ABSTRACT
Bioenergy necessary for cell growth, cell division, and cell life is mainly provided as light energy from the sun or as food nutrients. However, most living organisms can use energy only in the form of chemical energy released by hydrolysis of ATP. Thus, their energy source must be converted into chemical energy in the form of ATP. The cellular organelles in higher organisms such as chloroplasts and mitochondria are responsible for this conversion. In these organelles, energy obtained in the form of light or nutrients is first converted into a proton electrochemical gradient (∆µH) across their membranes by a redox enzyme complex consisting of the electron transport chain (in the case of mitochondria, also called the respiratory chain). Using the H gradient generated as a driving force, ATP can be synthesized from ADP and inorganic phosphate (Pi) by ATP synthase. Energy conversions in chloroplasts and mitochondria are referred to as photophosphorylation and oxidative phosphorylation, respectively.
