end result is the absorption of amino acids that get used, converted into fats, or turned into acetyl CoA for the Krebs cycle. The process of breaking down proteins into amino acids is called proteolysis. The urea cycle takes the nitrogenous waste products that come from the breakdown of amino acids and turns it into a molecule that is safer in the body than the ammonium ions made in amino acid breakdown. This cycle occurs in the liver and the kidneys. Since the amino group in the amino acid is not a part of any metabolic pathway, to become a metabolically-active substance it must undergo transamination, which turns the amino group into a keto group. This creates a molecule that can enter the Krebs cycle plus an ammonium ion that goes into the urea cycle. While in the urea cycle, ammonium goes along with CO2 to make urea and water; the urea is excreted by the kidneys. Amino acids can be metabolized into several different molecules, including acetyl CoA, pyruvate, oxaloacetate, acetoacyl CoA, and alpha-ketoglutarate. Each of these can participate somewhere in the basic metabolic processes in the body that lead to the Krebs cycle and aerobic metabolism.
BASIC HUMAN METABOLISM Food gets absorbed during eating, used or stored as necessary, and then metabolized during times when a person isn’t eating. In other words, you don’t have to eat continually in order to have nutrients readily available. Right after eating, the body is in an absorptive state. The food is digested and transported into the body via the enterocytes. The sugars, amino acids, and lipids go to the liver, adipose tissue, or muscle tissue in order to be processed and used for energy. This absorptive state can last for up to four hours; insulin is released in order to put glucose into the muscle cells, fat cells, and liver cells. Glucose gets immediately turned into glucose-6-phosphate, which means there is a concentration gradient that pushes more glucose into the cell. Liver glucose goes to make glycogen; the same is true of muscle glucose that isn’t directly needed.
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