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|Chapter Two: The Chemistry of Biology|
Diffusion is a phenomenon which is essential to all unicellular organisms. Most people have had firsthand experience with diffusion without knowing it, but it's easy to see it in action. Have you ever been in another room, yet you were still able to smell the food cooking in the kitchen? You may not know it, but that's because of diffusion. Here's another example you can try. Take a cup of coffee without any milk in it. Using a spoon, drop a little bit of milk by one edge of the cup and watch as the milk spreads through the coffee. This example is also caused by the process of diffusion.
The scientific definition of diffusion is "the movement of molecules from an area of high concentration to an area of low concentration." Since the food was cooking in the kitchen, most of the molecules which cause the smell are located there (that is, the kitchen has a high concentration of molecules), and very few of them are located in the next room. By diffusion, the molecules which cause smell tend to move out of the kitchen and into other rooms. Similarly, the milk was highly concentrated near the edge where you placed it, so it diffused to areas of lower concentration, namely the rest of the cup. The difference in concentration in both of the examples is sometimes referred to as a concentration gradient. Molecules move "down" a concentration gradient; that is, they move toward areas with fewer molecules.
Gradients do not necessarily have to be of concentration. For example, if one side of a membrane is highly acidic while the other side is neutral we say that there is a pH gradient. The acidic molecules will tend to move "down" a pH gradient; that is, they move toward the neutral side until both sides are equally acidic.
The diffusion of water has a special name: osmosis. Most unicellular organisms live in aquatic environments such as an ocean or lake. The water outside of the cell is usually fairly pure, whereas the water inside the cell contains all sorts of organic and inorganic molecules. By diffusion, these molecules will tend to move out of the cell since they are less concentrated in the external environment. Also, the water in the environment will tend to move into the cell by osmosis. To prevent themselves from bursting because of too much water, many cells have developed specialized structures to keep excess water out of the cell. This structure is discussed in Chapter Three.