cell membranes Every cell has a
plasma membrane that encloses it and maintains differences between the cell contents and the outside environment that are crucial to the function of the cell. All biological membranes consist of assemblies of lipid and
protein molecules. The lipids are rod-shaped molecules arranged in a double layer so that their
hydrophobic ends, which repel water, point inwards and their
hydrophilic ends, which attract it, point towards the aqueous environment and the inside of the cell. This
lipid bilayer provides the basic structure of the membrane, and forms a barrier that is relatively impermeable to most water-soluble molecules. Proteins are embedded in the bilayer; they also have hydrophobic surfaces in contact with the lipids, and hydrophilic surfaces exposed on either side of the membrane. At physiological temperatures, the lipid bilayer is fluid, and so the proteins are able to move about within the plane of the membrane. The two leaflets of the bilayer contain different lipids, and different proteins are exposed on the two faces of the membrane.
The respiratory gases exchange freely across the membrane, because
oxygen and
carbon dioxide are soluble in lipid. Apart from this it is the proteins that span the membrane which act as pumps and channels for the exchange of materials between the inside and outside of the cell. They allow entry of nutrients into the cell and the exit of waste products. They are also responsible for generating differences in the ionic composition between the inside and the outside of the cell. Finally, proteins act as molecular sensors (
membrane receptors) allowing the cell to change its behaviour in response to external chemical signals. In addition to the plasma membrane, most cells contain a variety of
organelles — internal structures that are also surrounded by membranes. These include the nucleus, the endoplasmic reticulum, the Golgi complex, and the mitochondria.
Many membrane proteins are made on
ribosomes (granules of nucleoprotein) bound to the membrane of the endoplasmic reticulum. Those bound for the plasma membrane are recognized and then inserted into the lipid bilayer locally, before being transported to their final destination by a trafficking system that relies on further signals within the protein. Proteins for the mitochondrial membrane are recognized and then inserted directly from the cytoplasm.
The most fundamental difference between the inside and the outside of a typical cell is in the
ionic composition. In particular, the inside of the cell has a low concentration of sodium ions and a high concentration of potassium ions; the reverse is true of the fluid outside. This difference in ionic composition is generated by
ion ‘pumps’, which use energy in the form of ATP, produced by mitochondrial respiration, to drive sodium ions out of the cell and potassium ions in. In addition to this ion-pumping function, most membranes contain
ion channels that let ions diffuse across the membrane passively when they open. The concentration gradients for different ions across the membranes are exploited widely by cells to drive the movement of other molecules across the membrane. For example, glucose enters cells on a carrier protein that carries both sodium and glucose. Furthermore, in specialized cells, such as neurons, the
ion gradients are also used to generate electrical signals that propagate along their axons and allow neurons to ‘talk’ to each other through the release of ‘neurotransmitter’ molecules.
The ability of many proteins to diffuse freely within the plane of the membrane allows them to interact transiently with protein partners, which is often crucial to their function. For instance, many receptor proteins recognize signals outside the cell and then pass the signals on to other proteins that affect cell behaviour. In other cases, though, it is important for the cell to cluster proteins at a particular region of the membrane; this is seen where receptors are localized adjacent to the site of neurotransmitter release at a
synapse. This localization involves the coupling of the membrane proteins to a ‘scaffold’ within the cell, known as the
cytoskeleton, via specialized anchoring proteins recruited from the cytoplasm.
Although the many organelles within the cell are enclosed by membranes, they are highly dynamic, and many are in constant communication with each other. Proteins are transported in
membrane vesicles that bud from one organelle and fuse with the other; for example between the endoplasmic reticulum and the Golgi complex, and between the Golgi complex and the plasma membrane. The
budding process involves the selection of proteins to be transported and the formation of a protein scaffolding that is able to pinch off a patch of membrane to form a vesicle. The vesicle must then locate and fuse with its target membrane. It is the specificity of these membrane budding-and-fusion events that permits organelles to maintain their integrity despite extensive communication between them.
A particularly good example of the specificity of membrane traffic is found in
epithelial cells, such as those that form the tubules of the
kidney, where the plasma membrane contains two domains that perform different functions and contain different proteins (one facing outwards to the lumen of the tubule where the urine is being formed, and the other facing inwards to the tissue fluid and the blood). The two sets of proteins are synthesized together on the membrane of the endoplasmic reticulum and are later segregated into two populations of vesicles. These vesicles are then able to recognize and fuse with the two separate domains of the plasma membrane. Without this specific targeting of proteins, epithelial polarity would break down, and epithelial secretory and absorptive function would be lost.
Cell membranes are continually in a state of flux. The delivery of new membrane into the plasma membrane is balanced by the removal of membrane by the process of
endocytosis: inward budding of vesicles. Endocytosis is responsible also for the internalization of important molecules from the outside of the cell, such as
cholesterol in the form of low density lipoprotein, and iron in the form of the protein transferrin. Endocytosis is also used as a route of access into the cell by rogue invaders: certain toxins, such as botulinum toxin, or enveloped viruses, such as the influenza virus.
Michael Edwardson
See also
cell;
ion channels;
neurotransmitters;
transport.
