The membranes of bacteria function to give the bacterium its shape, allow the passage of molecules from the outside in and from the inside out, and to prevent the internal contents from leaking out. Gram-negative bacteria have two membranes that make up their cell wall, whereas Gram-positive bacteria have a single membrane as a component of their cell wall. Yeasts and fungi have another specialized nuclear membrane that compartmentalizes the genetic material of the cell.

For all these functions, the membrane must be fluid. For example, if the interior of a bacterial membrane was crystalline, the movement of molecules across the membrane would be extremely difficult and the bacterium would not survive.

Membrane fluidity is assured by the construction of a typical membrane. This construction can be described by the fluid mosaic model. The mosaic consists of objects, such as proteins, which are embedded in a...