Introduction

The application of basic physics ideas to the study of biological molecules is one of the major growth areas of modern physics, and emphasizes well how physics principles ultimately underpin the whole of Nature. This chapter focuses on the collective properties of biological molecules showing examples of hierarchical structures, and, in some instances, how structure and dynamics enable biological function. In addition, supramolecular biophysics casts a wide web with contributions to a broad range of fields. Among them are, in medicine and genetics, the design of carriers of large pieces of DNA containing genes for gene therapy and for characterizing chromosome structure and function; in molecular neurosciences, elucidating the structure and dynamics of the nerve-cell cytoskeleton; and in molecular cell biology, characterizing the forces responsible for condensation of DNA in vivo, to name a few. Concepts and new materials emerging from research in the field continue to have a large impact in industries as diverse as cosmetics and optoelectronics. A separate branch of biophysics dealing with the properties of single molecules is not described here due to space limitations and the availability of excellent reviews published in the past few years.

If one looks at research in biophysics over the last few decades one finds that a large part has been dedicated to studies of the structure and phase behavior of biological membranes. Membranes of living organisms are astoundingly complex structures, with the lipid bilayer containing membrane-protein inclusions and carbohydrate-chain decorations as shown in a cartoon of a section of the plasma membrane of a eukaryotic cell, which separates the interior contents of the cell from the region outside of the cell (Figure 16.1). The common lipids in membranes are amphiphilic molecules, meaning that the molecules contain both hydrophilic (“water-liking”) polar head groups and hydrophobic (“water-avoiding”) double tail hydrocarbon chains. Plasma membranes contain a large number of distinct membrane-associated proteins, which may traverse the lipid bilayer, be partially inserted into the bilayer, or interact with the membrane but not penetrate the bilayer.