Cell Membrane / Plasma Membrane: Structure and Function

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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

Cell Membrane, Cell Membrane Function, Cell Membrane Structure, What is Cell Membrane,

Types of Protein in Plasma Membrane

o Two types of proteins are involved with the plasma membrane:

1) Integral Proteins: 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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