What systems biology is

Studying a cell part by part (each gene, each enzyme) is called reductionism. It tells us what the parts are, but not how they behave together. Systems biology joins the parts back up. It builds mathematical and computer models, from stored data, that copy how a complex biological system works. It is interdisciplinary, combining life science, information science and system science.

Such models, of metabolic or signalling networks for example, can predict emergent properties: behaviour of a cell, tissue or organism that cannot be seen in any single part. They are used to understand health and disease and to plan treatments. As the saying goes, the whole is more than the sum of its parts.

A short history:

• 1952: Hodgkin and Huxley made a mathematical model of the nerve impulse.
• 1960: Denis Noble built a computer model of the heart's pacemaker.
• 1966: Mesarovic published "Systems Theory and Biology"; in 1968 von Bertalanffy described general systems theory.
• 1960s-70s: metabolic control analysis was developed.

Since the 1990s, functional genomics has supplied the data for much more realistic models. In 2012, the first whole-cell model, of the bacterium Mycoplasma genitalium, predicted how mutations affect whether the cell survives. The Physiome project links models across scales, from single ion channels to the whole heart.

To build such models, systems biology uses network maps of how parts interact and the kinetics (speeds) of those interactions, often described by partial differential equations.