Electron transport chain is involved in a series of reactions in the cristae of the mitochondria where hydrogen is oxidised to water and 34 ATP are produced.

• A. Electron transport chain
• B. Krebs cycle
• C. Glycolysis
• D. Phosphocreatine

Answer: (A)

The process being described is oxidative phosphorylation through the electron transport chain on the inner mitochondrial membrane (cristae). Here, electrons from NADH and FADH2 move through a series of protein complexes, releasing energy to pump protons across the membrane. This creates a proton gradient that drives ATP synthase to convert ADP and Pi into ATP. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. Because this chain efficiently converts the energy carried by NADH and FADH2 into ATP, it accounts for the majority of ATP produced in cellular respiration, often cited as around 34 ATP per glucose, depending on conditions.

The Krebs cycle occurs in the mitochondrial matrix and mainly produces reduced carriers and CO2, setting up the supplies for the electron transport chain but not producing most ATP directly. Glycolysis takes place in the cytoplasm, yields a smaller amount of ATP directly and provides the NADH that feeds into later stages. Phosphocreatine serves as a rapid, short-term energy reserve in muscle, not part of the mitochondrial oxidative phosphorylation pathway.