(iii) Metabolically-oriented nature. The relative movements of man y substances, in or out of the cell, cannot be explained by reason of their size or lipid
solubility and indeed their direction of movement ma y, on simple ph ysico-chemical grounds, be somewhat unexpected. This suggests that substances ma y interact
chemically with the membrane in such a way as to facilitate or even promote their
transfer.
(iv) Excitable nature. Many substances interact with cells to change their function and permeability. This ma y be manifested as an internal metabolic transmutation, the conduction of an electrical impulse, contraction of a muscle or the
secretion of a gland. Such responses are usually highly specific, both with respect
to the chemical structure of the excitant, and the cellular component in the reaction
(the receptor).
The considerable diversity that exists in the transit of water and divers solutes
between different cells and their bathing solutions suggests that the cell membrane
is a mosaic of all these properties including pores, lipid solvents and metabolically
mediated processes. This is combined with an ability to interact and respond to
excitants, including metabolites and hormones.
The 'cell membrane' may be viewed as an array of physico-chemical and biochemical properties not necessarily located at the cell surface or even present in a discrete structure. However, biologists and especiall y anatomists, usually shrink from
such ethereal concepts so that a considerable effort has been made to find if the
molecular structure and ph ysico-chemical properties at the cell surface correspond
with those attributed to the 'cell membrane'. In the laboratory, the external cell
membranes ma y be ph ysicall y separated from other cell constituents by procedures
that involve exposure to hypoosmotic solutions, as with red blood cells, or by carefully breaking up the tissue and separating the components by differential centrifugation. Such isolated pieces of membranes ma y be analyzed (see STEIN 1967) to
determine their chemical composition and enzymatic activities. Mammalian red
cell membranes are generall y found to contain about equal proportions of protein
and lipid, the latter consisting of similar amounts of cholesterol and phospholipids.
The proportional composition is, however, found to vary in cells from different
tissues and species. Such differences may contribute to the distinctive permeabilities
of diverse cell types. The external cell membrane of the red cell is also found to
possess Na-K activated ATPase and acetylcholinesterase activity.
Molecular arrangement of the cell membrane. The membrane at the border of
the cell can be seen under the electron microscope but is too small to observe in
detail by light microscopy. Even before the advent of the electron microscope DAVSON and DANIELLI, (see DAVSON and DANIELLI, 1952) on the basis of the then current chemical and physical information, suggested that the lipids in the membrane
were arranged as a bimolecular leaflet oriented with the ends of their hydrophobic
hydrocarbon chains facing each other inwardly, while the opposing hydrophilic
heads of the molecules faced outwards. These hydrophilic 'heads' were both
thought to be associated electrostatically with a globular protein layer. The cell
membrane was thus conceived as being a sandwich: lipid berweeen protein.
The electron microscope reveals structures which are fairly consistent with such
a model; two electron dense lines separated by an electron transparent region having a total width of about 100 A. This structure has been called the unit membrane
16
solubility and indeed their direction of movement ma y, on simple ph ysico-chemical grounds, be somewhat unexpected. This suggests that substances ma y interact
chemically with the membrane in such a way as to facilitate or even promote their
transfer.
(iv) Excitable nature. Many substances interact with cells to change their function and permeability. This ma y be manifested as an internal metabolic transmutation, the conduction of an electrical impulse, contraction of a muscle or the
secretion of a gland. Such responses are usually highly specific, both with respect
to the chemical structure of the excitant, and the cellular component in the reaction
(the receptor).
The considerable diversity that exists in the transit of water and divers solutes
between different cells and their bathing solutions suggests that the cell membrane
is a mosaic of all these properties including pores, lipid solvents and metabolically
mediated processes. This is combined with an ability to interact and respond to
excitants, including metabolites and hormones.
The 'cell membrane' may be viewed as an array of physico-chemical and biochemical properties not necessarily located at the cell surface or even present in a discrete structure. However, biologists and especiall y anatomists, usually shrink from
such ethereal concepts so that a considerable effort has been made to find if the
molecular structure and ph ysico-chemical properties at the cell surface correspond
with those attributed to the 'cell membrane'. In the laboratory, the external cell
membranes ma y be ph ysicall y separated from other cell constituents by procedures
that involve exposure to hypoosmotic solutions, as with red blood cells, or by carefully breaking up the tissue and separating the components by differential centrifugation. Such isolated pieces of membranes ma y be analyzed (see STEIN 1967) to
determine their chemical composition and enzymatic activities. Mammalian red
cell membranes are generall y found to contain about equal proportions of protein
and lipid, the latter consisting of similar amounts of cholesterol and phospholipids.
The proportional composition is, however, found to vary in cells from different
tissues and species. Such differences may contribute to the distinctive permeabilities
of diverse cell types. The external cell membrane of the red cell is also found to
possess Na-K activated ATPase and acetylcholinesterase activity.
Molecular arrangement of the cell membrane. The membrane at the border of
the cell can be seen under the electron microscope but is too small to observe in
detail by light microscopy. Even before the advent of the electron microscope DAVSON and DANIELLI, (see DAVSON and DANIELLI, 1952) on the basis of the then current chemical and physical information, suggested that the lipids in the membrane
were arranged as a bimolecular leaflet oriented with the ends of their hydrophobic
hydrocarbon chains facing each other inwardly, while the opposing hydrophilic
heads of the molecules faced outwards. These hydrophilic 'heads' were both
thought to be associated electrostatically with a globular protein layer. The cell
membrane was thus conceived as being a sandwich: lipid berweeen protein.
The electron microscope reveals structures which are fairly consistent with such
a model; two electron dense lines separated by an electron transparent region having a total width of about 100 A. This structure has been called the unit membrane
16