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Diffusion And Osmosis – A brief look


All substances are made up from very small particles called molecules. When these molecules are packed relatively together with little or no freedom to move, it forms a solid substance. In liquids the molecules are spaced further apart and are also free to move about. However a in a gaseous substance, the molecules are much further apart again, they have complete freedom of movement at random which collide into walls or whatever contains the gaseous substance. Due to this random movement molecules of gas are able to distribute themselves evenly throughout any space to which they are confined. This is the same principle for molecules that can be dissolved in a liquid. For example, if there were to be a solution of water in a lab beaker and Copper Sulfate was added to the water, then the water would turn blue. However it doesn’t all turn blue at once at the same time. The Copper Sulfate that is dissolved will eventually spread throughout the water as the Copper Sulfate dissolves. The movement of gasses or liquid molecules from the example results in a uniform distribution, which is called Diffusion.

Rate of Diffusion of a Substance

The rate of diffusion of a substance is dependent on several factors; size of the molecule; temperature of the substance and its concentration. The larger the molecule, the longer it will take to diffuse. The warmer the substance, the more rapidly it diffuses.

Diffusion in Living Organisms

Diffusion is a major part of uptake or expulsion of substances within an organism or between the organism and its surrounding environment. Sometimes diffusion may be rapid enough to account entirely for the uptake of oxygen and the removal of carbon dioxide and other excretory substances. This is most common in small unicellular organisms such as bacteria. But diffusion is too slow to meet particular demands of living tissues when the size is on the large scale such as the mammal circulatory system hence why there is also active transport, which is another story…


Osmosis can be referred to as a special case of diffusion; the diffusion of water from a weaker to stronger solution. For example, a weak solution of salt has relatively more water than salt and vise versa for a strong solution. This causes the diffusion gradient between the strong and weak solution from the strong to the weak. But the water diffusion gradient would be from the weak to strong solution. If two solutions were in contact together then the water would move one way and the salt in the other way until the solution if uniformly distributed. However, with a semi permeable membrane, which allows water to pass but not salt to pass, only water would be able to diffuse.

Osmotic potential of a solution is a measure of the tendency for water molecules to diffuse out of it. A very concentrated solution, having relatively few water molecules, has a low osmotic potential, while a dilute solution with a larger proportion of water molecules has a high osmotic potential. Pure water is the only highest osmotic potential.

The selectively permeable membrane

It is unclear about the properties of what make a membrane selectively permeable. There is a theory that suggests that there are tiny pores that are too small to allow large molecules to pass through, such as sugar (C6H12O6), but large enough to let small water molecules (H2O) to pass through. The surfaces of plants and animals and the membranes in the cells tend to have selectively permeable properties. This is a very common membrane type to control the rate of diffusion in and out of the cell. When the organisms or their individual cells are surrounded by liquid, in particular water, there are osmotic forces sets up.


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