The upper limit of cell size is probably set by the rate of diffusion of solute molecules in aqueous systems. - Albert L. Lehninger

Biochemistry asks how the remarkable properties of living organisms arise from the thousands of different biomolecules.

The remarkable similarity of metabolic pathways and gene sequences across the phyla argues strongly that all modern organisms are derived from a common evolutionary progenitor by a series of small changes (mutations), each of which conferred a selective advantage to some organism in some ecological niche.

The current understanding that all organisms share a common evolutionary origin is based in part on this observed universality of chemical intermediates and transformations, often termed "biochemical unity."