In 1827, English botanist Robert Brown observed, under a microscope, pollen granules suspended in water. He noted that the granules darted randomly about and thought that he was observing the life force inherent in the pollen granules. He dismissed that idea when he observed the same behavior with dye particles in water or dust particles in air. The movement that he observed, subsequently referred to as Brownian motion, is a vital energy source for life. The movement of the particles that Brown observed is due to the random fluctuation of the energy content of the environment—thermal noise. The water and gas molecules of the environment are bouncing randomly about at a rate determined only by the temperature. When these molecules collide with pollen granules or dust motes, the particles move randomly themselves.

Brownian motion is responsible for initiating many biochemical interactions. In the context of the cell, water is the most common medium for the thermal noise of Brownian motion. Water is the lubricant that facilitates the flow of energy and information transformations through Brownian motion. Enzymes find their substrates; fuels can be progressively modified to yield energy, and signal molecules can diffuse from their sites of origin to their sites of effect, all through Brownian motion.

To be sure, the environment inside the cell is not as simple as just implied. Cells are not simply water-filled sacks with biomolecules bouncing about. As described in Chapter 1, a great deal of organization facilitates the Brownian-motion-driven exchange of metabolites and signal molecules. Examples of this organization will come up again many times in the course of our study of biochemistry.

Water is the medium for which Brownian motion provides the motive force for biochemical interactions. What are the properties of water that make it the perfect environment for life?