Understanding biological systems through statistical physics

One of the trends in current physics researches is to apply the principles and knowledge of physics to problems usually considered to be outside the conventional boundary of physics. Biological physics is one of the interdisciplinary efforts leading this trend, and statistical physics, originally formulated to deal with a large number of degrees of freedom, may provide a valuable framework for deeper understanding of complex biological systems. Here I will introduce a few examples of theoretical approaches to biology-related problems. Biological systems in vivo usually exhibit nonequilibrium fluctuation behaviors, understanding of which is one of the most fundamental issues in statistical physics. In this respect, fluctuation theorems (FT) that allow to infer equilibrium properties of a system from nonequilibrium processes have recently attracted considerable attention. Considering a Gaussian chain model, I will illustrate the fundamental limitation in practical applications of FT, which is related to time asymmetry and the second law violation of thermodynamics. Secondly, the dynamics of micro-objects, far from equilibrium, in hydrodynamic medium such as ciliary propulsion using hydrodynamic synchronization and chiral molecule separation using shear flow will be discussed. Almost any living organism is immersed in watery solution where biological functions occur to maintain the everyday life. Despite its apparent importance, many aspects of micro-hydrodynamics in nonequilibrium situations are yet to be explored. Thirdly, selective gating function of nuclear pore complex (NPC) will be discussed. NPC is an important macromolecular structure in eukaryotes controlling all nucleo-cytoplasmic traffic and NPC proteins interact with each other forming a highly selective barrier that allows receptor-facilitated transports. Despite numerous studies, the three dimensional structure of this barrier and its mechanism of operation are still unrevealed. I will briefly sketch how simple polymer physics sheds light on the selective gating function of NPC.