Summary

Summary

The body’s many chemical reactions constitute its metabolism. A chemical reaction stores and uses energy to maintain homeostasis and to perform all the body’s essential functions. The two major types of metabolic reactions that control the use of energy by the cells are anabolism and catabolism. Anabolism is the building of larger and new molecules. Catabo-lism is the breakdown of larger molecules into smaller ones. Every type of cell in the body performs basic chemical reactions.

During oxidation, glucose burns in the cells to release energy that fuels the process of anabo-lism. Inside the cells, enzymes reduce the amount of activation energy needed for oxidation as part of ­cellular respiration. Redox reactions occur when one substance­ gains oxygen or loses hydrogen (the pro-cess of oxidation), causing another substance to gain electrons (the process of reduction). These are coupled reactions that are also referred to as redox reactions. When the bonds between the atoms of molecules are broken, chemical energy is released.

The process of cellular respiration requires three types of reactions: glycolysis, the citric acid cycle, and the electron transport chain. Glycolysis involves the breakdown of glucose to yield lactic acid or pyru-vic acid, releasing energy as ATP. The citric acid cycle or Krebs cycle involves metabolism of carbon chains of glucose, fatty acids, and amino acids. It yields carbon dioxide, water, and high-energy phosphate bonds (ATP). In the electron transport chain, the high-energy electrons still contain most of the chem-ical energy of the original glucose molecule. Because oxygen is the final electron acceptor, this overall process is called aerobic respiration. Aerobic respiration generates­ energy in the mitochondria. Anaer-obic ­respiration occurs outside of the mitochondria, releasing less energy than aerobic respiration.

Specific sequences of enzymatic actions control cellular respiration, anabolic reactions, and catabolic­ reactions. Each sequence is called a metabolic pathway. Glycogenesis joins glucose molecules in long chains to form glycogen. The splitting (lysis) of glycogen occurs when blood glucose levels drop, which is known as gly-cogenolysis. Gluconeogenesis is the process of forming new glucose from noncarbohydrate molecules. Fats are the body’s most concentrated energy source, yet tri-glycerides are the only type of fats regularly oxidized for energy. Lipogenesis is also known as triglyceride syn-thesis, which occurs when ATP and glucose levels are high in the cells. Lipolysis is basically the opposite of lipogenesis and is defined as the breakdown of stored fats into fatty acids and glycerol. Because the body can-not store excess protein, it is broken down and replaced before it deteriorates. Amino acids must be deaminated before they can be oxidized for energy, with glutamic acid being the key molecule used for this purpose. The most important anabolic nutrients are amino acids, which form all protein structures and most of the body’s functional molecules. Protein synthesis occurs on ribosomes, regulated by hormones such as growth hormone, sex hormones, thyroxine, and others.