Cellular Respiration and Fermentation
...arbohydrate catabolism, and serves two principal functions: generation of high-energy ATP molecules, and production of a variety of 6- or 3-carbon intermediate metabolites which may be removed at various steps in the process for other intracellular purposes. Glycolysis is one of the most universal metabolic processes known, and occurs (with minor variations) in many types of cells in nearly all types of organisms from bacteria to plants to animals and humans. Although glycolysis produces less energy per glucose molecule than complete aerobic oxidation, it can occur at great speed and is anaerobic (i.e., it does not require oxygen). The individual steps of the conversion of glucose into pyruvate are: 1. A glucose molecule from the hydrolysation of starch or glycogen is phosphorylated using one ATP molecule to give glucose-6-phosphate. 2. The glucose-6-phosphate is converted to fructose-6-phosphate by isomerisation. 3. Fructose-6-phosphate is again phosphorylated to give fructose-1,6-diphosphate with the use of another ATP molecule. 4. Next, the fructose-1,6-diphosphate is then lysed into two molecules of 3-carbon sugar (dihydroxyacetone phosphate and glyceraldehyde-3-phosphate) which are interconvertible. 5. The 3-carbon sugars are dehydrogenated and inorganic phosphate is added to them, forming two molecules of 1,3 diphosphoglycerate. 6. The hydrogen is used to reduce two molecules of NAD, a hydrogen carrier, to give NADH+H+. NADH+H+ later proceeds to the mitochondria for use in the electron transport chain. 7. The two molecules of 1,3 diphosphoglycerate lose two phosphate groups to form two molecules of glycerate-3-phosphate (3-phosphoglycerate), converting two molecules of ADP to ATP. 8. The two molecules of glycerate-3-phosphate again lose phosphate forming two molecules of pyruvate, with the production of another two ATP molecules. There are now two ways to break down the resulting pyruvate: Krebs cycle: The Krebs cycle is the second step in carbohydrate catabolism (the breakdown of sugars). Glycolysis breaks glucose (a six-carbon-molecule) down into pyruvate (a three-carbon molecule). In eukaryotes, pyruvate moves into the mitochondria. It is converted into acetyl-CoA and enters the Krebs cycle. In protein catabolism, proteins are broken down by protease enzymes into their constituent amino acids. These amino acids are brought into the cells and can be a source of energy by being funneled into the Krebs cycle. In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. In the liver the glycerol can be converted into glucose via dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by way of gluconeogenesis. The Acetyl Co-A (2-C) is attached to a 4-C chemical (oxaloacetic acid). The Co-A is released and returns to await another pyruvic acid. The 2-C and 4-C make another chemical known as Citric acid, a 6-C. Kreb¡¦s Cycle is also known as the Citric Acid Cycle. The process after Citric Acid is essentially removing carbon dioxide, getting out energy in the form of ATP, GTP, NADH and FADH2, and lastly regenerating the cycle. Between Isocitric Acid and ƒÑ-Ketoglutaric Acid, carbon dioxide is given off and NAD+ is converted into NADH. Between ƒÑ-Ketoglutaric Acid and Succinic Acid the release of carbon dioxide and reduction of NAD+ into NADH happens again, resulting in a 4-C chemical, succinic acid. GTP (Guanine Triphosphate, which transfers its energy to ATP) is also formed here (GTP is formed by attaching a phosphate to GDP). The remaining energy carrier-generating steps involve the shifting of atomic arrangements within the 4-C molecules. Between Succinic Acid and Fumaric Acid, the molecular shifting releases not enough energy to make ATP or NADH outright, but instead this energy is captured by a new energy carrier, Flavin adenine dinucleotide (FAD). FAD is reduced by the addition of two H's to become FADH2. FADH2 is not as rich an energy carrier as NADH, yielding less ATP than the latter. The last step, between Malic Acid and Oxaloacetic Acid reforms OA to complete the cycle. Energy is given off and trapped by the reduction of NAD+ to NADH. The carbon dioxide released by cells is generated by the Kreb¡¦s Cycle, as are the energy carriers (NADH and FADH2) which play a role in the next step. Electron Transpo...