This thematic unit gathers (presents) few chapters on the study of energy transformation in living beings. The first topic introduces the basic concepts in the study of energy transformation that occur in the cell. Special attention is paid to the concepts of free energy and the formation and consumption of ATP. The second topic reviews the concept of oxidation-reduction and then links it with the description of the respiratory chain of the inner mitochondrial membrane. The third topic or chapter describes the process of oxidative phosphorylation. The fourth topic deals with the photosynthesis and analyzes both common and differential characteristics between the photosyntheis and the oxidative phosphorylation. Finally, the last chapter goes over the cycle of tricarboxylic acids.