Editing Lipid bilayer (section)

===Containment and separation===
The primary role of the lipid bilayer in biology is to separate [[Aqueous solution|aqueous]] compartments from their surroundings. Without some form of barrier delineating “self” from “non-self”, it is difficult to even define the concept of an organism or of life. This barrier takes the form of a lipid bilayer in all known life forms except for a few species of [[archaea]] that utilize a specially adapted lipid monolayer.<ref name=Brock2003>{{cite book |vauthors=Parker J, Madigan MT, Brock TD, Martinko JM |title=Brock biology of microorganisms |url=https://archive.org/details/brockbiologyofmi00madi |url-access=registration |publisher=Prentice Hall |location=Englewood Cliffs, N.J |year=2003 |edition=10th |isbn=978-0-13-049147-3 }}</ref> It has even been proposed that the very first form of life may have been a simple [[lipid vesicle]] with virtually its sole [[Biosynthesis|biosynthetic]] capability being the production of more [[phospholipid]]s.<ref name=Koch1985>{{cite journal |author=Koch AL |title=Primeval cells: possible energy-generating and cell-division mechanisms |journal=J. Mol. Evol. |volume=21 |issue=3 |pages=270–7 |year=1984 |pmid=6242168 |doi=10.1007/BF02102359 |s2cid=21635206 }}</ref> The partitioning ability of the lipid bilayer is based on the fact that [[hydrophilic]] molecules cannot easily cross the [[hydrophobic]] bilayer core, as discussed in Transport across the bilayer below. The nucleus, mitochondria and chloroplasts have two lipid bilayers, while other sub-cellular structures are surrounded by a single lipid bilayer (such as the plasma membrane, endoplasmic reticula, Golgi apparatus and lysosomes). See [[Organelle]].<ref>{{Cite web |url=http://csls-text.c.u-tokyo.ac.jp/active/05_01.html |title=5.1 Cell Membrane Structure {{!}} Life Science {{!}} University of Tokyo<!-- Bot generated title --> |access-date=10 November 2012 |archive-url=https://web.archive.org/web/20140222160647/http://csls-text.c.u-tokyo.ac.jp/active/05_01.html |archive-date=22 February 2014 |url-status=dead }}</ref>

[[Prokaryote]]s have only one lipid bilayer - the [[cell membrane]] (also known as the plasma membrane). Many prokaryotes also have a [[cell wall]], but the cell wall is composed of [[protein]]s or long chain [[carbohydrate]]s, not lipids. In contrast, [[eukaryote]]s have a range of [[organelle]]s including the [[Cell nucleus|nucleus]], [[mitochondria]], [[lysosome]]s and [[endoplasmic reticulum]]. All of these sub-cellular compartments are surrounded by one or more lipid bilayers and, together, typically comprise the majority of the bilayer area present in the cell. In liver [[hepatocyte]]s for example, the plasma membrane accounts for only two percent of the total bilayer area of the cell, whereas the endoplasmic reticulum contains more than fifty percent and the mitochondria a further thirty percent.<ref name=Alberts2002/>
[[File:PDB 1hzx 7TM Sketch Membrane.png|thumb|260px|Illustration of a GPCR signaling protein. In response to a molecule such as a [[hormone]] binding to the exterior domain (blue) the GPCR changes shape and [[catalyzes]] a chemical reaction on the interior domain (red). The gray feature is the surrounding bilayer.]]