= Biological membrane
A \b[biological membrane] is a thin, organized boundary built from <lipids> and associated <proteins> that separates cellular compartments while controlling exchange and communication. The <cell membrane> bounds the whole <biological cell>; internal <biological membranes> surround structures such as the <endoplasmic reticulum> and <mitochondrion>. A <biological membrane> is therefore more general than the outer boundary of a cell.
The basic <lipid bilayer> puts water-compatible groups toward the surrounding water and hydrophobic portions toward its interior. Its barrier properties make transport selective: crossing a <biological membrane> through the lipid interior differs from moving through an <ion channel> or a transporter <protein>. Such selectivity allows different chemical conditions on the two sides, including an <electrochemical gradient> that can store usable energy. <Membrane proteins> also provide receptors, enzymes and attachment sites.
A <biological membrane> is organized but can remain fluid. In the <mouse-human heterokaryon membrane-mixing experiment>, <Larry Frye> and <Michael Edidin> fused cells whose surface <antigens> could be distinguished by different fluorescent antibody labels. Initially separate patterns became intermixed, providing evidence for <lateral mobility of membrane proteins>. This helped motivate the <fluid mosaic model>: many components can move within a shared membrane rather than occupying a completely rigid shell.<The rapid intermixing of cell surface antigens after formation of mouse-human heterokaryons by Larry Frye and Michael Edidin>[Journal of Cell Science 7 (1970), 319–335: distinguishable parental surface antigens redistribute after cell fusion.]{ref}
The experiment tests lateral redistribution, not unrestricted movement across the <lipid bilayer> or unlimited motion of every <membrane protein>. The distinction is useful: a boundary can preserve different environments across its thickness while permitting substantial rearrangement within its plane.
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