A study of reverse osmosis system, or the science of life, and, specifically, the cell, is not complete without at least an introduction to the principles of diffusion and osmosis. These very basic principles not only govern how many substances are transported throughout the cell, and the body, but also give an important grounding in the fundamentals of how many life processes, be they in animals or plants, work.

The cell contains a selectively permeable membrane, illustrated through the fluid mosaic model. Diffusion is the movement of a substance from a higher concentration to a lower concentration, down their concentration gradient. While this may sound rather complex, it is not, and may be explained in the following manner. First, picture a large glass containing only water. Then, picture a drop of ink being released into that water. The ink hits the water as a small droplet, but then spreads out, eventually mixing with all the water. The ink is moving from an area of high ink concentration, and lower water concentration, to an equal distribution, or at least an attempt to, throughout the glass. This is similar to how diffusion works. Stuff moves from a higher concentration to a lower concentration.

Osmosis is a similar concept, built off of the idea of diffusion. Osmosis deals only with water, and the solutes in it. A solution is used to describe a mixture of solvent and solute-for instances, in a glass of salt water, the solvent (substances that does the dissolving) is the water, while the solute (substance that gets dissolved) is the salt, and the solution is the entire mixture. Thus, in osmosis, substance will move from a lower concentration of solute to a higher concentration of solute. While at first glance this may seem to be in contradiction to the principle of diffusion, it isn’t. Instead, think of it this way-when there is a low concentration of solute, there is a high concentration of water. When there is a high concentration of solute, there is a low concentration of water. Thus, water is still following the principles of diffusion, and is moving from a area of high concentration to a region of lower concentration.

A cell usually tries to be in an isotonic environment, where it is most stable. Isotonic is when there is an even distribution of solute on both sides of the cell membrane, inside and out. Thus, there is not a sudden rush of water in, or out. However, this theoretical environment is not always the case, and the cell may be hypotonic (have less solute inside than outside the cell) and water may leave the cell, as the cell is hypotonic to its environment, while the outside of the cell is hyptertonic (has a greater concentration of solute, and less water) to the cell. Or, it may be hypertonic (have more solute in the cell than in its outside environment) and water may rush inside the cell, possibly causing it to lyse, or explode from too much water.

Many, many biological processes require an understanding of the concepts of diffusion, and osmosis before they can be understood. Fish use these principles to regulate their body environment to the outside water. Substances are moved between the mother and her unborn baby in the blood across the placenta according to these concepts. The list goes on and on of examples of diffusion and osmosis, pointing out exactly how necessary these concepts are for a study in biology.


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