There are three major kinds of membrane lipids: phospholipids, glycolipids,
and sterols. Both phospholipids and glycolipids readily associate
spontaneously to form a lipid bilayer (Figure 2-2). Cellular
membranes behave as two-dimensional, semifluid structures, allowing
embedded protein molecules to constantly move about rather freely by
lateral diffusion. The fluidity of prokaryotic membranes is regulated by
varying the number of double bonds in, and the lengths of, the fatty acid
chains of the lipid molecules constituting the membrane. In animals, the
quantity of the sterol lipid cholesterol is a key regulator of membrane fluidity.
|Figure 2-2 Lipid bilayer membrane.
The plasma membrane is a selective filter that controls the entry of
nutrients and other molecules needed for cellular processes. Waste products
of metabolism pass out of the cell through this membrane. Due to
their composition, membranes have a low permeability for ions and
most polar molecules, thus these molecules must pass through channels
formed from integral membrane proteins. If a substance is moving
against its concentration gradient (i.e., from an area of lower concentration
to an area of higher concentration), then energy must be expended.
This is termed active transport.