A.3 Cell membranes
Cells are broadly classified as eitherprokaryotesoreukaryotes. Prokaryotic cells
are found in simpler organisms, such as bacteria. They do not have a membrane
enclosed nucleus, but their DNA is dispersed in the cytoplasm of the cell.
Eukaryotic cells are found in all members of the animal kingdom. Their struc-
tures contain distinct membrane encapsulated compartments, such as the
nucleus, which contains their DNA, mitochondria and lysomes. These separate
compartments are known asorganelles.
All cells have a membrane, known as theplasmaorcytoplasmic membrane,
that separates the internal medium of a cell (intracellular fluid) from its sur-
rounding medium (extracellular fluid). Membranes also form the boundaries
between the various internal regions of the cell that retain the intracellular fluid
in separate compartments in the cell. The plasma membrane maintains the
integrity of the cell in its environment and also regulates the movement of
substances into and out of the cell. These movements control health, as well
as the flow of information between and within cells. The plasma membrane of a
cell is also involved in both the generation and receipt of chemical and electrical
signals, cell adhesion, which is responsible for tissue formation, cell locomotion,
biochemical reactions and cell reproduction. The internal cell membranes have
similar functions and, in addition, are often actively involved in the function of
organelles. Most drugs have to pass through one or more membranes to reach
their site of action.
The currently accepted structure of membranes (Figure A3.1) is a fluid-like
bilayer arrangement of phospholipids with proteins and other substances such
as steroids and glycolipids either associated with its surface or embedded in it to
varying degrees. This structure is an intermediate state between the true liquid
and solid states, with the lipid and protein molecules having a limited degree of
rotational and lateral movement.
X-ray diffraction studies have shown that many naturally occurring mem-
branes are about 5 nm thick. Experimental work has also shown that a potential
difference exists across most membranes due to the movement of ions through
pores in the membrane, known asion channels, and the action of so called
244 APPENDIX 3 CELL MEMBRANES