Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, evolution, distribution, and taxonomy. Biology has many subdisciplines unified by five so-called axioms of modern biology: Cells are the basic unit of life Genes are the basic unit of heredity New species and inherited traits are the product of evolution An organism regulates its internal environment to maintain a stable and constant condition Living organisms consume and transform energy Subdisciplines of biology are defined by the scale at which organisms are studied and the methods used to study them: biochemistry examines the rudimentary chemistry of life; molecular biology studies the complex interactions among biological molecules; cellular biology examines the basic building block of all life, the cell; physiology examines the physical and chemical functions of tissues, organs, and organ systems of an organism; evolutionary biology examines the processes that produced the diversity of life; and ecology examines how organisms interact in their environment.