Organisation of phospholipids in biomembranes

Abstract

Phospholipids consist of a short polar group (α) and two comparatively long hydrocarbon chains (β and γ) connected to a glycerol residue. Molecular orbital calculations indicate that the possible conformations of phospholipids are highly restricted. When considering conformations relevant to structures in cell membranes, a further selection is possible because of the fact that in aqueous soIutions hydrophobic interacrions stabilise an arrangement where the β and γ chains are packed parallel to one another leading to a bilayer structure. Various models which satisfy these conditions have been compared and it has been found that only four are favoured by energy considerations. These arrangements differ from one anotherin the orientation of β and γ chains, close to its linkage with the glycerol group.Similarly, the polar group (α chain) can exist in four possible conformations. A low energy pathway connects these conformations and thus a phospholipid molecule can easily flip from one preferred conformer to the other.The proposed model provide explanations to a number of dynamic and static properties of cell membranes in addition to a theoretical basis for ligid bilayers.More interesting however, is the fact that based on the conformational freedom of phospholipids, it is possible to postulate a model for Na⁺ and K⁺ channels and their passive movement across biomembranes.