Phospholipid
= Phospholipid
{wiki}
<Phospholipids> are <lipids> containing a <phosphate> group as part of their structure linked by <covalent bonds>. The familiar membrane-forming <phospholipids> are <amphiphilic>: a <hydrophilic> head region with <chemical polarity> coexists with a <hydrophobic>, largely <nonpolar> <hydrocarbon> region. These descriptions refer to different parts of the same <molecule>, not to two separate substances. See https://goldbook.iupac.org/terms/view/P04559[IUPAC Gold Book: phospholipids] and https://goldbook.iupac.org/terms/view/A00303[IUPAC Gold Book: amphiphilic].
A typical two-chain <glycerophospholipid> provides a useful example: a <glycerol> scaffold carries two <fatty acid>-derived <acyl groups> through <ester> linkages, plus a <phosphate>-containing head region. <Phosphatidylcholine> is one example. This scaffold describes an important subclass, rather than all <phospholipids>: <sphingomyelin> has a different backbone, and even <glycerophospholipids> need not all have two <acyl groups>. See https://iupac.qmul.ac.uk/lipid/lip1n2.html[IUPAC–IUB, Nomenclature of Lipids (1976), Lip-1.5 and Lip-2.1–2.6].
\Image[/phospholipid-amphiphilic-schematic.png]
{title=Polar head and <hydrophobic> chains of an illustrative <glycerophospholipid>}
{description=Colored regions represent a <phosphate>-containing head, a <glycerol> scaffold, and two <fatty acid>-derived chains. The small paired-chain symbol is shorthand for this illustrative subclass. Shapes and connectors show organization, not <chemical atoms>, exact <chemical bonds>, chain lengths, or a measured <molecular geometry>. The head is not assigned a universal <electric charge>.}
Polar does not mean negatively charged overall. At near-neutral <pH>, <phosphatidylcholine> is <zwitterionic>: its <phosphate> region carries negative <electric charge> and its <choline> region positive <electric charge>, with zero net <electric charge> for the <molecule>. Counting those opposite charges gives $(-1)+(+1)=0$; it does not remove their local interactions with <water>. See https://www.ncbi.nlm.nih.gov/books/NBK21726/[Lodish et al., Molecular Cell Biology: membrane lipids].
In a <lipid bilayer>, two <membrane leaflets> orient their <hydrophilic> heads toward the <water> on either side and their <hydrophobic> chains toward the shared interior. The <hydrophobic effect> helps explain this organization through the <free energy> of the <water> and <lipids> together. It is not a special repulsive <chemical bond> between chains and <water>. Which organization is favored also depends on <molecular geometry>, <temperature>, <concentration>, and the surrounding <chemical solution>; a <phospholipid> label alone does not guarantee a <lipid bilayer>. See https://www.ncbi.nlm.nih.gov/books/NBK26871/[Alberts et al., Molecular Biology of the Cell: the lipid bilayer].
\Image[/phospholipid-bilayer-organization.png]
{title=Two <membrane leaflets> shelter a <hydrophobic> interior between water-facing heads}
{description=An illustrative <phospholipid>-based <lipid bilayer> has <water> on both sides. Individual <phospholipids> can undergo <diffusion> within a fluid <membrane leaflet>. The heads and chains are schematic regions; spacing, arrows, and chain shapes are unscaled. This is a local cross-section, without assigning either side to a particular <biological cell> compartment.}
This organization supplies a flexible barrier for compartments. A fluid <lipid bilayer> allows <lipids> to undergo <diffusion> within each <membrane leaflet>; barrier function therefore coexists with motion of <molecules>, as in the <fluid mosaic model>. It is not completely impermeable: small <nonpolar> <molecules> such as <molecular oxygen> and <carbon dioxide> cross much more readily than <ions>. In a <cell membrane>, <ion channels> and other <membrane transport proteins> provide selective routes; the existing <membrane transport> article explains their driving forces. See https://www.ncbi.nlm.nih.gov/books/NBK26815/[Alberts et al.: principles of membrane transport].
In 1925, E. Gorter and F. Grendel extracted <lipids> from <red blood cells> and compared the <surface area> of a one-<molecule>-thick film on <water> with the estimated total <surface area> of the <red blood cells>. Their table gave ratios near two, supporting a two-layer covering; they also proposed heads facing the aqueous surroundings on both sides. The inference depends on recovering the relevant <lipids>, estimating the <red blood cell> <surface area> correctly, and comparing compatible packing in the film and the <cell membrane>. It was indirect evidence, not an image of the <lipid bilayer>. The paper itself notes possible losses during evaporation. See https://doi.org/10.1084/jem.41.4.439[Gorter and Grendel, On Bimolecular Layers of Lipoids on the Chromocytes of the Blood, Journal of Experimental Medicine 41 (1925), 439–443].