Physiology of Finfish and Shellfish (2+1)

4.3.2.8 The Electron Transport Chain

Very little energy has been produced during glycolysis and the Krebs cycle. Most of the energy locked in the original glucose molecule will be released by the electron transport chain and oxidative phosphorylation. The electron transport chain is a network of electron-carrying proteins located in the inner membrane of the mitochondrion. These proteins transfer electrons from one to another, down the chain, much in the way a bucket brigade passes buckets of water. These electrons will eventually be added, along with protons, to oxygen, which is the final electron acceptor. This produces water, but does not produce any ATP. The ATP is actually produced by a proton motive force. This force is a store of potential energy created by the gradient formed when hydrogen (protons) are moved across a biological membrane. Therefore, the electron transport chain merely produces a gradient through which ATP can be made (this is known as chemiosmosis). The electron transport chain produces the remaining 32-34 ATP).

SOURCE: DeSilva S.S & Anderson, T.A. (1995) & Fish Nutrition in Aquaculture, Chapman& Hall

The interrelationships between protein, lipid and carbohydrate metabolism. All pathways interconnect at acetyl Co A and citric acid cycle.

Under anaerobic conditions, in active muscle, glycolysis produces lactate from pyruvate. When energy demands exceed oxygen supply this reaction is important in the muscle. The lactate moves through the blood stream to the liver, where it is oxidized back to pyruvate. Glyconeogenesis converts pyruvate to glucose, which is carried back to the muscles. The Cori cycle is the flow of lactate and glucose between the muscles and the liver.

Glycogenesis is the conversion of excess glucose to glycogen. Glycogenolysis is the conversion of glycogen to glucose which could occur several hours after a meal or overnight in the liver or in the absence of glucose-6-phosphate in the muscle to lactate. Glyconeogenesis is the formation of glucose from non carbohydrate sources, such as certain amino acids and the glycerol fraction of fats when carbohydrate intake is limited. Liver is the main site for glyconeogenesis, except during starvation, when the kidney becomes important in the process. Disorders of carbohydrate metabolism include diabetes mellitus, lactose intolerance and galactosemia.