Boundary layer between cell sectors with different (biochemical) functions.

Membranes are 5–10 nanometres thick boundary layers formed of lipids containing intercalated proteins (not shown here). The amphiphilic lipids coalign within the membranes by their hydrophobic parts (marked in blue), their hydrophilic constituents (marked in red) are situated at the outer face of the membrane. Membranes enclose the cells (plasma membrane) or various compartments within the cell (organelles).