This chapter discusses the mechanisms of mitochondrial respiration and photosynthesis by chloroplasts, which are processes involved in the production of biological energy. ATP synthesis by respiration is known as oxidative phosphorylation, and that carried out by photosynthesis is called photophosphorylation. These processes, combined with the substrate-level phosphorylation discussed in the previous chapter, represent almost all the energy production pathways found in cells. This chapter also outlines the production of organic compounds by photosynthesis. Reactions in which organisms use energy to perform activities are diverse, but the production methods for energy and organic compounds are universal and very simple. Energy production systems have been studied as a central pillar of biology since the early 20th century. The energy production system mediated by oxidation-reduction reactions in the biological membrane is particularly well understood from the atomic and molecular levels to the cellular level, and is an active area in which cutting-edge research remains ongoing.