Biophysics is that branch of knowledge that applies the principles of physics and chemistry and the methods of mathematical analysis and computer modeling to understand how the mechanisms of biological systems work.
Biophysics is a molecular science. It seeks to explain biological function in terms of the molecular structures and properties of specific molecules. The size of these molecules varies dramatically, from small fatty acids and sugars (~1 nm = 10-9 m), to macromolecules like proteins (5-10 nm), starches (>1000 nm), and the enormously elongated DNA molecules (over 10,000,000 nm = 1 cm long but only 20 nm wide; imagine a piece of string 45 miles long!).
These molecules, the sole building blocks of living organisms, assemble into cells, tissues, and whole organisms by forming complex individual structures with dimensions of 10, 100, 1000, 10,000 nm and larger. Proteins assemble into the casein micelles of milk, which aggregate to form the curd of cheese; proteins and ribonucleic acids assemble into ribosomes, the machinery for building proteins; lipids and proteins assemble into cell membranes, the external barriers and internal surfaces of cells; proteins and DNA wind up into chromosomes, the carriers of the genetic code; and so on.
Consequently, much effort in biophysics is directed to determining the structure of specific biological molecules and of the larger structures into which they assemble. Some of this effort involves inventing new methods and in building new instruments for viewing these structures.
Note: Biophysics is an important topic for pg entrance exam for All India Institute of Medical Scinces, New Delhi. Invariable few questions are asked from biphysics in this exam.