Editing Lipid bilayer (section)

=== Phases and phase transitions ===

{{further|Lipid bilayer phase behavior}}

[[File:Lipid unsaturation effect.svg|right|thumb|350px|Diagram showing the effect of unsaturated lipids on a bilayer. The lipids with an unsaturated tail (blue) disrupt the packing of those with only saturated tails (black). The resulting bilayer has more free space and is, as a consequence, more permeable to water and other small molecules.]]

At a given temperature a lipid bilayer can exist in either a liquid or a gel (solid) phase. All lipids have a characteristic temperature at which they transition (melt) from the gel to liquid phase. In both phases the lipid molecules are prevented from flip-flopping across the bilayer, but in liquid phase bilayers a given lipid will exchange locations with its neighbor millions of times a second. This [[random walk]] exchange allows lipid to [[diffusion|diffuse]] and thus wander across the surface of the membrane.Unlike liquid phase bilayers, the lipids in a gel phase bilayer have less mobility.<ref name=Berg1993>{{cite book |author=Berg, Howard C. |title=Random walks in biology |publisher=Princeton University Press |location=Princeton, N.J |year=1993 |isbn=978-0-691-00064-0 |edition=Extended Paperback}}</ref> 

The phase behavior of lipid bilayers is determined largely by the strength of the attractive [[van der Waals force|Van der Waals]] interactions between adjacent lipid molecules. Longer-tailed lipids have more area over which to interact, increasing the strength of this interaction and, as a consequence, decreasing the lipid mobility. Thus, at a given temperature, a short-tailed lipid will be more fluid than an otherwise identical long-tailed lipid.<ref name=Rawicz2000/> Transition temperature can also be affected by the [[degree of unsaturation]] of the lipid tails. An unsaturated [[double bond]] can produce a kink in the [[alkane]] chain, disrupting the lipid packing. This disruption creates extra free space within the bilayer that allows additional flexibility in the adjacent chains.<ref name=Rawicz2000/> An example of this effect can be noted in everyday life as butter, which has a large percentage saturated fats, is solid at room temperature while vegetable oil, which is mostly unsaturated, is liquid.<ref>{{cite web |title=Fats and oils |url=https://www.heartuk.org.uk/low-cholesterol-foods/fats-and-oils |website=Heart UK: The Cholesterol Charity |access-date=1 December 2024}}</ref>

Most natural membranes are a complex mixture of different lipid molecules. If some of the components are liquid at a given temperature while others are in the gel phase, the two phases can coexist in spatially separated regions, rather like an iceberg floating in the ocean. This phase separation plays a critical role in biochemical phenomena because membrane components such as proteins can partition into one or the other phase and thus be locally concentrated or activated.<ref name=Dietrich2001>{{cite journal |vauthors=Dietrich C, Volovyk ZN, Levi M, Thompson NL, Jacobson K |title=Partitioning of Thy-1, GM1, and cross-linked phospholipid analogs into lipid rafts reconstituted in supported model membrane monolayers |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=98 |issue=19 |pages=10642–7 |date=September 2001 |pmid=11535814 |pmc=58519 |doi=10.1073/pnas.191168698 |bibcode=2001PNAS...9810642D |doi-access=free }}</ref> One particularly important component of many mixed phase systems is [[cholesterol]], which modulates bilayer permeability, mechanical strength, and biochemical interactions.<ref name="isbn1-4292-4646-4">{{cite book |vauthors=Sadava D, Hillis DM, Heller HC, Berenbaum MR |chapter=Cell Membranes |title=Life: The Science of Biology |edition=9th |publisher=Freeman |location=San Francisco |year=2011 |pages=105–114 |isbn=978-1-4292-4646-0 }}</ref>