Trypan blue is an organic dye used as a membrane-exclusion indicator in cell studies. Cells with sufficiently intact plasma membranes generally exclude the dye, whereas membrane-compromised cells can become stained.
The observation reports membrane permeability under the measurement conditions. It does not by itself measure every aspect of cellular function, making it a useful example of the distinction between an experimental readout and the broader biological property inferred from it.
New to topics? Read the docs here!