Editing Lipid (section)

==Biological functions==

===Component of biological membranes===
[[Eukaryote|Eukaryotic]] cells feature the compartmentalized membrane-bound [[organelle]]s that carry out different biological functions. The [[glycerophospholipids]] are the main structural component of [[biological membranes]], as the cellular [[plasma membrane]] and the intracellular membranes of organelles; in animal cells, the plasma membrane physically separates the [[intracellular]] components from the [[extracellular]] environment.{{citation needed|date= January 2015}} The glycerophospholipids are [[amphipathic]] molecules (containing both hydrophobic and hydrophilic regions) that contain a glycerol core linked to two fatty acid-derived "tails" by ester linkages and to one "head" group by a [[phosphate]] ester linkage.{{citation needed|date= January 2015}} While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as [[sphingomyelin]] and [[sterol]]s (mainly cholesterol in animal cell membranes) are also found in biological membranes.<ref>{{cite journal | vauthors = Coones RT, Green RJ, Frazier RA | title = Investigating lipid headgroup composition within epithelial membranes: a systematic review | journal = Soft Matter | volume = 17 | issue = 28 | pages = 6773–6786 | date = July 2021 | pmid = 34212942 | doi = 10.1039/D1SM00703C|issn=1744-683X | s2cid = 235708094 | bibcode = 2021SMat...17.6773C | doi-access = free }}</ref><ref name = "Stryer_2007" />{{rp|329–331}} In plants and algae, the galactosyldiacylglycerols,<ref name="Heinz">Heinz E. (1996). "Plant glycolipids: structure, isolation and analysis", pp. 211–332 in ''Advances in Lipid Methodology'', Vol. 3. W.W. Christie (ed.). Oily Press, Dundee. {{ISBN|978-0-9514171-6-4}}</ref> and sulfoquinovosyldiacylglycerol,<ref name="Hölzl_2007"/> which lack a phosphate group, are important components of membranes of chloroplasts and related organelles and are among the most abundant lipids in photosynthetic tissues, including those of higher plants, algae and certain bacteria.<ref name="Lyu et al. 2021">{{cite journal |last1=Lyu |first1=Jiabao |last2=Gao |first2=Renjun |last3=Guo |first3=Zheng |title=Galactosyldiacylglycerols: From a photosynthesis-associated apparatus to structure-defined ''in vitro'' assembling |journal=Journal of Agricultural and Food Chemistry |volume=69 |issue=32 |year=2021 |doi=10.1021/acs.jafc.1c00204 |pages=8910–8928|pmid=33793221 |bibcode=2021JAFC...69.8910L |s2cid=232761961 }}</ref>

Plant thylakoid membranes have the largest lipid component of a non-bilayer forming monogalactosyl diglyceride (MGDG), and little phospholipids; despite this unique lipid composition, chloroplast thylakoid membranes have been shown to contain a dynamic lipid-bilayer matrix as revealed by magnetic resonance and electron microscope studies.<ref name="Yashroy_1990"/>

[[File:Phospholipids aqueous solution structures.svg|thumb|250px|Self-organization of [[phospholipid]]s: a spherical [[liposome]], a [[micelle]], and a [[lipid bilayer]].]]

A biological membrane is a form of [[lamellar phase]] [[lipid bilayer]]. The formation of lipid bilayers is an energetically preferred process when the [[glycerophospholipids]] described above are in an aqueous environment.<ref name = "Stryer_2007" />{{rp|333–4}} This is known as the [[hydrophobic effect]]. In an aqueous system, the polar heads of lipids align towards the polar, aqueous environment, while the hydrophobic tails minimize their contact with water and tend to cluster together, forming a [[vesicle (biology)|vesicle]]; depending on the [[critical micelle concentration|concentration]] of the lipid, this biophysical interaction may result in the formation of [[micelle]]s, [[liposomes]], or [[lipid bilayer]]s. Other aggregations are also observed and form part of the polymorphism of [[amphiphile]] (lipid) behavior. [[Phase behaviour|Phase behavior]] is an area of study within [[biophysics]].<ref name="van_Meer_2008"/><ref name="Feigenson_2006"/> Micelles and bilayers form in the polar medium by a process known as the hydrophobic effect.<ref name="Wiggins_1990"/> When dissolving a lipophilic or amphiphilic substance in a polar environment, the polar molecules (i.e., water in an aqueous solution) become more ordered around the dissolved lipophilic substance, since the polar molecules cannot form [[hydrogen bond]]s to the lipophilic areas of the amphiphile. So in an aqueous environment, the water molecules form an ordered "[[clathrate]]" cage around the dissolved lipophilic molecule.<ref name="Raschke_2005"/>

The formation of lipids into [[protocell]] membranes represents a key step in models of [[abiogenesis]], the origin of life.<ref name="Segré 2001"/>

===Energy storage===
Triglycerides, stored in adipose tissue, are a major form of energy storage both in animals and plants. They are a major source of energy in aerobic respiration. The complete oxidation of fatty acids releases about 38&nbsp;kJ/g (9&nbsp;[[Calorie#Kilogram and gram calories|kcal/g]]), compared with only 17&nbsp;kJ/g (4&nbsp;kcal/g) for the oxidative breakdown of [[carbohydrate]]s and [[protein]]s. The [[adipocyte]], or fat cell, is designed for continuous synthesis and breakdown of triglycerides in animals, with breakdown controlled mainly by the activation of hormone-sensitive enzyme [[lipase]].<ref name="Brasaemle_2007"/> Migratory birds that must fly long distances without eating use triglycerides to fuel their flights.<ref name = "Stryer_2007" />{{rp|619}}

===Signaling===
Evidence has emerged showing that [[lipid signaling]] is a vital part of the [[cell signaling]].<ref name="pmid21743455">{{cite journal | vauthors = Malinauskas T, Aricescu AR, Lu W, Siebold C, Jones EY | title = Modular mechanism of Wnt signaling inhibition by Wnt inhibitory factor 1 | journal = Nature Structural & Molecular Biology | volume = 18 | issue = 8 | pages = 886–893 | date = July 2011 | pmid = 21743455 | pmc = 3430870 | doi = 10.1038/nsmb.2081 }}</ref><ref name="pmid=18256869">{{cite journal | vauthors = Malinauskas T | title = Docking of fatty acids into the WIF domain of the human Wnt inhibitory factor-1 | journal = Lipids | volume = 43 | issue = 3 | pages = 227–230 | date = March 2008 | pmid = 18256869 | doi = 10.1007/s11745-007-3144-3 | s2cid = 31357937 }}</ref><ref name="Wang_2004"/><ref name="Dinasarapu_2011"/> Lipid signaling may occur via activation of [[G protein-coupled receptor|G protein-coupled]] or [[nuclear receptor]]s, and members of several different lipid categories have been identified as signaling molecules and [[Second messenger system|cellular messengers]].<ref name="Eyster_2007"/> These include [[sphingosine-1-phosphate]], a sphingolipid derived from ceramide that is a potent messenger molecule involved in regulating calcium mobilization,<ref name="Hinkovska-Galcheva_2008"/> cell growth, and apoptosis;<ref name="Saddoughi_2008"/> [[diacylglycerol]] and the [[phosphatidylinositol]] phosphates (PIPs), involved in calcium-mediated activation of [[protein kinase C]];<ref name="Klein_2008"/> the [[prostaglandins]], which are one type of fatty-acid derived eicosanoid involved in [[inflammation]] and [[immunity (medical)|immunity]];<ref name="Boyce_2008"/> the steroid hormones such as [[estrogen]], [[testosterone]] and [[cortisol]], which modulate a host of functions such as reproduction, metabolism and blood pressure; and the [[oxysterol]]s such as 25-hydroxy-cholesterol that are [[liver X receptor]] [[agonist]]s.<ref name="Bełtowski 2008"/> Phosphatidylserine lipids are known to be involved in signaling for the phagocytosis of apoptotic cells or pieces of cells. They accomplish this by being exposed to the extracellular face of the cell membrane after the inactivation of [[flippase]]s which place them exclusively on the cytosolic side and the activation of scramblases, which scramble the orientation of the phospholipids. After this occurs, other cells recognize the phosphatidylserines and phagocytosize the cells or cell fragments exposing them.<ref name="Biermann 2013"/>

===Other functions===
The "fat-soluble" vitamins ([[retinol|A]], [[Calciferol|D]], [[tocopherol|E]] and [[Phylloquinone|K]])&nbsp;– which are [[isoprene]]-based lipids&nbsp;– are essential nutrients stored in the liver and fatty tissues, with a diverse range of functions. [[Carnitine#Role in fatty acid metabolism|Acyl-carnitines]] are involved in the transport and metabolism of fatty acids in and out of mitochondria, where they undergo [[beta oxidation]].<ref>{{cite journal | vauthors = Indiveri C, Tonazzi A, Palmieri F | title = Characterization of the unidirectional transport of carnitine catalyzed by the reconstituted carnitine carrier from rat liver mitochondria | journal = Biochimica et Biophysica Acta (BBA) - Biomembranes | volume = 1069 | issue = 1 | pages = 110–116 | date = October 1991 | pmid = 1932043 | doi = 10.1016/0005-2736(91)90110-t }}</ref> Polyprenols and their phosphorylated derivatives also play important transport roles, in this case the transport of [[oligosaccharide]]s across membranes. Polyprenol phosphate sugars and polyprenol diphosphate sugars function in extra-cytoplasmic glycosylation reactions, in extracellular polysaccharide biosynthesis (for instance, [[peptidoglycan]] polymerization in bacteria), and in eukaryotic protein N-[[glycosylation]].<ref name="Parodi_1979"/><ref name="Helenius_2001"/> [[Cardiolipin]]s are a subclass of glycerophospholipids containing four acyl chains and three glycerol groups that are particularly abundant in the inner mitochondrial membrane.<ref name="Nowicki_2005"/><ref name="Gohil_2009"/> They are believed to activate enzymes involved with [[oxidative phosphorylation]].<ref name="Hoch_1992"/> Lipids also form the basis of steroid hormones.<ref>{{cite web | url = http://www.elmhurst.edu/~chm/vchembook/556steroids.html | title = Steroids | archive-url =  https://web.archive.org/web/20111023232815/http://www.elmhurst.edu/~chm/vchembook/556steroids.html |archive-date=2011-10-23 | work = Elmhurst. edu. | access-date = 2013-10-10 }}</ref>