In the nonliving world complex things degrade naturally to simpler things: Gradients of temperature or concentration disappear, chemical reactions approach equilibrium, and uniformity triumphs. Living things do not appear to follow these trends. Cells are complex and divide to make other complex cells: A fertilized egg differentiates to make a whole complex organism. Living things must obey the laws of thermodynamics. The escape from the behavior of nonliving systems is allowed because living systems take matter and energy from the environment and use it to grow, to reproduce, and to repair themselves. Living systems are open systems while nonliving systems are closed.
A chemical reaction that has reached equilibrium can do no work. A good definition of death is the state at which all of the chemical reactions in a cell/organism have reached equilibrium. In a living organism the concentrations of metabolites are often very far from the equilibrium concentrations and yet are more or less constant: This is said to be a steady state. Cells can do this because they are open systems taking energy and matter from their environment.
We can use an analogy with the world of economics. It is unlikely that people would spontaneously repair our houses, or feed us, or give us this book, but we can drive these otherwise unlikely processes by spending money. In a similar way, cells can drive otherwise unlikely processes by using up one of four energy currencies that are then replaced using energy taken from the outside world.
Cell Biology: A Short Course, Second Edition, by Stephen R. Bolsover, Jeremy S. Hyams, Elizabeth A. Shephard, Hugh A. White, Claudia G. Wiedemann ISBN 0-471-26393-1 Copyright © 2004 by John Wiley & Sons, Inc.
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