<P> There are four fundamental steps in the fusion process . First, the involved membranes must aggregate, approaching each other to within several nanometers . Second, the two bilayers must come into very close contact (within a few angstroms). To achieve this close contact, the two surfaces must become at least partially dehydrated, as the bound surface water normally present causes bilayers to strongly repel . The presence of ions, in particular divalent cations like magnesium and calcium, strongly affects this step . One of the critical roles of calcium in the body is regulating membrane fusion . Third, a destabilization must form at one point between the two bilayers, locally distorting their structures . The exact nature of this distortion is not known . One theory is that a highly curved "stalk" must form between the two bilayers . Proponents of this theory believe that it explains why phosphatidylethanolamine, a highly curved lipid, promotes fusion . Finally, in the last step of fusion, this point defect grows and the components of the two bilayers mix and diffuse away from the site of contact . </P> <P> The situation is further complicated when considering fusion in vivo since biological fusion is almost always regulated by the action of membrane - associated proteins . The first of these proteins to be studied were the viral fusion proteins, which allow an enveloped virus to insert its genetic material into the host cell (enveloped viruses are those surrounded by a lipid bilayer; some others have only a protein coat). Eukaryotic cells also use fusion proteins, the best - studied of which are the SNAREs . SNARE proteins are used to direct all vesicular intracellular trafficking . Despite years of study, much is still unknown about the function of this protein class . In fact, there is still an active debate regarding whether SNAREs are linked to early docking or participate later in the fusion process by facilitating hemifusion . </P> <P> In studies of molecular and cellular biology it is often desirable to artificially induce fusion . The addition of polyethylene glycol (PEG) causes fusion without significant aggregation or biochemical disruption . This procedure is now used extensively, for example by fusing B - cells with myeloma cells . The resulting "hybridoma" from this combination expresses a desired antibody as determined by the B - cell involved, but is immortalized due to the melanoma component . Fusion can also be artificially induced through electroporation in a process known as electrofusion . It is believed that this phenomenon results from the energetically active edges formed during electroporation, which can act as the local defect point to nucleate stalk growth between two bilayers . </P> <P> Lipid bilayers can be created artificially in the lab to allow researchers to perform experiments that cannot be done with natural bilayers . These synthetic systems are called model lipid bilayers . There are many different types of model bilayers, each having experimental advantages and disadvantages . They can be made with either synthetic or natural lipids . Among the most common model systems are: </P>

What lipid do animal cells have in their cell membrane