The cell membrane envelops the entire cell and is semipermeable. The aqueous interior
of the cell has an ionic composition markedly different from that of the extracellular
fluid. Traditionally, the cell membrane is described as a lipid bilayer composed of two
layers of lipids, usually phospholipids, with their anionic head groups oriented either
outwards to the extracellular environment or inwards to the intracellular environment.
The long alkyl chain tails of the lipids face each other within the interior of the cell
membrane. The membrane is 10 nm in thickness. Within this lipid bilayer, a wide vari-
ety of proteins are interspersed between various lipid molecules: “protein icebergs
floating in a sea of fat.” Singer and Nicolson envisioned the cell membrane as a lipid–
proteinfluid mosaic made up of a discontinuous bimolecular array of phospholipids in
which globular proteins are erratically embedded. Some of these proteins extend across
the entire expanse of the lipid (integral ortransmembraneproteins), whilst others
extend only partly into the membrane lipids and are associated with only one face of
the membrane (peripheral proteins) (see figure 7.2).
ENDOGENOUS CELLULAR STRUCTURES 407Figure 7.1 Drug targets at the level of cellular structure. The mammalian cell presents a variety of
druggable targets. The most important ones are located at the level of the cell membrane. Within
the cell, cytoplasmic organelles, such as mitochondria, are beginning to be exploited as potential
drug targets. The nucleus, at the center of the cell, is an important target for the development of
antineoplastic agents for the treatment of cancer.
Figure 7.2 The cell membrane consists of protein “icebergs” floating in a sea of lipid. The proteins
are a collection of ion channels, ion pumps, and G-proteins. These afford a rich diversity of potential
drug targets.