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 supporting—but mobile—structure of lipid.
The fluid mosaic model for membrane construction was proposed in 1972 by S. J. Singer of the University of California at San Diego and G. L. Nicolson of the Salk Institute. Since that time, the evidence in support of a fluid membrane has become irrefutable.
In a fluid membrane, proteins may be exposed on the inner surface of the membrane, the outer surface, or at both surfaces. Depending on their association with neighbouring molecules, the proteins may be held in place or may capable of a slow drifting movement within the membrane. Some proteins associate together to form pores through which molecules can pass in a regulated fashion (such as by the charge or size of the molecule).
The fluid nature of the membrane rest with the supporting structure of the lipids. Membrane lipids of microorganisms tend to be a type of lipid termed phospholipid. A phospholipid consists of fatty acid chains that terminate at one end in a phosphate group. The fatty acid chains are not charged, and so do not tend to associate with water. In other words they are hydrophobic . On the other hand, the charged phosphate head group does tend to associate with water. In other words they are hydrophilic. The way to reconcile these chemistry differences in the membrane are to orient the phospholipids with the water-phobic tails pointing inside and the water-phyllic heads oriented to the watery external environment. This creates two so-called leaflets, or a bilayer, of phospholipid. Essentially the membrane is a two dimensional fluid that is made mostly of phospholipids. The consistency of the membrane is about that of olive oil.
Regions of the membrane will consist solely of the lipid bilayer. Molecules that are more hydrophobic will tend to dissolve into these regions, and so can move across the membrane passively. Additionally, some of the proteins embedded in the bilayer will have a transport function, to actively pump or move molecules across the membrane.
The fluidity of microbial membranes also allows the constituent proteins to adopt new configurations, as happens when molecules bind to receptor portions of the protein. These configurational changes are an important mechanism of signaling other proteins and initiating a response to, for example, the presence of a food source. For example, a protein that binds a molecule may rotate, carrying the molecule across the membrane and releasing the molecule on the other side. In bacteria, the membrane proteins tend to be located more in one leaflet of the membrane than the other. This asymmetric arrangement largely drives the various transport and other functions that the membrane can perform.
The phospholipids are capable of a drifting movement laterally on whatever side of the membrane they happen to be. Measurements of this movement have shown that the drifting can actually be quite rapid. A flip-flop motion across to the other side of the membrane is rare. The fluid motion of the phospholipids increases if the hydrophobic tail portion contains more double bonds, which cause the tail to be kinked instead of straight. Such alteration of the phospholipid tails can occur in response to temperature change. For example if the temperature decreases, a bacterium may alter the phospholipid chemistry so as to increase the fluidity of the membrane.
See also Bacterial membranes and cell wall