The structure of biological membranes typically includes several types of molecules in addition to the phospholipids comprising the bilayer. A particularly important example in animal cells is cholesterol, which helps strengthen the bilayer and decrease its permeability. Cholesterol also helps regulate the activity of certain integral membrane proteins. Integral membrane proteins function when incorporated into a lipid bilayer, and they are held tightly to the lipid bilayer with the help of an annular lipid shell. Because bilayers define the boundaries of the cell and its compartments, these membrane proteins are involved in many intra- and inter-cellular signaling processes. Certain kinds of membrane proteins are involved in the process of fusing two bilayers together. This fusion allows the joining of two distinct structures as in the acrosome reaction during fertilization of an egg by a sperm, or the entry of a virus into a cell. Because lipid bilayers are fragile and invisible in a traditional microscope, they are a challenge to study. Experiments on bilayers often require advanced techniques like electron microscopy and atomic force microscopy.
the sTruCturE of bIolOGiCal meMbRanes typically IncLuDES SeVerAl TYpES of moleCules in addiTion to thE phoSphoLipidS comPrisIng thE Bilayer. a particularLY iMpOrtaNt EXampLe in animal CellS is ChoLesterol, wHich helPs sTrengthen thE BilAyeR AnD DeCrEaSE its pERmeabIlitY. chOLesterol aLso HelPs ReGulAte thE ACtIvity oF cErTain INTEgRAl MembRAne PRoteInS. iNtegrAl meMBraNE PRoteINs fuNCtioN WHen incoRporAtEd inTo a lipid bILaYer, aNd THey aRE held Tightly To thE lipId biLAyer With the helP Of an aNnuLAr liPId sHelL. BecAUsE bilaYERs DefIne the boUnDarieS of thE cell anD its cOmParTMEnTS, tHeSe meMBRANe PrOTeINs arE involVed in many intra- aNd intER-cellular siGNaLInG pRocesSEs. ceRtaiN kinds of meMbrane pRoteiNS aRe iNvolVed In the process of fUsIng Two bilayErs tOgetHer. thIs fusIon alLowS tHE jOInIng OF twO DistiNcT sTrUcTurEs as In the acROsomE ReactIon DURing ferTiliZatIOn Of an egg by A Sperm, or The entry Of a virus inTO a cell. beCaUse lIpId bIlaYers are fragile and invisiblE In a TrAditional miCroscope, they arE A cHAlLenge to study. ExperimeNts oN bilayers OFTEN rEQUiRe advaNCEd teCHNiques like elecTrOn mIcroscopY and aToMIc FoRce micrOscoPy.