= Cell membrane
{wiki}
= Plasma membrane
{synonym}
The <cell membrane>, also called the <plasma membrane>, is the selective boundary separating a <biological cell>'s <cytoplasm> from its surroundings. Both <prokaryotes> and <eukaryotes> have a <cell membrane>. Where a <cell wall> is present, it is a distinct structure outside the <cell membrane>; the two names are not synonyms. This separation lets a <biological cell> maintain internal conditions while exchanging material through <membrane transport>. See https://www.genome.gov/genetics-glossary/Cell-Membrane-Plasma-Membrane[NHGRI: cell membrane].
The familiar architecture is a <lipid bilayer> containing <membrane proteins>. In a <phospholipid>-based <lipid bilayer>, two <membrane leaflets> face opposite ways: <hydrophilic> heads face the <water> on each side, while <hydrophobic> tails face one another in the interior. The heads are therefore exposed both outside and inside the <biological cell>. This arrangement combines a water-facing surface with a barrier to many water-soluble <solutes>; the schematic represents organization rather than an atomistic structure. See https://www.ncbi.nlm.nih.gov/books/NBK26871/[Alberts et al., Molecular Biology of the Cell: the lipid bilayer].
\Image[/cell-membrane-selective-barrier.png]
{title=Two <membrane leaflets> and selective routes through a <cell membrane>}
{description=Original <cell membrane> schematic showing <phospholipids>, <hydrophilic> heads, <hydrophobic> tails, an <ion channel>, and a <carrier protein>. Arrows illustrate possible passage, not measured rates. Shapes, sizes, spacing and numbers of <molecules> are illustrative.}
<Selective permeability> means that different substances cross at different rates under comparable conditions. Small <nonpolar> <molecules> can pass through a <lipid bilayer> by <diffusion>, while <ions> encounter a strong barrier without a suitable route. An <ion channel> supplies a selective water-filled pathway when open; a <carrier protein> binds a <solute> and changes shape to expose it to the other side. These <membrane proteins> therefore add specific routes to the barrier. An open <ion channel> permits <passive transport>, whereas a <carrier protein> can mediate either <passive transport> or <active transport>. See https://www.ncbi.nlm.nih.gov/books/NBK26815/[Alberts et al.: principles of membrane transport].
The useful combination is retention plus controlled exchange: a completely impermeable boundary would prevent uptake, whereas unrestricted mixing would erase differences in <concentration> and <membrane potential>. The <fluid mosaic model> connects these functions to an organized but mobile <lipid bilayer> containing <proteins>.
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