far enough apart to allow the passage of the comparatively tiny H2O molecules. This is why the membrane is called semi-permeable. Semi is a prefix meaning "partly" or "half". Permeability is the ability of something to allow a fluid to pass through.

In the event that you dropped a plant cell into a dish of water, one of several things could happen.

  1. If the concentration of salt in the water in the dish is greater, water will pass out of the cell through the cell wall. The cell will actually loose pressure and may shrink
  2. If the concentration of salt in the water in the dish is less than in the cell, the water will pass into the cell, causing the cell to swell with pressure.
  3. If the concentration of salt is equal in the cell to the water in the dish, then there will be no discernible change in the pressure within the cell.

The water molecules naturally pass from one side to the other of the membrane. They are actually passing both ways, but in the case of a cell with a high concentration of salt-water, the bigger salt molecules block a number of the passages out of the cell. With more inflow than outflow the pressure in the cell naturally builds. The flow of water will continue until the salt concentration is equalized or the pressure within the cell builds to a point that the amount of water that can make its way against that internal pressure is equal to the number of water molecules that can make their way out of the cell, around the salt molecule. In this case we would say that the pressure within the cell was now equal to the osmotic pressure.

Osmosis normally refers to cell activity in plants and animals, though it does have wider application as in reverse osmosis filters.

Water, Water Everywhere...

Osmosis is the reason that people who are shipwrecked, on a life-boat, cannot drink the sea water that is all about them. Sea water has a much higher concentration of salt than the body needs. When sea water is consumed, the body cannot get rid of the salt from the body fast enough. The cells lose water and shrivel up, causing severe dehydration. In the human body, it is the kidney (controlled by the hypothalamus in the brain) that regulates the body's fluid. It expels excess water and salt to maintain just the right concentration to keep the cells healthy and operating smoothly. Remember too much water in the body, without some salt intake, can also cause health problems.

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