Lipid bilayer

Definition

A lipid bilayer is a thin membrane (surrounding a cell or organelle) that is two molecules thick, with both molecules being lipids with a hydrophilic head and hydrophobic tail. The hydrophilic head of the outer molecule points outward to the (usually aqueous) external environment, and the hydrophilic head of the inner molecule points inward to the cell or organelle being surrounded by the bilayer. The hydrophobic tails both point inward toward each other.

Lipid bilayers constitute cell membranes (in both prokaryotic cells and eukaryotic cells) as well as the membranes of cellular organelles (mostly in eukaryotic cells). The most typical examples of lipid bilayers that occur in cell membranes and organelle membranes are those where the lipid is a phospholipid. A phospholipid is a two-tailed lipid. Note however that this two describes two tails of each single molecule and is distinct from the two that is part of the bilayer definition.

Also, when people talk of double membranes (as in the case of the nucleus, where the membrane is folded onto itself, and the mitochondrion, where there is a distinct inner mitochondrial membrane and outer mitochondrial membrane), this refers to an additional doubling over and above the bilayer, so that there are effectively four layers of molecules.