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After studying this chapter, you should be able to:

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  • State the first and second laws of thermodynamics and understand how they apply to biologic systems.
  • Explain what is meant by the terms free energy, entropy, enthalpy, exergonic, and endergonic.
  • Appreciate how reactions that are endergonic may be driven by coupling to those that are exergonic in biologic systems.
  • Understand the role of high-energy phosphates, ATP, and other nucleotide triphosphates in the transfer of free energy from exergonic to endergonic processes, enabling them to act as the “energy currency” of cells.

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Bioenergetics, or biochemical thermodynamics, is the study of the energy changes accompanying biochemical reactions. Biologic systems are essentially isothermic and use chemical energy to power living processes. How an animal obtains suitable fuel from its food to provide this energy is basic to the understanding of normal nutrition and metabolism. Death from starvation occurs when available energy reserves are depleted, and certain forms of malnutrition are associated with energy imbalance (marasmus). Thyroid hormones control the rate of energy release (metabolic rate), and disease results when they malfunction. Excess storage of surplus energy causes obesity, an increasingly common disease of Western society, which predisposes to many diseases, including cardiovascular disease and diabetes mellitus type 2, and lowers life expectancy.

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Gibbs change in free energy (ΔG) is that portion of the total energy change in a system that is available for doing work—ie, the useful energy, also known as the chemical potential.

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Biologic Systems Conform to the General Laws of Thermodynamics

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The first law of thermodynamics states that the total energy of a system, including its surroundings, remains constant. It implies that within the total system, energy is neither lost nor gained during any change. However, energy may be transferred from one part of the system to another or may be transformed into another form of energy. In living systems, chemical energy may be transformed into heat or into electrical, radiant, or mechanical energy.

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The second law of thermodynamics states that the total entropy of a system must increase if a process is to occur spontaneously. Entropy is the extent of disorder or randomness of the system and becomes maximum as equilibrium is approached. Under conditions of constant temperature and pressure, the relationship between the free-energy change (ΔG) of a reacting system and the change in entropy (ΔS) is expressed by the following equation, which combines the two laws of thermodynamics:

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where ΔH is the change in enthalpy (heat) and T is the absolute temperature.

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In biochemical reactions, since ΔH is approximately equal to ΔE, the total change in internal energy of the reaction, the above relationship may be expressed in the following way:

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If ΔG is negative, the reaction proceeds spontaneously with ...

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