Unlike carbohydrates and proteins, lipids, or fats, are not digested until they reach the small intestine where they are broken down into small micelles and digested by the bile salts and acids, as well as enzymes called lipases. The lipids are then absorbed and stored in adipose tissue, the liver and other areas of the body. Stored lipids are important sources of energy for the body. Laboratory MedNews notes that during fasting or long periods without food, the body uses up glucose stores in the liver and other tissues, and the breakdown, or catabolism, of stored lipids begins.
Lipolysis
Lipids are primarily stored as triglycerides in the body. In order for the body to use these fats for energy productions, lipolysis or break down of the triglycerides must first occur. Lipolysis separates the storage form of lipids in the adipose tissues into fatty acids that can be metabolized into energy, as explained by the faculty of biochemistry at Elmhurst College.
Beta Oxidation
Beta oxidation is the next step in lipid catabolism to produce energy for the body functions. The products from lipolysis are transported to the energy-producing "factories" inside cells called mitochondria. Several enzymes, including one called transferase I, transport the fatty acids across the membrane of the outer mitochondria and into its inner membrane called the matrix. This is where the beta oxidation process occurs. The end result of this process results in the fatty acid being cleaved into two-carbon molecules called acetyl-CoA. Laboratory MedNews notes that lipids provide 9 kilocalories of energy per gram, while carbohydrates and protein provide only 4 kilocalories of energy per gram.
Electron Transport Chain
The products of the beta oxidation process then enter the next step in lipid catabolism, which is called the electron transport chain. This step is also called the Krebs Cycle, and is the final part of the process to break down lipids to produce energy. Information from the faculty of biochemistry at Elmhurst College indicates that the electron transport chain results in the production of the energy molecule ATP, or adenosine triphosphate, which the body uses as fuel. Other byproducts of this process include carbon dioxide and water.
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