Editing Lipid bilayer (section)

=== Signaling ===

{{see also|Neurotransmission}}

The most familiar form of cellular signaling is likely [[synaptic transmission]], whereby a nerve impulse that has reached the end of one [[neuron]] is conveyed to an adjacent neuron via the release of [[neurotransmitter]]s. This transmission is made possible by the action of [[synaptic vesicle]]s which are, inside the cell, loaded with the neurotransmitters to be released later. These loaded vesicles [[Lipid bilayer fusion|fuse]] with the cell membrane at the pre-synaptic terminal and their contents are released into the space outside the cell. The contents then diffuse across the synapse to the post-synaptic terminal.<ref>{{cite book |first1=Bryan |last1=Kolb |first2=Ian Q. |last2=Whishaw |name-list-style=vanc |year=2003 |title=Fundamentals of Human Neuropsychology |edition=5th |pages=102–104 |publisher=Worth |isbn=978-0-7167-5300-1 }}</ref>

Lipid bilayers are also involved in signal transduction through their role as the home of [[integral membrane protein]]s. This is an extremely broad and important class of biomolecule. It is estimated that up to a third of the human [[proteome]] are membrane proteins.<ref name=Martelli2003>{{cite journal |vauthors=Martelli PL, Fariselli P, Casadio R |title=An ENSEMBLE machine learning approach for the prediction of all-alpha membrane proteins |journal=Bioinformatics |volume=19 |issue=Suppl 1|pages=i205–11 |year=2003 |pmid=12855459 |doi=10.1093/bioinformatics/btg1027|doi-access=free }}</ref> Some of these proteins are linked to the exterior of the cell membrane. An example of this is the [[CD59]] protein, which identifies cells as “self” and thus inhibits their destruction by the immune system. The [[HIV]] virus evades the [[immune system]] in part by grafting these proteins from the host membrane onto its own surface.<ref name=Alberts2002>{{cite book |author=Alberts, Bruce |title=Molecular biology of the cell |publisher=Garland Science |location=New York |year=2002 |edition=4th |isbn=978-0-8153-4072-0 }}</ref> Alternatively, some membrane proteins penetrate all the way through the bilayer and serve to relay individual signal events from the outside to the inside of the cell. The most common class of this type of protein is the [[G protein-coupled receptor]] (GPCR). GPCRs are responsible for much of the cell's ability to sense its surroundings and, because of this important role, approximately 40% of all modern drugs are targeted at GPCRs.<ref name=Filmore2004>{{cite journal |author=Filmore D |title=It's A GPCR World |journal=Modern Drug Discovery |volume=11 |pages=24–9 |year=2004}}</ref>

In addition to protein- and solution-mediated processes, it is also possible for lipid bilayers to participate directly in signaling. A classic example of this is [[phosphatidylserine]]-triggered [[phagocytosis]]. Normally, phosphatidylserine is asymmetrically distributed in the cell membrane and is present only on the interior side. During programmed cell death a protein called a [[scramblase]] equilibrates this distribution, displaying phosphatidylserine on the extracellular bilayer face. The presence of phosphatidylserine then triggers phagocytosis to remove the dead or dying cell.<ref>{{Cite journal |last=Fadok |first=V A |last2=Voelker |first2=D R |last3=Campbell |first3=P A |last4=Cohen |first4=J J |last5=Bratton |first5=D L |last6=Henson |first6=P M |date=1992-04-01 |title=Exposure of phosphatidylserine on the surface of apoptotic lymphocytes triggers specific recognition and removal by macrophages. |url=http://dx.doi.org/10.4049/jimmunol.148.7.2207 |journal=The Journal of Immunology |volume=148 |issue=7 |pages=2207–2216 |doi=10.4049/jimmunol.148.7.2207 |issn=0022-1767}}</ref>