Lipid catabolism ( ketogenesis and oxidation of glycerol) and Lipid anabolism (lipogenesis)




• During excessive beta oxidation, the two-carbon fatty acid fragments are converted into acidic ketone bodies. • Ketosis, the overproduction of ketone bodies, can lead to acidosis (ketoacidosis) of the blood. • After lipolysis, glycerol is converted to pyruvic acid. • Pyruvic acid will then be used to form acetyl CoA, which enters the Krebs cycle. • The electron transport chain uses the resulting reduced coenzymes to generate ATP. • Glycerol is also directed into the glycolytic pathway and converted to glucose. • Gluconeogenesis is stimulated by the hormones glucagon and cortisol. • Lipid anabolism is the process called lipogenesis. • Stimulated by the hormone insulin, lipogenesis occurs in liver and adipose cells. • Lipogenesis synthesizes new lipids from Acetyl CoA and glucose molecules. • Acetyl CoA molecules, produced from amino acids, glucose, and ketone bodies, are converted to fatty acids. • Glucose is converted to glycerol via the formation of glyceraldehyde phosphate. • Glycerol and fatty acids become triglycerides and phospholipids. Because most lipids are nonpolar (hydrophobic), they do not dissolve in water. Because blood plasma is over 90% water, lipids must be transported combined with proteins produced by the liver and intestines. These are lipoproteins. There are four classes of lipoproteins: Chylomicrons—transport dietary lipids to adipose tissue Very-low-density lipoproteins (VLDLs)—transport triglycerides from hepatocytes to adipocytes Low-density lipoproteins (LDLs)—carry about 75% of the total cholesterol in blood and deliver it to cells High-density lipoproteins (HDLs)—remove excess cholesterol from body cells and the blood and transport it to the liver for elimination Cholesterol comes from some foods (eggs, dairy, organ meats), but most is synthesized by hepatocytes. Increases in total cholesterol levels are associated with a greater risk of coronary artery disease. Exercise, diet and certain drugs are used to reduce high cholesterol levels Lipids may be oxidized to produce ATP. If the body does not need lipids at any given time, they get stored in adipose tissue. Some are used as structural molecules or to synthesize other essential substances. Adipose tissue is used to remove triglycerides from chylomicrons and VLDLs. These triglycerides constitute 98% of all body energy reserves. Lipid catabolism (lipolysis) is the process of splitting triglycerides into fatty acids and glycerol. Lipid anabolism (lipogenesis) is the process of synthesizing lipids from glucose or amino acids. It occurs when individuals consume more calories then needed.



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