This lecture covers the metabolic diversity of organisms in terms of energy generation, focusing on oxidative and substrate-level phosphorylation. It also delves into the intricate cycles of iron (Fe) and manganese (Mn) in biological systems, detailing the processes of oxidation, reduction, and electron transfer. The mechanisms of Fe(III) reduction in microorganisms like Shewanella MR-1 are explored, highlighting the remarkable feat of exporting electrons outside the cell. Environmental examples such as acid mine drainage and ore leaching are discussed, showcasing the impact of these cycles in nature.