A discussion of thermodynamics begins this unit to establish a solid foundation for the subsequent topics: the nature of biological energy, the laws governing it and its transformations as well as mechanisms by which the biological system can function effectively under conditions of constant temperature and pressure. The role and action of enzymes, ATP and coupled reactions address the unique thermodynamic problems of biological systems. The energy-releasing reactions of anaerobic and aerobic cellular respiration will be studied by examining their thermodynamic efficiency. The chemiosmotic hypothesis provides an excellent explanation for ATP synthesis, linking together what we know of the structure of the cristae and thylakoid membranes (current research) and coupled reactions. It also allows comparison of the ATP synthesis in both the electron transport chain of the mitochondria and the light reaction of photosynthesis. The energy-acquiring reactions of photosynthesis will be studied by comparing them with the reactions of cellular respiration.