Question

Explain the three pathways of the breakdown in living organisms?

Answer 1

Hint: The three pathways of the breakdown of glucose in living organisms depend upon the presence or absence of oxygen. The first step common to all pathways is glycogenesis which produces pyruvic acid, further, the breakdown of pyruvic acid occurs depending upon if oxygen present aerobic breakdown in mitochondria and if oxygen is absent then anaerobic respiration happen in yeast for microorganism and in muscle cells for humans.

Complete answer: Respiration of glucose occurs in two ways i.e. aerobic breakdown (in presence of oxygen) or anaerobic breakdown (in absence of oxygen). Aerobic breakdown involves the breakdown of glucose into pyruvic acid through glycolysis and then anaerobic breakdown involves the breakdown of glucose into pyruvic acid through glycolysis and then a breakdown of pyruvic acid into carbon dioxide and alcohol in alcoholic fermentation. In organisms that perform cellular respiration, glycolysis is the first stage of this process. The first step is common to all pathways. In this process, the breakdown of glucose into a 6 carbon molecule into 2 molecules of 3 carbon pyruvate happens in the cytoplasm which is a 10 step process. After the 2 molecules of 3 carbon pyruvate is formed the subsequent pathways depend upon the presence or absence of oxygen
1. In Mitochondria: After the breakdown of glucose into pyruvic acid through glycolysis the breakdown of pyruvic acid into carbon dioxide and water with the release of energy happens through Krebs’s cycle. The transition reaction connects glycolysis to the citric acid (Krebs) cycle. Through a process called oxidative decarboxylation, the transition reaction converts the two molecules of the 3-carbon pyruvate from glycolysis (and other pathways) into two molecules of the 2-carbon molecule acetyl Coenzyme A (acetyl-CoA) and 2 molecules of carbon dioxide. First, a carboxyl group of each pyruvate is removed as carbon dioxide and then the remaining acetyl group combines with coenzyme A (CoA) to form acetyl-CoA. The Krebs cycle occurs in the mitochondrial matrix and generates a pool of chemical energy (ATP, NADH, and FADH$_2$, and gives carbon dioxide and water as end products.
2. Fermentation in yeast: After the breakdown of glucose into pyruvic acid through glycolysis the breakdown of pyruvic acid into ethanol and carbon dioxide with energy takes place in yeast.
3.In muscle cells, : After the breakdown of glucose into pyruvic acid through glycolysis the breakdown of pyruvic acid into lactic acid with energy takes place in muscle cells.

Note: In presence of oxygen both single and multicellular organism breaks down food in mitochondria and gives carbon dioxide and water with energy as the product, whereas in absence/lack of oxygen pyruvic acid is converted either into carbon dioxide and ethanol (in case of microbes, i.e., fermentation in yeast) or into lactic acid (in the case of muscle cells during sudden activity in humans.)