Cellular Respiration and Enzymes

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. 

Kreb cycle is also known as the citric acid cycle or tricarboxylic acid cycle takes place in the inner membrane of mitochondria. It occurs there because the necessary enzyme for the Krebs cycle, succinic dehydrogenase is only found in the inner membrane of the mitochondria.

• Glycolysis and Krebs cycle yield certain reduced coenzymes like NADH+H+ and FADH2.
• ETS is metabolic pathway of electron transport that oxidizes these coenzymes to release energy from them.

• NADH+H+ gets oxidized at Complex I of ETS and as the electrons are transferred to other Complexes, it creates a proton gradient for ATP production. One NADH+H+ yields 3 ATPs and one FADH2 yields 2 ATPs.
  
• So the correct answer is '3 ATPs'.

The electron transport system is located in the inner membrane of the mitochondria.

The process of formation of ATP during oxidation of reduced coenzymes in the electron transport chain is known as oxidative phosphorylation. This process takes place in the mitochondrial membrane because in mitochondrial membrane there are specialized particles which are known as oxysomes which contain coenzymes for electron transfer.

Formation of ATP from ADP in the presence of oxygen during the Krebs cycle is known as oxidative phosphorylation.

ATP is produced during both cyclic and non-cyclic photophosphorylation. During non-cyclic photophosphorylation when electrons move from PS II to PS I, then ATP and $NADPH _2$ is generated and during cyclic photophosphorylation only ATP is generated.

In the electron transport chain, one FADH2 molecule yields 2 ATP molecules upon oxidation.

Pentose phosphate pathway or HMP is an oxidative phosphate pathway of cellular respiration in which glucose 6-phosphate acts as the substrate. After its complete oxidation through HMP, 12 NADPH + H+ and 6 CO2 are formed. This pathway is an alternate pathway to glycolysis and is seen in RBC, adrenal cortex, liver, etc. A net gain of 35 ATP is achieved through this pathway.

Each NADH produces 3 ATP and each FADH2 produces 2 ATP. For 2 NADH and 3 FADH2: (2 * 3) + (3 * 2) = 6 + 6 = 12 ATP.

Reduced molecules like NADH+H+ and FADH2 yield energy in the form of ATP when they undergo ETC in mitochondria. NADH+H+ enter ETC at higher complex than FADH2 so they release energy as 3 ATP and 2 ATP respectively.

The Krebs cycle is the condensation of acetyl group with oxaloacetic acid and water to yield citric acid and leads to the formation of Oxalo acetic acid.

Phosphorylation is the addition of a phosphate (PO$ _{4}^{3-}$) group to a protein or other organic molecule. Phosphorylation results in the formation of ATP, which is the energy currency of cell. Phosphorylation occurs as a part of glycolysis, Kreb's cycle and HMP pathway.

Oxidative phosphorylation refers to synthesis of ATP by ADP and Pi driven by energy released from oxidation of reducing powers NADH and FADH2 by transfer of their electrons through electron trasnport chain to oxygen. Electron trasnport chain is present in inner mitochondrial membrane and passes the electrons down a series of carriers and pump protons into the intermembrane space. Oxygen serves as final electron acceptor in oxidation of NADH and FADH2 and the resulting proton gradient serves as energy source for the enzyme ATP synthase to synthesize ATP. So final products of oxidative phosphorylation are ATP, NAD and FAD. DPN (diphosphopyridine nucleotide), also called as NAD. Correct answers are D and B. NADP is coenzyme of photosynthetic light reactions. Puruvic acid is product of glycolysis. 

Oxidative phosphorylation is the metabolic pathway in which the mitochondria in cells use their structure, enzymes and energy released by the oxidation of nutrients to reform ATP in respiration.