- All cells are enclosed by a thin, film-like membrane called the plasmalemma or more popularly as the plasma membrane
- Danielli and Davson (1935) proposed a “trilaminar model” according to which, the plasma membrane is formed of a bimolecular layer of phospholipids (35 Å thick) sandwitched between two layers of proteins (each 20 Å thick). The model was proposed even before the plasma membrane was seen under the electron microscope
- J.D. Robertson (1959) proposed a “unit membrane concept” according to which, all biological membranes shared the same basic structure:
- thickness of about 75 Å
- a characteristic trilaminar appearance when viewed with electron microscope
- the three layers are a result of the same arrangement of proteins and lipids as proposed by Danielli and Davson
- S. J. Singer and G. Nicolson (1972) put forward the “fluid mosaic model” of membrane structure which is presently the most widely accepted model.
- According to the fluid mosaic model, the cell membrane consists of a highly viscous fluid matrix of two layers of phospholipid molecules which serve as a relatively impermeable barrier to the passage of most water soluble molecules
- The plasma membrane contains lipids (32%), proteins (42%), carbohydrates (6%) and water (20%) although variations are always there
- Protein molecules or their complexes occur in the membrane, but not in continuous layer; instead, these occur as separate particles asymmetrically arranged in a mosaic pattern
- some of these proteins (peripheral or extrinsic proteins) are loosely bound at the polar surfaces of lipid layers
- some proteins (integral or intrinsic proteins) penetrate deeply into the lipid layer
- other proteins (transmembrane or tunnel proteins) penetrate through the phospholipid layers and project on both the surfaces
Illustration of a transmembrane protein - The carbohydrates occur only at the outer surface of the membrane and their molecules are covalently linked to other components of the plasma membrane thus constituting the glycocalyx of the cell surface:
- the polar heads of some lipid molecules (forming glycolipids)
- most of the proteins exposed at outer surface (forming glycoproteins)
- Cholesterol, found particularly in animal cell membranes, is an amphipathic lipid that is found in lipid bilayers that serves as a temperature-stability buffer
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| The Plasma membrane structure |
Nature of the Plasma Membrane
Membrane proteins
- Integral membrane proteins
- membrane proteins differ in the degree to which they span lipid bilayers and most of the integral membrane proteins completely span the lipid bilayer
- they are typically hydrophobic where they interact with the hydrophobic portion of the membrane and are typically hydrophilic where they interact with the hydrophilic portion of the membrane and overlying (and underlying) H2O
- Peripheral membrane proteins
- contrasting with integral membrane proteins, peripheral membrane proteins do not enter the lipid bilayer but are instead attached to the outside of the membrane via attachment to portions of integral membrane proteins jutting out of the membrane interior
- Functions of membrane proteins
- transport of substances across membranes
- enzymatic activity (e.g., smooth endoplasmic reticulum)
- signal transduction (e.g., cell communication)
- intracellular joining (e.g., intercellular junctions in animals)
- cell-cell recognition (e.g., cell communication)
- attachment to the cytoskeleton and extracellular matrix
- Fluidity of membrane proteins
- membrane proteins are capable of diffusing within the membrane, a diffusion that is similar to that of phospholipids within membranes, though not as rapid. Other membrane proteins are tied in place by attachment to the cytoskeleton or the extracellular matrix
- many eukaryotic membrane proteins are glycoproteins, proteins to which carbohydrate molecules of intermediate length (oligosaccharides) have been covalently attached. The attached oligosaccharides are always found on the extracellular side of the plasma membrane. The extracellular placement of oligosaccharides on membrane proteins makes intuitive sense since the oligosaccharides are added to these proteins within the lumen of the endomembrane system
- oligosaccharides play important roles in cell-cell recognition (i.e., oligosacherides of specific monomer sequence and branching pattern are recognized by other cells)
- a typical cell membrane tends to have a different composition on one leaflet (monolayer) than on the other. Differences between leaflets tend to include different ratios or types of amphipathic lipid-based molecules found in each leaflet, different kinds of proteins facing in or facing out, or fixed orientations of proteins spanning the membrane.
- this asymmetry allows the cell to automatically differ its intracellular environment from that existing extracellularly. Thus, asymmetries tend to be rigidly maintained via minimal flip-flopping
Membrane Fluidity
- There are typically 3 types of phospholipid mobility in a lipid bilayer which are required for the membrane to function properly:
- lateral diffusion → lipid molecules within a monolayer constantly exchange places with their neighbors
- rotation → lipid molecules within a monolayer rotate very rapidly around their long axis
- “flip-flop” → movement lipid molecules very rarely flip from one monolayer to the other
- The fluidity of a lipid bilayer depends on the nature of the hydrocarbon tails → the closer and more regular the packing of the tails, the less fluid the bilayer will be
- Membrane fluidity depends on:
- length of the hydrocarbon tails → a shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another and therefore increases the fluidity of the bilayer
- level of saturation of the hydrocarbon tails with respect to hydrogen → lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are more fluid than those with lower proportions
- presence of cholesterol → at higher temperatures, cholesterol serves to impede phospholipid fluidity whereas at at lower temperatures, cholesterol interferes with solidification of membranes (i.e. functions similarly to the effect of unsaturated fatty acids)
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