Contents:

• Background Vocabulary
• Fundamental Concepts

Equilibrium

The point at which the number of molecules leaving a system equals the number of molecules entering it.

Selectively permeable

The characteristic of only allowing certain substances to pass through.

Permeable

The characteristic of allowing to pass through.

Homeostasis

The process of maintaining a constant internal environment despite changing external conditions.

Turgor pressure

Osmatic pressure exerted by the contents of a plant cell against the cell wall.

Osmotic pressure

The pressure created against the cell membrane by the movement of water molecules in osmosis.

Plasmolysis

Loss of turgor pressure in plant cells.

Active Transport

The movement of particles across a membrane that requires cellular energy.

Passive Transport

The movement of molecules across a membrane without using cellular energy.

Hypertonic Solution

Contains a greater concentration of dissolved material than the solution to which it is being compared.

Hypotonic Solution

Contains a lower concentration of dissolved material than the solution to which it is being compared.

Isotonic Solution

Is in equilibrium with the substance to which it is being compared.

The process of osmosis.

Osmosis is the diffusion of water molecules through a selectively permeable membrane.
Osmosis can be demonstrated by using a sack of cellulose filled with starch immersed in water. The water molecules will pass through the cellulose in both directions, yet more will enter than leave, as diffusion dictates that molecules will move to an area of lower concentration. The starch molecules, however, will remain within the sack as they are too large to leave. As water molecules continue to enter the sack, the pressure inside it builds. This pressure is called osmotic pressure. This pressure will also push some of the molecules out of the sack. Eventually, the rates of molecules entering and leaving the sack will be even, e.g. equilibrium. Osmatic pressure increases inside the sack, as the starch molecules cannot escape and the water continues to move in.

The process of diffusion.

Diffusion is the process of molecules moving form an area of higher concentration to an area of lower concentration so as to equalize the distribution in both areas. This is caused by the motion of the molecules. Molecules packed into a small group collide often, knocking themselves off in different directions. Eventually the group of molecules is broken up, and they are spread over a larger area.
This can be seen easily by dissolving a spoonful of sugar into a cup of tea. The sugar will be concentrated at the bottom at first, eventually it will be distributed evenly throughout the tea.
After time passes, the food coloring diffuses evenly throughout the water.

Several ways these processes are used within the body system.

The aforementioned processes are quite important for humans, especially when concerning the movement of nutrients and wastes.
For example, when food is digested and drawn into the intestines, the villi on the intestinal walls absorb the nutrients through their cell membranes, through active transport. Water is also drawn off from these wastes through osmosis.
The lungs work on the same principle: the red blood cells draw in oxygen from the air in the lungs through active transport as well. The molecules are drawn through the blood cells1 plasma membranes to provide oxygen for the other body systems.
All in all, the processes of osmosis and diffusion are essential to life, and it is rather doubtful that humans (or any other life) could live without them.

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