Levels of Organization Image Map
What other important jobs do membranes perform?
  • Separate chemicals. Very big molecules, such as proteins, cannot escape the cell, but smaller chemicals may.
  • Generate electricity. Proteins, which are electrically negative, are trapped inside a cell make the inside of the cell relatively negative, with respect to the outside. It is like a battery.

Charged Ion and Membrane Image

The electrical field influences the movement of electrically charged ions across the membrane. Remember, like charges repel and unlike charges attract. Thus, positive ions accumulate inside cells and negative ions accumulate outside.

Example of electric charges:

  1. Dust is attracted to TV screens by electrical force.
  2. Touching metal on a very dry day may cause sparks to fly from your hand.

Why is charge separation important?


Separated electrical charges are a form of stored energy (like a battery). Anything that changes permeability (porousness) of a membrane would change the distribution of charged atoms, for two reasons:

  1. Charged atoms move from areas of high concentration to areas of low concentration (like a drop of ink in a glass of water).

  2. Charged atoms are attracted to regions of opposite electrical charge. 

This is how nerve and muscle cells generate electricity. The electricity is actually the movement of electrically charged atoms that move according to electrical and chemical forces. Think of it as "liquid-state electronics," as opposed to the solid-state electronics of computer chips. A good narrated and animated explanation of ion movements across a membrane can be found at

http://entochem.tamu.edu/Gibbs-Donnan/index.html

This explanation uses some advanced vocabulary, but the animated illustrations should prove very helpful.