What are the key reactants for the pathway and key products

What are the key reactants for the pathway and key products of the pathway For each process of cellular respiration – Glycolysis, Fermentation, Pyruvate Oxidation, Citric acid cycle, Electron transport, Oxidative phosphorylation

Solution

Cellular respiration is the utilization of oxygen by the cell for the synthesis of metabolic products such as sugars, fats, proteins etc. In humans, cellular respiration takes place through the mitochondria (power hoses of the cell), in which the most of the metabolic processes takes place. Blood carries the oxygen to each and every cell in the body and again collects the carbon dioxide.

C₆H₁₂O₆ (s) + 6 O₂ (g) 6 CO₂ (g) + 6 H₂O (liq) + heat

The first step in cellular respiration is glycolysis.

Glycolysis is an anaerobic process, through which one glucose molecules is breakdown into two molecules of three carbon pyruvate. The glycolysis of each glucose molecule generates 2 ATP molecules.

Glucose + 2 NAD⁺ + 2 P_i + 2 ADP 2 pyruvate + 2 NADH + 2 ATP + 2 H⁺ + 2 H₂O + heat.

Citric acid cycle:

The pyruvate generated by the glycolysis is converted into acetyl-CoA that enters into the citric acid cycle. Citric acid cycle involves a series of reactions that occur in the presence of oxygen. Citric acid cycle generates NADH, which enters into the oxidative phosphorylation process. This cycle occurs in mitochondrial matrix and generates one ATP molecule only.

Oxidative phosphorylation:

Glycolysis (breakdown of glucose), oxidation of fatty acids and citric acid cycle releases NADH (hydrohenated nicotinamide adenine dinucleotide) and FADH₂(hydrogenated flavin adenine dinucleotide) molecules. These molecules carry electrons with high transfer potential. The oxygen present in the mitochondria is reduced by these electrons to water molecules; this process produces a very high amount of energy (in the form of ATP (adenosine triphosphate)).

“Oxidative phosphorylation (occurs in cristae of mitochondria)” is an aerobic process that involves a series of reactions, which include the transfer of electrons from the NADH or FADH₂to oxygen. This process involves the transfer of electrons (oxidation) by a series of electron carriers (electron transport system (ETS)) and attachment of phosphate group (phosphorylation).

The free energy released by oxidative phosphorylation contributes to the major part (nearly above 90%) of energy expenditure of our daily activities.

Key reactants and products of fermentaiton: Fermentation is a process of anaerobic catabolism. It is the continuation of glycolysis in the absence of oxygen. However, this process needs the supply of NAD+ as an electron acceptor. NAD+ is produced in aerobic conditions, fermentation occurs as long as NAD+ reserves are available. During fermentation, pyruvate is metabolized to lactic acid and produces 2 molecules of ATP.