Thus our structural database is heavily biased towards globular proteins. The tertiary structure of globular proteins reflects their interaction with their aqueous solvent. At a simple level, a globular protein may be considered to consist of a hydrophobic core surrounded by a hydrophilic external surface which interacts with water. The tertiary fold of the polypeptide is such that those residues with apolar side chains are buried in the centre, while the polar residues remain exposed. A patchwork arrangement of hydrophobic and polar surface allows solvent molecules close to apolar regions to interact favourably with neighbouring polar surfaces.
Every cell is enclosed by a plasma membrane .
In addition, the cytoplasm of eukaryotic cells contains a variety of organelles, each bounded by an internal membrane . All biological membranes are assemblies of lipid and protein molecules held together by non-covalent interactions. The lipids are amphipathic , ie they have a hydrophilic polar 'head' and a hydrophobic non-polar 'tail', and they therefore spontaneously form bilayers in aqueous solution. A membrane is not simply a physical barrier between the cell cytoplasm and its external environment, or between different compartments of a cell. The functions of membranes depend on the proteins within them.