24 November 2012
Describe the metabolic changes, with reference to specific pathways, that accompany (i) missing breakfast, and (ii) subsequently having a large lunch. Briefly, contrast the changes occurring in (i) to those in an extended fast (this needs the lectures in part II of Chemistry of Life but again do some reading to help you with the answer) Metabolic pathways and mechanisms react well-matched to the nutrients, which are absorbed by the body. They allow a steady supply of energy to the body’s cells. ATP is the “currency” of energy transfer. Its phosphorylation is Gibbs free energy negative therefore it provides the energy for almost all vital reactions. Its steady production or availability after missing out breakfast is essential for “surviving” breakfast-free times. A breakfast is generally carbohydrate-rich so it contains monosaccharides, disaccharides, oligosaccharides and polysaccharides, which all can be broken down to Glucose by intestine digestive enzymes. The blood from the gastrointestinal tract (including pancreas) enters the liver through the hepatic portal vein and is rich in nutrient, mainly Glucose in this case. The concentration of the antagonistic hormones insulin and glucagon, which are produced in the pancreas changes with blood sugar level: If there is much glucose in the blood, insulin is released, if the glucose level is low glucagon is secreted. These two hormones control the blood sugar level and have different effects on different receptors of organs. If the breakfast is missed out (i) the blood sugar level is low and glucagon is released. In both liver and muscle cells glycogen, a polysaccharide, which is originally synthesized from glucose acting as energy storage, is cleaved by glycogen phosphorylase to form glucose 6phosphate which continues on the glycolysis pathway. Liver glycogen in form of glucose can be used throughout the whole body. Muscle glycogen is less abundant and just provides glucose...
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