The outer limiting membrane present in most animal cells is plasmamembrane, plasmalemma or cell membrane. It lies between the cell wall and the cytoplasm in plant cells. In addition, all eucaryotic cells possess a number of intracellular membrane systems that surround the vacuole and other organelles. The plasmamembrane and the subcellular membrane are grouped together as biological membranes.
The plasmamembrane is mainly composed of proteins, lipids and a small amount of (1-5%) oligosaccharides that may either be attached to lipids (glycolipids) or the proteins (glycoproteins). Proteins constitute about 60-80% of the substance which are present in the form of structural, carrier and enzymatic proteins. Lipids constitute about 20-40%, which are mostly present in the form of phospholipids, galactolipids or cholesterol.
All biological membranes, including the plasmamembrane and the internal membranes of eucaryotic cells exhibit a common overall structure. Since biological membranes are very thin and difficult to see, much of the early information on their structure are obtained from indirect approaches based on their physicochemical properties. They are made up of protein and lipid molecules held together by non-covalent interactions. The first indication that biological membranes may include bilayered lipid molecules came from a study in 1925, by Gorter and Grendel. Harvey and Cole (1931) proposed a lipid-protein model of the cell membrane, according to which the cell membrane consists of two layers of lipid molecules.
The lipid molecules have their polar regions towards outer side and are associated with globulin proteins. Danielli and Davson (1935) proposed that the plasmamembrane is made up of lipid molecules sandwiched between two continuous layers of proteins. With the development of the electron microscope, it became possible to understand the internal structure of cell membrane. J.David Robertson (1959) proposed the Unit Membrane Concept. According to this, all biological membranes are trilamellar two outer electron dense protein layers separated by a lighter middle phospholipid layer. Thus, all the membranes in the cell are now known as unit membrane although different membranes show many structural, physico-chemical and functional difference.
Fluid mosaic model
In 1972, Singer and Nicholson proposed the Fluid Mosaic Model for plasmamembrane which is the most accepted model. They published their paper in the reputed journal Science in 1972. They described the plasmamembrane as protein icebergs in a sea of lipids. The model describes a central, continuous bilayer of lipid molecules that forms the structural framework of the plasmamembrane with their spherical, polar heads towards outer surface. The two non-polar tails of each molecule point inwards facing each other. In this way, the membrane forms a water resistant barrier, through which only lipid soluble substances can pass.
The protein molecules are arranged in two different manners. Some proteins are located exclusively adjacent to the outer and inner surfaces of lipid bilayer and are called extrinsic or peripheral proteins. These can be easily dislodged from the membrane. The other proteins penetrate the lipid bilayer partially or wholly and are called integral or intrinsic proteins. These proteins are either large, globular and project beyond the lipid layer on both sides or are smaller, partially penetrate the lipid layer and are exposed on one surface only.
This arrangement explains why the active, sites of enzymes and antigenic glycoproteins are exposed to the outer surface of the membranes. The quasi-fluid structure of plasmamembrane explains the movement of cluster of protein molecules of considerable size across the membrane.
Some lipids and integral proteins in the plasmamembrane are bound to oligosaccharide which project into the extracellular fluid from the outer surface of the membrane. Cells recognise one another on the basis of these glycoproteins and glycolipids. This remarkable ability of cell membranes to recognise their own kind plays a distinct role in blood-grouping, immune response, cancer and rejection of transplanted organs.
The proteins play an important role, not only in the mechanical structure of the membrane, but also as carriers or channels, serving for transport. Specific proteins in the form of membrane receptors can mediate the flow of materials and information into the cell by binding with specific molecules reaching the cell surface. Some receptors which bind with adrenaline hormone can be taken as an example. In addition, some membrane proteins function as enzymes. More than thirty enzymes have been detected in isolated plasmamembrane. The electron transport enzymes in respiration which are located in the mitochondrial membrane are good examples of membrane proteins serving as enzymes.
The primary function of plasmamembrane is the regulation of movement of various substances in and out of the cell. Besides, it may perform several other functions.
Cell membrane is selectively permeable. The permeability across the membrane may be passive, if it obeys only physical laws as in the case of diffusion, or active, if it involves the expenditure of energy. We know, when a concentrated solution of a soluble substance (sugar) is placed in water and is separated by a semipermeable membrane there will be a movement of solutes along the concentration gradient, i.e., from the region of high to that of low concentration. Materials, that are lipid soluble have an easy access into the membrane than that of water soluble substance.
Exocytosis and endocytosis are two other important functions associated with plasmamembrane. The cell throws out the macromolecules through the process of exocytosis and ingests them through endocytosis. In exocytosis a vesicle in the cytoplasm moves to the inside surface of the plasmamembrane and fuses with it, releasing the vesicular contents outside membrane.
In endocytosis a region of the plasmamembrane invaginates, enclosing a small volume of extracellular fluid within a bud that pinches off inside the cell by membrane fission. The resulting small vesicle, called endosome, can move into the interior of the cell, delivering its contents to another organelle bounded by single membrane. When the foreign substances ingested are fluids, the process is called pinacocytosis or pinocytosis and, when are solids, the process is called phagocytosis.
Presence of glycoproteins and glycolipids are specific in plasmamembrane of different cells. This plays an important role in selective adhesion of cells. Hence, the cells of heart and liver, do not adhere. Besides, rejection or acceptance of transplanted tissues is also determined by these specific components of the membrane. Also, the plasmamembrane participates in cellular locomotion either by the formation of pseudopodia or by undulation of the plasmamembrane.