Concepts of Human Anatomy & Physiology   5/e   Van De Graaff/Fox
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I. Diffusion and Osmosis

Concept: Net diffusion of a molecule or ion through a cell membrane always occurs in the direction of its lower concentration. Nonpolar molecules can penetrate the phospholipid barrier, and small inorganic ions can pass through channels in the membrane. The net diffusion of water through a membrane is known as osmosis.

II. Carrier-Mediated Transport

Concept: Molecules such as glucose are transported across the cell membranes by special protein carriers. Carrier-mediated transport in which the net movement is down a concentration gradient, and which is therefore passive, is called facilitated diffusion. Carrier-mediated transport that occurs against a concentration gradient, and which therefore requires metabolic energy, is called active transport.

III. The Membrane Potential

Concept: As a result of the permeability properties of the cell membrane, the presence of nondiffusible negatively charged molecules inside the cell, and the action of the Na/K pumps, there is an unequal distribution of charges across the membrane. Consequently, the inside of the cell is negatively charged compared to the outside. This difference in charge, or potential difference, is known as the membrane potential.

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