Thermodynamics

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Biomolecule BCH 3101
Thermodynamics of Biological Systems • Movement, growth, synthesis of biomolecules, and the transport of ions and molecules across membranes all requires energy. • All organisms acquire energy from their surroundings and utilize that energy efficiently to carry out life processes. • In order to study these bio-energetic phenomena we will require knowledge of thermodynamics. BCH3101 1

• Thermodynamics: defined as a collection of laws and principles describing the flows and interchanges of heat, energy and matter in systems of interest . • Thermodynamics also allows us to determine whether or not chemical processes and reactions can occur spontaneously.

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The ease for a reaction to occur and the direction of reaction is determined by the Laws of Thermodynamics (refer to Mathews and van Holde, 1996 ed. p.62).

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• Several basic thermodynamic principles considered including the analysis of heat flow, entropy production, and free energy functions and the relationship between entropy and information.

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• In any consideration of thermodynamics, a distinction must be made between the system and the surroundings. The system is that portion of the universe with which we are concerned, whereas the surroundings include everything else in the universe. The nature of the system must also be specified.

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• There are three basic systems: isolated, closed, and open. – An isolated system cannot exchange matter or energy with its surroundings. – A closed system may exchange energy, but not matter, with the surroundings. – An open system may exchange matter, energy, or both with the surroundings. • Living things are typically open systems that exchange matter (nutrients and waste products) and heat (from metabolism) with their BCH3101 6 surroundings.

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The First Law: Heat, Work, and Other Forms of Energy • It was realized early in the development of thermodynamics that heat could be converted into other forms of energy, and moreover that all forms of energy could ultimately be converted to some other form. The first law of thermodynamics states that the total energy of an isolated system is conserved. – Enthalpy ( H): A More Useful Function for Biological Systems. BCH3101 8

The Second Law and Entropy: An Orderly Way of Thinking About Disorder • The second law of thermodynamics has been described and expressed in many different ways, including the following. • 1. Systems tend to proceed from ordered (low entropy or low probability) states to disordered (high entropy or high probability) states. • 2. The entropy of the system plus surroundings is unchanged by reversible processes; the entropy of the system plus surroundings increases for irreversible processes. • 3. All naturally occurring processes proceed toward equilibrium, that is, to a state of minimum BCH3101 9 potential energy.

• The third law of thermodynamics states that the entropy of any crystalline, perfectly ordered substance must approach zero as the temperature approaches 0 K, and at T = 0 K entropy is exactly zero.

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• An important question for chemists, and particularly for biochemists, is, “Will the reaction proceed in the direction written?” • J. Willard Gibbs, one of the founders of thermodynamics, realized that the answer to this question lay in a comparison of the enthalpy change and the entropy change for a reaction at a given temperature.

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• To characterize a chemical reaction, the Gibbs Free Energy equation is used and the change in Gibbs free energy (ΔG) is defined as: ΔG = ΔH –TΔS

where ΔH is the change in enthalpy, T is the temperature (Kelvins) and ΔS the change in entropy.

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• Enthalpy changes for biochemical processes can be determined experimentally by measuring the heat absorbed (or given off) by the process in a bomb calorimeter

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• When a thermodynamic...
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