Editing Lipid bilayer (section)

=== Cross-section analysis ===

[[File:Bilayer hydration profile.svg|right|thumb|310px |Schematic cross sectional profile of a typical lipid bilayer. There are three distinct regions: the fully hydrated headgroups, the fully dehydrated alkane core and a short intermediate region with partial hydration. Although the head groups are neutral, they have significant dipole moments that influence the molecular arrangement.<ref>Mashaghi et al. Hydration strongly affects the molecular and electronic structure of membrane phospholipids. 136, 114709 (2012){{cite web |url=http://jcp.aip.org/resource/1/jcpsa6/v136/i11/p114709_s1 |title=The Journal of Chemical Physics |access-date=2012-05-17 |url-status=dead |archive-url=http://arquivo.pt/wayback/20160515021109/http://jcp.aip.org/resource/1/jcpsa6/v136/i11/p114709_s1 |archive-date=15 May 2016 |df=dmy-all }}</ref>]]

The lipid bilayer is very thin compared to its lateral dimensions. If a typical mammalian cell (diameter ~10 micrometers) were magnified to the size of a watermelon (~1&nbsp;ft/30&nbsp;cm), the lipid bilayer making up the [[plasma membrane]] would be about as thick as a piece of office paper. Despite being only a few nanometers thick, the bilayer is composed of several distinct chemical regions across its cross-section. These regions and their interactions with the surrounding water have been characterized over the past several decades with [[x-ray reflectometry]],<ref name=Lewis1983>{{cite journal |vauthors=Lewis BA, Engelman DM |title=Lipid bilayer thickness varies linearly with acyl chain length in fluid phosphatidylcholine vesicles |journal=Journal of Molecular Biology |volume=166 |issue=2 |pages=211–7 |date=May 1983 |pmid=6854644 |doi=10.1016/S0022-2836(83)80007-2 }}</ref> [[neutron scattering]],<ref name=Zaccai1975>{{cite journal |vauthors=Zaccai G, Blasie JK, Schoenborn BP |title=Neutron Diffraction Studies on the Location of Water in Lecithin Bilayer Model Membranes |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=72 |issue=1 |pages=376–380 |date=January 1975 |pmid=16592215 |pmc=432308 |doi=10.1073/pnas.72.1.376 |bibcode=1975PNAS...72..376Z |doi-access=free }}</ref> and [[nuclear magnetic resonance]] techniques.<ref name="Skarjune 1982">{{cite journal |last=Skarjune |first=Robert |last2=Oldfield |first2=Eric |title=Physical studies of cell surface and cell membrane structure. Deuterium nuclear magnetic resonance studies of N-palmitoylglucosylceramide (cerebroside) head group structure |journal=Biochemistry |volume=21 |issue=13 |date=22 June 1982 |issn=0006-2960 |doi=10.1021/bi00256a019 |pages=3154–3160}}</ref>

The first region on either side of the bilayer is the hydrophilic headgroup. This portion of the membrane is completely hydrated and is typically around 0.8-0.9&nbsp;nm thick. In [[phospholipid]] bilayers the [[phosphate]] group is located within this hydrated region, approximately 0.5&nbsp;nm outside the hydrophobic core.<ref name=Nagle2000>{{cite journal |vauthors=Nagle JF, Tristram-Nagle S |title=Structure of lipid bilayers |journal=Biochim. Biophys. Acta |volume=1469 |issue=3 |pages=159–95 |date=November 2000 |pmid=11063882 |pmc=2747654 |doi=10.1016/S0304-4157(00)00016-2}}</ref> In some cases, the hydrated region can extend much further, for instance in lipids with a large protein or long sugar chain grafted to the head. One common example of such a modification in nature is the [[lipopolysaccharide]] coat on a bacterial outer membrane.<ref name="Avila-Calderón_2021">{{cite journal |vauthors=Avila-Calderón ED, Ruiz-Palma MD, Aguilera-Arreola MG, Velázquez-Guadarrama N, Ruiz EA, Gomez-Lunar Z, Witonsky S, Contreras-Rodríguez A |display-authors=6 |title=Outer Membrane Vesicles of Gram-Negative Bacteria: An Outlook on Biogenesis |journal=Frontiers in Microbiology |volume=12 |pages=557902 |year=2021 |pmid=33746909 |pmc=7969528 |doi=10.3389/fmicb.2021.557902 |doi-access=free }}</ref>

[[File:Bacillus subtilis.jpg|thumb|240px |[[Transmission electron microscopy|TEM]] image of a bacterium. The furry appearance on the outside is due to a coat of long-chain sugars attached to the cell membrane. This coating helps trap water to prevent the bacterium from becoming dehydrated.]]

Next to the hydrated region is an intermediate region that is only partially hydrated. This boundary layer is approximately 0.3&nbsp;nm thick. Within this short distance, the water concentration drops from 2M on the headgroup side to nearly zero on the tail (core) side.<ref name=Marsh2001>{{cite journal |author=Marsh D |title=Polarity and permeation profiles in lipid membranes |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=98 |issue=14 |pages=7777–82 |date=July 2001 |pmid=11438731 |pmc=35418 |doi=10.1073/pnas.131023798 |bibcode=2001PNAS...98.7777M |doi-access=free }}</ref><ref name=Marsh2002>{{cite journal |author=Marsh D |title=Membrane water-penetration profiles from spin labels |journal=Eur. Biophys. J. |volume=31 |issue=7 |pages=559–62 |date=December 2002 |pmid=12602343 |doi=10.1007/s00249-002-0245-z |s2cid=36212541 }}</ref> The hydrophobic core of the bilayer is typically 3-4&nbsp;nm thick, but this value varies with chain length and chemistry.<ref name=Lewis1983/><ref name=Rawicz2000>{{cite journal |vauthors=Rawicz W, Olbrich KC, McIntosh T, Needham D, Evans E |title=Effect of chain length and unsaturation on elasticity of lipid bilayers |journal=Biophys. J. |volume=79 |issue=1 |pages=328–39 |date=July 2000 |pmid=10866959 |pmc=1300937 |doi=10.1016/S0006-3495(00)76295-3 |bibcode=2000BpJ....79..328R}}</ref> Core thickness also varies significantly with temperature, in particular near a phase transition.<ref name=Trauble1971>{{cite journal |vauthors=Trauble H, Haynes DH |title=The volume change in lipid bilayer lamellae at the crystalline-liquid crystalline phase transition |journal=Chem. Phys. Lipids |volume=7 |issue=4 |pages=324–35 |year=1971 |doi=10.1016/0009-3084(71)90010-7}}</ref>