Tag: glycolysis

Respiration is a process in which respiratory substrates are broken down through oxidation accompanying with the release of energy and its storage or conservation in the form of ATP with the release of carbon dioxide as a consequence. The actual mechanism of respiration is a stepwise process in which each step is catalyzed by a specific enzyme and the energy stored in the substrate is released in a stepwise series of reactions. The energy released in the process is used up in various energy-requiring processes of organisms. 

The respiratory breakdown of glucose in the presence of oxygen is an oxidative process. During this process, several intermediates such as pyruvic acid, isocitric acid, succinic acid and oxalic acid are oxidized. Each oxidation step involves the release of 2 H which goes to reduce various coenzymes i.e. NAD+ and FAD. Reduced NAD+ and FAD released in the glycolysis and Krebs cycle finally reduce oxygen to water. This transfer of H+ and e- from NADH + H+ or FADH2 to oxygen is not a simple process and the direct transfer of electrons from coenzymes to oxygen is thermodynamically not possible. To facilitate this transfer, many intermediate cytochromes and other carriers are arranged in a series which transport electrons from NADH or FADH2 to oxygen. This sequence of electron carriers constitutes electron transport system. The electron transport proceeds from carriers that have low redox potential to those having high redox potential. The electron transport down to the energy gradient through electron transport system leads to the formation of ATP from ADP and inorganic phosphate. This generation of ATP is called oxidative phosphorylation. 

A large amount of NADH produced during glycolysis and Krebs cycle undergoes oxidation with phosphorylation of ADP to form ATP which is supported by mitochondrial electron transport assembly and ATP synthase which are integral protein complexes of the inner mitochondrial membrane. The electron transport assembly is comprised of a series of protein complexes that catalyze sequential oxidation-reduction reactions.

In  the cytoplasm, one glucose molecule breaks down into two pyruvate molecules, and creates two NADH molecules by glycolysis. Transformation of one pyruvate from cytosol to one acetyl CoA produces one NADH molecule in the mitochondrial matrix. During  each turn of Krebs cycle, each acetyl CoA is systematically processed through the cycle and produces three NADH molecules.

• Oxidative phosphorylation is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing energy which is used to produce ATP. In most eukaryotes, this takes place inside mitochondria. This was explained by Leininger

In Kreb's cycle the oxidation or dehydrogenation reaction occurs 4 times as follows:

Oxidative phosphorylation-