Romanian Reports in Physics, Vol. 65, No. 3, P. 1063–1077, 2013 Dedicated to Professor Valentin I. Vlad’s 70 th Anniversary BIOPHYSICAL INTERACTIONS: THEIR PARAMOUNT IMPORTANCE FOR LIFE A.I. POPESCU 1 , D.M. GĂZDARU 1 , C.G. CHILOM 1 , M. BACALUM 1,2 1 University of Bucharest, Department of Electricity, Solid State and Biophysics, Faculty of Physics, 405 Atomistilor, Măgurele-Ilfov, ROMANIA 2 “Horia Hulubei” National Institute of Physics and Nuclear Engineering, Department of Life and Environmental Physics, Măgurele, PO Box MG-6, 077125, ROMANIA Corresponding author: prof.aurel.popescu@gmail.com Received June 12, 2013 Abstract. Life processes are manifesting properly only by correlated and multiple biomolecular interactions taking permanently place in all the cells. The sine qua non molecular interactions for life processes are the specific interactions, that is, those conducting to preferential associations between partners. These interactions are the result of many synergic factors. The common specific interactions are the enzyme- substrate and the protein-ligand interactions. In the case of enzymes, the natural ligands are the specific substrates bound to enzyme active sites before they are split into products. Quantitatively, the specific interactions are characterized by the association (affinity) constant, a high affinity meaning a great value for this constant. The specific interactions are also ubicuitary both at supramolecular and cellular levels facilitating a coherent interplay of all the living matter components. Indeed, a large variety of subtle biological phenomena taking place at cellular level, are involving cell-to-cell interactions, more specifically, membrane-to-membrane interactions of the apposing cells. The cell-to-cell interactions are mainly determined by the composition and spatial structure of their membrane and their membrane peripheries (i.e. glycocalyx) and also by the particular composition of the liquid micro-environment. Some specific interactions both at molecular (first part) and cellular (second part) level are shortly reviewed and commented. Key words: biomolecular interactions, steric and electrostatic complementarity, protein- ligand, affinity constant, cell-to-cell interactions, electrostatic and electrodynamic interactions, hydration forces. 1. INTRODUCTION A fundamental question addressed by biophysics is the following: which are the simplest processes considered as “elementary” ones sustaining the Life processes? Alike any other natural phenomena Life is maintained by myriad of