Here is a compilation of notes on Cell Membrane. After reading these notes you will learn about: 1. Composition of Cell Membrane 2. Structure of Cell Membrane 3. Function 4. Constituents.
Note # 1. Composition of Cell Membrane:
Cell membrane essentially consists of lipids and proteins. Carbohydrate is present in the form of glycoproteins and glycolipids. Membranes contain three different classes of proteins—structural proteins, enzymes and carrier proteins of which structural proteins form the backbone of the cell membrane and are extremely lipophilic.
The plasma membrane proteins fall in two categories, intrinsic or integral proteins and extrinsic or peripheral proteins. The former are firmly associated with the membrane, while the latter have a weaker association and are bound by electrostatic interaction. The lipids in the membrane consist mainly of phospholipids in addition to glycolipids and sterols.
Polar lipids contain hydrophilic heads and hydrophobic tails, bridged by glycerol moiety.
Note # 2. Structure of Cell Membrane:
Several models have been proposed to explain the physical and biological features of cell membranes.
(a) Trilaminar Sandwich Model (Danielli-Davson):
According to this model the bimolecular lipid layer consists of two layers of molecules with the polar regions on the outer side. Globular proteins are thought to be associated with the polar groups of the lipid (Fig. 2.35). The proteins are of two types—tangentially arranged proteins in contact with the lipid, and globular proteins on the outer surface.
The lipids in the membrane consist mainly of phospholipids, with their non-polar groups near each other and their polar groups directed outwards. The lipid layer in many cases consists of a phospholipid lecithin alternating with a steroid molecule cholesterol. The lecithin molecule consists of two lipid chains of glycerol and a polar head containing phosphate and choline.
(b) Unit Membrane (Robertson):
The basic unit membrane structure was considered to be general for a wide variety of plant and animal, cells. Membranes of cell organelles like mitochondria, lysosomes, plastids, Golgi complex, the endoplasmic reticulum and the nuclear envelope were thought to have the unit membrane structure, indicating its cellular universality.
The unit membrane is considered to be trilaminar, with a bimolecular lipid layer between two protein layers (Fig. 2.36).
Under the electron microscope, after osmium fixation, the cell membrane appears like two dense osmiophilic bands separated by a clear zone. Each dense band is made up of protein (20A) and the polar groups of the lipids (SA), and is thus 25A thick (Fig. 2.37). The clear zone is 25A thick and consists of the bimolecular lipid layer without the polar groups.
Thus the unit membrane is 75A, with a 35A lipid layer between two protein layers, each 20A m thickness.
(c) Fluid-Mosaic Model (Singer and Nicolson):
Membrane is considered to be quasifluid structure in which the lipids and integral proteins are arranged in a mosaic manner (Fig. 2.38). The fluidity of the membrane is the result of the hydrophobic interaction between lipids and proteins. There is a continuous bilayer of phospholipid molecules in which are embedded globular proteins.
The globular proteins of the membrane are considered to be of two different types, extrinsic (peripheral) proteins and intrinsic (integral) proteins. The peripheral proteins are soluble, readily dissociated from the membrane and are entirely outside the lipid bilayer.
The integral proteins are relatively insoluble and penetrate either surface of the lipid bilayer. The integral proteins are amphipathic with hydrophilic polar heads protrude from the surface of the membrane, while the non-polar regions are embedded in the interior of the membrane.
The integral proteins are capable of lateral diffusion in the lipid bilayer. The lipid bilayer has many dynamic motional properties — rapid internal motion involving flexing, lateral diffusion of the lipids, transfer of lipid molecules from one side of the bilayer to the other, rotation about their axes. Because of the rapid movement of the lipid and protein molecules the membrane is considered to be highly fluid.
Note # 3. Function of Cell Membrane:
The plasma membrane acts as a barrier which, however, permits the movement of certain substances in and out of the cell. Cell membranes are selectively permeable rather than semi-permeable. Transport of molecules across the membrane may be active or passive.
Thus the membrane regulates the passage of certain nutrient molecules into the cell, the removal of waste products and the release of secretary product from the cell.
It also protects various organelles of the cytoplasm and gives shape to the cell and sometimes, it gives origin of certain cell organelles. The cell membrane also contains receptors which recognize specific hormones; molecules responding to a variety of stimuli and the sites for cell recognition. The plasma membrane of bacteria contains the electron transport chain which plays an important role in cell respiration.
Note # 4. Constituents of Cell Membrane:
i. Membrane Lipids:
Membrane lipids are amphipathic molecules containing both hydrophobic fatty acid chains and a hydrophilic polar head group. Three major classes of lipids are commonly present in membrane: Glycerophospholipids, Sphingolipids and Sterols.
The glycerophospholipids have a glycerol backbone that is attached to two fatty acid hydrocarbon chains and a phosphorylated head group. These includes phosphatidate, phosphatidyl choline, phosphatidyl ethanolamine, phosphatidyl glycerol, phosphatidyl inositol, phosphatidyl serine and di-phosphatidyl glycerol.
The Sphingolipids are based on sphingosine to which a single fatty acid chain and either a phosphorylated head group (sphingomyelin) or sugar residues (glycosphingolipids) are attached. Sterols include cholesterol, stigma-sterol and β- sitosterol.
Role of Lipids:
Lipid molecules play a major role in maintaining fluid property of membrane. Because of the absence of covalent bonds between the lipids in the bilayer, the membrane has fluidity. The flip-flop movement of lipid molecules occurs very rarely from one lipid monolayer to other monolayer of lipid bimolecular layer.
However, they readily exchange places with their neighbours within a monolayer (~107 times a second) which results in their rapid lateral diffusion. Individual lipid molecules rotate very rapidly about their long axes and the hydrocarbon chains are flexible causing greatest degree of flexion near the centre of bilayer (Fig. 2.39).
The double bonds in unsaturated hydrocarbon chains tend to increase the fluidity of phospholipid bilayer by making it more difficult to pack the chains together. Sterols are thought to enhance both the flexibility and mechanical stability of the bilayer.
Sterol molecules orient themselves in the bilayer in such a way that their hydroxyl groups remain close to polar head groups of the phospholipids, their rigid plate-like steroid rings interact with and partly immobilize those regions of hydrocarbon chains that are closest to the polar head groups, leaving the rest of the chain flexible (Fig. 2.40).
Sterol inhibits phase transition by preventing hydrocarbon chain coming together. Inositol phospholipids are functionally very important particularly in cell signalling. Glycolipids help in cell recognition.
ii. Membrane Proteins:
The amount and types of protein in the membranes are highly variable. According to their position, the proteins are intrinsic (integral) or extrinsic (peripheral). Intrinsic proteins are tightly associated with the hydrophobic core of the lipid bilayer.
Most of them have the region of polypeptide chains that traverse the lipid bilayer by non-covalent interactions, while some proteins are attached covalently and do not traverse the membrane. Integral proteins are asymmetrically distributed across the membrane.
Extrinsic Proteins are loosely bound to the surface of the plasma membrane by non-covalent ionic and hydrogen bonds, no part of it interacts within the hydrophobic interior of the bilayer.
Role of Proteins:
Structural proteins of membrane are extremely lipophilic and form the main bulk, i.e., backbone of the plasma membrane and impart mechanical strength. Integral proteins are usually free to move in the plane of the bilayer by lateral and rotational movement, but are unable to flip from one side of the membrane to the other (transverse movement).
Transport proteins (permeases carriers) transport specific substances across the plasma membrane, either behaving as mobile carriers (carrier proteins) or transport channels (channel proteins) (Fig. 2.41 A). Channel proteins form open pores through the membrane allowing the free passage of any molecule of appropriate size. Carrier proteins selectively bind and transport specific small molecules, such as glucose.
Uniport, Symport and Antiport:
Carrier proteins involved in facilitated diffusion are uniporters (transporting single solute), symporters (transport of one solute depends on simultaneous transfer of a second solute in the same direction) and anti-porters (transport of one solute depends on simultaneous transfer of a second solute, but in opposite direction) (Fig. 2.41 B).
Ion fluxes involving passive transport are facilitated by ion channels (each is a single channel protein). Some trans membrane proteins catalyse the transport of anions; some like bacteriorhodopsin can pump proton in efficient way (light driven active transport); porins allow selected hydrophilic solutes to pass across the lipid bilayer.
Many proteins in the membrane serve as enzyme catalysts. The enzymes of plasma membrane are either endoenzymes or ecto-enzymes and are of about 30 types (Table 2.3). Some of the membrane proteins may act as receptors (e.g., glycoprotein), regulatory molecules and may also act as antigens.
iii. Membrane Carbohydrates:
Membrane carbohydrates are present as short un-branched or branched chains of oligosaccharides, confined mostly to the external side of the plasma membrane, in the form of covalently linked molecules with either lipids to form glycolipids or with proteins to form glycoproteins.
In glycoproteins the sugar residues are attached either to the hydroxyl group of serine or threonine to form O-linked oligosaccharides or to the amide group of asparagine to form N-linked oligosaccharides.
The common sugars associated with the proteins are D-glucose, D-galactose, D-mannose, D-xylose, L-fucose, L-arabinose as well as sugar derivatives like N-acetyl- D-glucosamine, N-acetyl-D-galactosamine, N- acetyl-muramic acid. In glycolipids carbohydrate is attached to the glycerol molecules of the lipid through glycosidic bonds.
Role of Carbohydrates:
The carbohydrates on the external surface of the membrane not only serves the protective role but is also involved in intercellular recognition and in maintaining the asymmetry of the membrane.
It has been suggested that because of their presence on the outer surface, the membrane is negatively charged, so the positively charged proteins may remain bound to plasma membrane through electrostatic interaction. Recent work has shown that glycoproteins have the capacity to bind to hormones. Glycolipids help in cell recognition.