Chapter 5 - Thermodynamics, Chemical Kinetics, and Energy Metabolism

Abstract

This chapter discusses the thermodynamics, chemical kinetics, and energy metabolism involved in a living system. Thermodynamics deals with the changes in energy content between reactants and products, whereas kinetics is concerned with the reaction rates. The study of chemical reaction rates is called chemical kinetics. Nearly all chemical changes involving breakage and formation of covalent bonds in a living organism are catalyzed by enzymes, without which the reactions would proceed too slowly for biological systems to function at any significant rate. The energy changes that occur in cellular reactions obey the two laws of thermodynamics. The first law states that energy can be neither created nor destroyed but can be converted from one form into another, and the second law states that in all processes involving energy changes under a given set of conditions of temperature and pressure, the entropy of the system and the surroundings increases and attains a maximum value at equilibrium. The term “metabolism” encompasses the numerous chemical transformations that occur within the human body and comprises anabolism and catabolism. Anabolism is concerned with the synthesis of new molecules, usually larger than the reactants, and is an energy-requiring process. Catabolism is concerned with degradation processes, usually the breaking down of large molecules into smaller ones, and is an energy-yielding process. Two main sources of energy for metabolism are carbohydrates and fats (lipids). In a living system, the energy derived from food is released as body heat and also used in the synthesis of ATP. The energy captured in ATP is then transformed into other forms such as chemical (synthesis of new compounds), mechanical (muscle contraction), electrical (nerve activity), electrochemical (various ion pumps), thermal (maintenance of body temperature), and informational (base sequences in nucleic acids, amino acids in proteins).