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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 15 Current Lecture Metabolism and Metabolic Fuel There are two states of the body: 1. you have just eaten and food is being absorbed. This is the absorptive/fed state, when there is an excess of fuel flooding blood. Also known as the anabolic state 2. Post absorptive/fasted state. You are not eating, and catabolic reactions are predominating. Flow of Molecules through Metabolism Food molecules enter the body as three major macromolecules: fats, carbs, and protein. The tubs immediately underneath them represent the immediately available pools in the blood plasma of free fatty acids, glucose, and amino acids. The pools at the bottom are what the body cells use for their needs, and just above them are the storage forms of these fuel molecules, with fatty acids stored as fat, glucose stored as glycogen, and the amino acids stored in the body protein (protein stores in the body are all the cells that have protein in them - mainly skeletal muscle, which can be broken down when necessary to restore the protein pool. This doesn’t greatly reduce their function – even if you got rid of half of the protein in your skeletal muscle, you would be weaker but still functional). Connecting the storage pool with the immediately available pools are the systems of reactions/processes that breaks one down into another. -Lipolysis breaks down fat to form free fatty acids. Free fatty acids and excess glucose can also be esterified into new fats through lipogenesis. -Glycogen(carbohydrate stores) is replenished by glycogenesis from the glucose pool, mostly down in the liver. And glycogen is broken down by glycogenolysis to break glucose. -Amino acids are broken down by proteolysis, which isn’t shown. Other lateral pipes exist, for example gluconeogenesis; glucose can be synthesized from amino acids. If you take the amino group off amino acids, they look like carbohydrates. The liver can transmogrify those amino acids and convert them into glucose (gluconeogenesis). For normal metabolism in most tissues, which is powered by glucose or fatty acids, any excess nutrients are recycled back into their storage forms. The red lines in the glucose pool represent the levels of normal plasma glucose. The pipe coming out of the left can only take glucose out of the pool when glucose levels are above the lower pipeline level. You can also see that the brain metabolism is privileged, and it is at a lower pipeline level, meaning it can continue drawing glucose even after levels have fallen down below the normal range. This is because brain metabolism is so important – it depends on glucose more than any other tissue. CNS tissues are highly metabolically active, and they do not store any form of energy – there are no glycogen or fats or proteins up there for fuel. Since it has such a high metabolic rate, it is completely dependent on an adequate supply of glucose. Control of Appetite and Satiety Satiety is the opposite of hunger. This is one of the ...

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