Our previous theoretical analysis has shown that the structure of a photosynthetic unit should be strictly optimized in vivo to ensure the high quantum yield values (∼90%) found experimentally for the primary photochemistry. We have already studied the basic principles of the structural organization of an optimal model light-converting systems with certain simple types of their lattices. However, in all the known photosynthetic organisms, the three-dimensional array of pigment-protein complexes form the cluster structure of the photosynthetic unit lattice. Does this cluster structure reflect an optimal space distribution of pigment molecules for ensuring the highest efficiency of excitation energy transfer from an antenna to a reaction centre? This problem is examined here by mathematical simulation of the light-harvesting process in model systems and has drawn two principal conclusions. o (1) In the case of weak interactions between all the photosynthetic unit molecules, the molecular cluster formation significantly lowers the rate of the delivery of the excitation, from the excited antenna molecules to the reaction centre, and hence increases the energy losses. (2) In the case of strong interactions within each cluster and weak interactions between adjacent clusters, the molecular cluster formation in the photosynthetic unit accelerates the delivery of the excitation from antenna to reaction centre and hence increases the efficiency of such a system as compared to that of a corresponding uniform isotropic system. As discussed here, this is the situation that is realized in natural systems. Consequently, the molecular cluster formation in photosynthetic systems in vivo, as an efficient strategy for the light harvesting in photosynthesis, is biologically expedient.