Abundant is partly due to its relatively
Abundant in living organisms and vital for the formation,survival and function of cells, biological membranes are important structuresthat define the space inside and outside cells. They control what can enter orleave cells due to their partial permeability, as well as some of the membraneproteins embedded or anchored to the membrane. Functions of membrane proteins canvary from cell-cell recognition & signal transduction to transport & enzymeactivity. There are also intracellular membranes that serve as organelleformation and more. We know that biological membranes are paramount to the lifeof every cell, in many different but connected ways, which is partly due to itsrelatively simple structure. 1 It is generally accepted that membranes takethe fluid mosaic structure, where every membrane is comprised of 2 layers oflipids, with the hydrophilic ‘heads’ pointing outwards and the hydrophobic’tails’ pointing inwards to form what is called a bilayer.
This minimises contactof the non-polar long carbon chains with the aqueous environment and maximisescontact of the aqueous environment with the polar region of the lipids. Thisconformation of lipids forms a bilayer sheet, which the plasma membrane adopts spontaneouslyin an aqueous environment, due to the strong hydrophobic repulsions of the longchains to water molecules. Hydrophobic long chains cluster together from Van derWaals forces forming between long chains. Also the polar hydrogen bondformation of water molecules and hydrophilic heads and dipole-dipoleinteractions indirectly attract hydrophilic heads to each other. Since anyinteraction between water and the fatty acid chains are energeticallyunfavourable, any gaps for a possible water molecule to pass through is blockedby a hydrophilic head, essentially ‘sealing’ the hydrophobic tails in thebilayer shape, resulting in a relativelystrong ‘wall’ blocking water soluble molecules. The bilayer sheet isn’t theonly bilayer structure formed as they can also form liposomes, a sphericalbilayer shape (vesicle membranes take this shape). The bilayer structure isconstantly moving (hence ‘fluid’), with membrane proteins and lipids free tomove around if not anchored to any part of a cell.
2One of the three main lipidsutilised in these membranes, phospholipids are the most abundant molecules inbiological membranes and is composed of one or two fatty acid chains attachedto the C1 (& C2) atoms (via esterification) of glycerol molecule with an alcohol-phosphategroup which is then attached to the C3 atom of glycerol. The long hydrocarbonchains (saturated or unsaturated) act as the hydrophobic ‘tails’ and the polar –OHgroups act as the hydrophilic ‘heads’. These lipids are calledphosphoglycerides, although they are not the only phospholipids in membranes.The platform molecule (that which connects the long chains and the phosphategroup) can be replaced with sphingosine, an alcohol with an amino group and along hydrocarbon chain to form sphingomyelin, another phospholipid. The alcoholgroups attached to phosphate are most commonly one of 5 alcohols: Serine,Ethanolamine, Choline, Glycerol or Inositol. 3Glycolipids are another type oflipid common in the bilayer. They are similar in shape to phospholipids, onlydiffering in structure by a saccharide group in place of the phosphate-alcoholgroup. They are often in the plasma membrane, with the saccharide group on theextracellular side of the membrane.
This is often to perform cell-cellsignalling, with the saccharide group able to bind to many binding sites, amongother functions. 2 The othercommon lipid molecules are sterols. These are steroids, with a framework ofmultiple joint hydrocarbon rings and a non polar chain joined at one end (tail),with a polar –OH on the ring framework (head). Sterols, such as cholesterol, playan important role in fluidity and adding strength to membranes. 4 Therings are attracted to adjacent fatty acid chains, and so end up preventing toomuch fluidity of the hydrophilic heads by holding phospholipids in placeleading to small water soluble molecules not being able to diffuse through theplasma membrane & making the membrane firmer.
They also maintain areasonably high level of fluidity, as the shape of the ring framework meansadjacent phospholipids are kept a certain distance from each other alongsidethe sterol. This is advantageous in cold conditions, where this space preventsthe lipids crystallising and rupturing the membrane.
