Cell Membrane / Plasma Membrane: Structure and Function
Plasma Membrane / Cell Membrane
o The cytosol of nearly all prokaryotes is surrounded by a phospholipid bilayer called the plasma membrane (the membranes of archaea differ in their lipid structure).
o It gives the cell its basic structure and serves as a permeability barrier.
o The phospholipid is composed of a phosphate group, two fatty acid chains, and a glycerol backbone.
o The phospholipid group is polar, while the fatty acid chains are nonpolar making the molecule amphipathic.
o When placed in aqueous solution, amphipathic molecules spontaneously aggregate, turning their polar ends toward the solution, and their nonpolar ends toward each other.
o The resulting spherical structure is called a micelle.
o If enough phospholipids exist, and the solution is subjected to ultrasonic vibrations, liposomes may form.
o A liposome is a vesicle surrounded and filled by aqueous solution.
o It contains a lipid bilayer like that of a plasma membrane.
o The inner and outer layers of a membrane are referred to as leaflets.
o The level of saturation in the fatty acids of the phospholipids also determines the membranes fluidity; an increase in the unsaturation of these fatty acids increases the fluidity of the membrane.
o If we increase the temperature as well the membrane fluidity increases.
o The plasma membrane contains other types of lipids such as glycolipids.
o Different lipid types are arranged asymmetrically between the leaflets.
o For instance, glycolipids are found on the outer leaflet only.
o Unlike eukaryotic membranes, prokaryotic membranes usually DON’T contain steroids such as cholesterol.
Instead, some bacterial membranes contain steroid like molecules called hopanoids.
o Cholesterol tends to stiffen the bilayer, making it more rigid and less permeable.
o Hoponoids probably reduce the fluidity of the membrane in Prokaryotes.
o In eukaryotes nearly all new membrane synthesis occurs in the ER.
o Also embedded within the plasma membrane are proteins.
o Most of the functional aspects of membranes are due to their proteins.
o Membrane proteins act as transporters, receptors, attachment sites, and enzymes.
Organization of Plasma Membrane
Types of Protein in Plasma Membrane
o Two types of proteins are involved with the plasma membrane:
1) Integral Proteins:Â Ampipathic proteins that transverse the membrane from inside the cell to the outside.
o Are usually ion channels.
2) Peripheral or Extrinsic Proteins: are situated entirely on the surfaces of the membrane.
o They are ionically bonded to integral proteins or the polar group of a lipid.
o Both integral and peripheral proteins may contain carbohydrate chains making them glycoproteins and the carbohydrate portion always protrudes toward the outside of the cell.
o Proteoglycans also exist on the membrane.
o It can thus form a carbohydrate layer (glycocalyx) which protects the cell surface from chemical and mechanical damage.
o Proteoglycans are also a mixture of proteins and carbohydrates, but they generally consist of more than 50% carbohydrates.
o Proteoglycans are a major component of extracellular matrix.
o Glycoproteins are proteins with a carbohydrate group attached and they are a component of cellular plasma membranes.
o Also serve as markers for cellular recognition.
o Lipoproteins also exist in some plasma membranes with their lipid portions embedded in the membrane and their protein portions at the surfaces.
o Membrane proteins are distributed asymmetrically throughout the membrane and between the leaflets.
o Neither proteins nor lipids easily flip from one leaflet to the other.
o The shape of the cell and the mechanical properties of the plasma membrane are determined by a meshwork of fibrous proteins, called the cell cortex, that is attached to the cytosolic surface of the membrane.
o It is made up primarily of spectrin.
o Since the forces holding the entire membrane are intermolecular, the membrane is fluid; its parts can move laterally but can’t separate.
o The model of the membrane as just described is known as the fluid mosaic model.
Plasma Membrane Citations
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