The branch of science concerned with the chemical and physicochemical processes that occur within living organisms.
See also Molecular biology, for the way Biochemistry drives biology at the molecular scale.
Biological Molecules - You Are What You Eat: Crash Course Biology #3
According to size
In most cases, new challenges that an organism faces, are solved by the adaptation of an existing macromolecule, rather than by the evolution of an entirely new one.
Structural biology has greatly benefited biochemistry, with the discovery of the detailed 3-dimensional structure of macromolecules. See Protein structure analysis
there is an intimate relation between structure (form) and function in biology
William Prout