The cytoplasmic membrane

In general with all biological membranes the Bacterial membrane among the cytoplasm and the cell wall is composed of a lipid bilayer in Figure , made up of two tiers

                            Figure  1.  A generalized diagram of a Bacterial or Archaeal cell.

of phospholipids. These phospholipids are arranged so in which their hydrophilic heads face into the cytoplasm or the outside world although the hydrophobic tails are embedded within the membrane. Several proteins are partially buried within the membrane or pass right through it. These proteins have many final functions but all mediate transport in one form or another – f₁ f₀ ATPase regulates the transport of H⁺, sugar transporters regulate the flow of carbohydrate and so on. The lipid bilayer is impermeable to H⁺, as with- out this property the organism would be unable to generate ATP. The present model of the cell membrane allows passage of:

? Gases O^2, CO², N² by passive diffusion directly by the membrane

? Water-soluble ions Na⁺, K⁺, except where these substitute for H⁺ in extremophiles by small pores in the membrane

? Water itself down or up the osmotic gradient

? Small molecules by facilitated transport through protein channels but following potential gradient or an osmotic, chemical

? Molecules by active transport normally at the expense of ATP

The cytoplasmic membrane does not give rigidity or shape to the cell as this is provided by the tougher cell wall. The Bacterial cell membranes lack the sterols eukaryotes have but instead stabilize membrane structure with hopanoids.The Archaeal cell membrane performs the similar roles as the Bacterial equivalent but it is unique in biology with a predominantly monolayer phospholipid structure.