A simple biomembrane like erythrocyte contains well over hundred lipid
species with diverse molecular shapes differing in the number of acyl
chains, chain length, unsaturation and head group composition. A deli
cate balance between these molecular shapes is necessary in order to h
ave a functional membrane. It is well established that the activities
of a number of membrane-bound enzymes and other properties such as agg
regation, spontaneous vesiculation, pathophysiological properties and
lipid-protein interactions of lipids depend on the acyl chain length,
unsaturation and head group composition. In fact, the molecular shape
of a phospholipid molecule, as modulated by changes in chain length, u
nsaturation and head group composition, is probably what is affecting
the above mentioned properties. The molecular shape of a lipid depends
on a dimensionless packing parameter, S, the value of which influence
s the size and shape of aggregate formed upon hydration. In fact, the
additivity of S values of lipid mixtures explains a number of experime
ntal observations. The molecular shape concept, although very simple,
explains many membrane phenomena like complementarity of molecular sha
pes of non-bilayer lipids to form stable bilayers. Membrane permeabili
ty is controlled to a large extent by lipid packing which depends upon
molecular shapes. In fact, membranes maintain their lamellar structur
e by delicately balancing the composition of bilayer-forming and non-b
ilayer-forming lipids indicating that complementarity of molecular sha
pes is essential to maintain the permeability barrier. Based on this,
the complementary molecular shape model of cell membrane is proposed.