passive transport diffusion concentration gradient equilibrium osmosis hypotonic hypertonic isotonic contractile vacuole turgor pressure plasmolysis cytolysis facilitated diffusion carrier protein ion channel

figure 5-1

Sugar molecules, initially in a high concentration at the bottom of a beaker, O, will move about randomly through diffusion, 0, and eventually reach equilibrium, 0.At equilibrium the sugar concentration will be the same throughout the beaker. Diffusion occurs naturally because of the kinetic energy the molecules possess.

It is important to understand that even at equilibrium the random movement of molecules continues. But because there is an equal concentration of molecules everywhere, molecules are just as likely to move in one direction as in any other. The random movements of many molecules in many directions balance one another, and equilibrium is maintained.

Diffusion Across Membranes

Cell membranes allow some molecules to pass through, but not others. If a molecule can pass through a cell membrane, it will diffuse from an area of higher concentration on one side of the membrane to an area of lower concentration on the other side. Diffusion across a membrane is also called simple diffusion, and only allows certain molecules to pass through the membrane.

The simple diffusion of a molecule across a cell membrane depends on the size and type of molecule and on the chemical nature of the membrane. A membrane can be made, in part, of a phospho-lipid bilayer, and certain proteins can form pores in the membrane. Molecules that can dissolve in lipids may pass directly through the membrane by diffusion. For example, because of their nonpolar nature, both carbon dioxide and oxygen dissolve in lipids. Molecules that are very small but not soluble in lipids may diffuse across the membrane by moving through the pores in the membrane.

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