Living organisms are not at equilibrium. Rather they require a continuous influx of free energy to maintain order in a universe bent on maximizing disorder.
Metabolism is the overall process through which living systems acquire and utilize the free energy they need to carry out their various functions. They do so by coupling the exergonic reactions of nutrient oxidation to the endergonic processes required to maintain the living state such as the performance of mechanical work, the active transport of molecules against gradients and the biosynthesis of complex molecules.
How do living things acquire this necessary free energy?
And what is the nature of the energy coupling process?
Phototrophs- plants and certain bacteria acquire free energy from the sun through photosynthesis, a process in which light energy powers the endergonic reactions of CO2 and H2O to form carbohydrates and O2.
Chemotrophs-obtains their free energy by oxidizing organic compounds(carbohydrates, lipids, proteins) obtained from other organisms, ultimately phototrophs.
This free energy is most often coupled to endergonic reactions through the intermediate synthesis of “high energy” phosphate compounds such as adenosine triphosphate- ATP. In addition to being completely oxidized, nutrients are broken down in a series of metabolic reactions to common intermediates that are used as precursors in the synthesis of other biological molecules.
A remarkable property of living systems is that, despite the complexity of their internal processes, they maintain a steady state. This is strikingly demonstrated by the observation that, over a 40 year time span, a normal human adult consume literally tons of nutrients and imbibes over 20,000 L of water, but does so without significant weight change. This steady state is maintained by a sophisticated set of metabolic controls.
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