6.1: Prelude

One of the most obvious and prominent processes in the Universe is diffusion, which is defined as “the movement of a substance from an area of high concentration to an area of low concentration.” For example, when you exhale air, it is composed principally of carbon dioxide (a toxic gas). As soon as it leaves your mouth, the carbon dioxide is at a high concentration (relative to the surrounding air) and so diffuses away from your mouth to all the surrounding air. It is important to understand the distinction between “concentration” and “abundance.” In the example, air is more abundant in the atmosphere than in a balloon. But the critical factor is “concentration.” There is more air per volume in a filled balloon than in the atmosphere. So the air in a filled balloon is at a higher concentration.

Everything, including water, is subject to diffusion. Osmosis is a specific type of diffusion.

Osmosis is the diffusion of water across a semi-permeable membrane.

A “semi-permeable membrane” is any object that allows free passage of only some substances. The prime example of a semi-permeable membrane is the covering around every cell in your body, i.e., the plasma membrane. The plasma membrane allows free passage of water, oxygen, carbon dioxide and other small compounds, but resists the crossing of larger molecules like sugars or salts (namely, chlorine, sodium and potassium ions).

Because many solutions contain different amount of compounds (e.g., pure freshwater has nothing in it; saltwater is water and salt), osmosis is a very important biological process.

For example, a walleye has saline (i.e., salty) solution coursing through its body, yet it swims in freshwater that has very little salt. This means that the salt is more concentrated in the walleye’s body than in the environment that it swims in. Conversely, the water in the walleye is less concentrated than it is in the lake (i.e., salt is taking up some of the space in the walleye where there could be water).

Because the walleye is composed of cells (which have cell membranes that are semi-permeable; they allow free passage of water but prevent the passage of salts), the water will move from the area of high concentration (the lake) to an area of low concentration (within the walleye’s body). The result would be a walleye that gains water and swells. Eventually the walleye would burst from all the excess water flowing in by osmosis. Fortunately, the walleye has kidneys that prevent this; walleyes urinate often and their urine in very dilute (i.e., rich in water and low in salts). The opposite process occurs in fishes that live in the ocean.

(Because freshwater fishes constantly have water pouring into their body by osmosis, there is little need to drink. However, marine fishes are constantly losing water and so must drink constantly. This has the interesting everyday result that the statement “drinks like a fish” needs to be modified to “drinks like a saltwater fish.”)

This example also applies to cells. In today’s procedure, you will use cells of carrot root to observe the cumulative effect of osmosis. Cells (in our case, pieces of carrot which contain thousands of small cells) will be placed in two “environments” that differ in amount of salt: no salt (distilled water) and 1 M solution of salt (1 M NaCl). You can observe osmosis in action by monitoring the change in length.