Where does membrane collide?

The thin sheet of lipid and protein that bounds a living cell and controls what crosses into and out of it. Singer and Nicolson set out the modern picture in their 1972 paper The Fluid Mosaic Model of the Structure of Cell Membranes: a two-dimensional fluid of phospholipid molecules, arranged tail to tail in a bilayer, with proteins floating in and through it. The referent is a physical barrier around a cell, a few nanometres thick, and what it names is a real material object made of matter, measured in units of thickness and composition. It shares the word with the other meanings because all mark a sheet or a boundary, but this one is a lipid bilayer you can isolate, image, and assay, not an abstract surface and not an engineered part.[1]

Unit

The lipid bilayer enclosing a living cell

Recovery means

Not applicable

Measured by

Observed by microscopy, spectroscopy, and transport assays of the lipids and embedded proteins

An extended object in string and M-theory, a surface that sweeps out a worldvolume as it moves, generalising the one-dimensional string to higher dimensions. Bergshoeff, Sezgin, and Townsend described the supermembrane in their 1987 paper Supermembranes and Eleven-Dimensional Supergravity, a two-dimensional object whose motion is governed by the area it traces and which couples consistently to eleven-dimensional supergravity; the general case is the p-brane, a p-dimensional object. Here the word membrane names a fundamental degree of freedom in a theory of quantum gravity, defined by its tension and its action, not a piece of matter you can hold. It shares only the notion of a two-dimensional sheet with the cell's bilayer; the two live in different worlds and are studied, if at all, in entirely different ways.[2]

Unit

A p-dimensional extended object in spacetime

Recovery means

Not applicable

Measured by

Described mathematically by the tension and worldvolume action of the object, not measured empirically

A selective barrier placed between two phases that lets some species through and holds others back, used to separate mixtures. Baker's Membrane Technology and Applications treats the membrane as an engineered material, a polymer film or ceramic layer, characterised by how fast a species permeates and how well it is rejected. The referent is a manufactured part in a process such as reverse osmosis, ultrafiltration, or gas separation, and the object of interest is the separation it performs, rated by flux and selectivity. It shares the word with the cell membrane, and both are thin barriers that pass some things and stop others, but this one is designed to a specification and judged by throughput, where the cell's is grown by the cell and studied as biology.[3]

Unit

An engineered semipermeable barrier between two phases

Recovery means

Not applicable

Measured by

Characterised by flux, rejection, pore size, and permselectivity in separation tests

A thin sheet held under tension that vibrates when struck or driven, such as a drumhead. Fletcher and Rossing treat the membrane in The Physics of Musical Instruments as an idealised two-dimensional elastic sheet whose motion obeys the wave equation, with modes set by its tension, its mass per unit area, and the shape of its boundary. The referent is a mechanical oscillator, and what matters is the set of frequencies it produces, not what passes through it. It shares the word with the engineered barrier and the cell's bilayer because all three are thin sheets, but here nothing crosses the sheet at all: the membrane is a surface that carries a standing wave, and its job is to sound, not to separate or to enclose.[4]

Unit

A thin tensioned sheet that vibrates in standing modes

Recovery means

Not applicable

Measured by

Characterised by its modal frequencies, set by tension, mass per area, and geometry