Editing Phospholipid (section)

== Phospholipids in biological membranes ==

===Arrangement===
The phospholipids are [[amphiphilic]]. The hydrophilic end usually contains a negatively charged phosphate group, and the hydrophobic end usually consists of two "tails" that are long [[fatty acid]] residues.<ref>{{Citation |last1=Alberts |first1=Bruce |title=The Lipid Bilayer |date=2002 |url=https://www.ncbi.nlm.nih.gov/books/NBK26871/ |work=Molecular Biology of the Cell. 4th edition |access-date=2023-05-25 |publisher=Garland Science |language=en |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter}}</ref>

In aqueous solutions, phospholipids are driven by [[hydrophobic interactions]], which result in the fatty acid tails aggregating to minimize interactions with the water molecules. The result is often a [[phospholipid bilayer]]: a membrane that consists of two layers of oppositely oriented phospholipid molecules, with their heads exposed to the liquid on both sides, and with the tails directed into the membrane. That is the dominant structural motif of the membranes of all [[cell (biology)|cells]] and of some other biological structures, such as [[vesicle (biology)|vesicles]] or [[virus]] coatings.<ref>{{Cite journal |last=Lenard |first=John |last2=Compans |first2=Richard W. |date=1974-04-08 |title=The membrane structure of lipid-containing viruses |url=https://linkinghub.elsevier.com/retrieve/pii/0304415774900082 |journal=Biochimica et Biophysica Acta (BBA) - Reviews on Biomembranes |volume=344 |issue=1 |pages=51–94 |doi=10.1016/0304-4157(74)90008-2 |issn=0304-4157 |pmc=7148776 |pmid=4598854}}</ref><ref>{{Cite journal |last=Posor |first=York |last2=Jang |first2=Wonyul |last3=Haucke |first3=Volker |date=Dec 2022 |title=Phosphoinositides as membrane organizers |url=https://www.nature.com/articles/s41580-022-00490-x |journal=Nature Reviews Molecular Cell Biology |language=en |volume=23 |issue=12 |pages=797–816 |doi=10.1038/s41580-022-00490-x |issn=1471-0080 |pmc=9117997 |pmid=35589852}}</ref>

[[File:Cell membrane detailed diagram 4.svg|thumb|185px|Phospholipid bilayers are the main structural component of the cell membranes.]]
In biological membranes, the phospholipids often occur with other molecules (e.g., [[protein]]s, [[glycolipid]]s, [[sterol]]s) in a [[bilayer]] such as a [[cell membrane]].<ref>{{cite book | last = Campbell | first = Neil A. | author2 = Brad Williamson | author3 = Robin J. Heyden | title = Biology: Exploring Life | publisher = Pearson Prentice Hall | year = 2006 | location = Boston, Massachusetts | url = http://www.phschool.com/el_marketing.html | isbn = 978-0-13-250882-7  | access-date = 2008-12-14 | archive-date = 2014-11-02 | archive-url = https://web.archive.org/web/20141102041816/http://www.phschool.com/el_marketing.html }}{{page needed|date=October 2012}}</ref> Lipid bilayers occur when hydrophobic tails line up against one another, forming a membrane of hydrophilic heads on both sides facing the water.<ref>{{Citation |last=Alberts |first=Bruce |title=The Lipid Bilayer |date=2002 |work=Molecular Biology of the Cell. 4th edition |url=https://www.ncbi.nlm.nih.gov/books/NBK26871/ |access-date=2024-07-13 |publisher=Garland Science |language=en |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter}}</ref>

===Dynamics===
These specific properties allow phospholipids to play an important role in the cell membrane. Their movement can be described by the [[fluid mosaic model]], which describes the membrane as a mosaic of lipid molecules that act as a solvent for all the substances and proteins within it, so proteins and lipid molecules are then free to diffuse laterally through the lipid matrix and migrate over the membrane. [[Sterol]]s contribute to membrane fluidity by hindering the packing together of phospholipids. However, this model has now been superseded, as through the study of [[lipid polymorphism]] it is now known that the behaviour of lipids under physiological (and other) conditions is not simple.{{citation needed|date=December 2014}}