Intermediary metabolism

Metabolism is the sum total of all the chemical reactions occurring at a cellular level in the body. Metabolic pathways can be classified as anabolic, catabolic and amphibolic (both catabolic and anabolic). Anabolic pathways synthesize complex end products from simple pre-cursors. For example, the synthesis of the polysaccharide, glycogen, from glucose. Anabolism is a divergent process, and the energy for anabolic reactions is provided by the breakdown of adenosine triphos-phate (ATP) to adenosine diphosphate (ADP) and inorganic phosphate. Catabolic reactions break down complex molecules to simple mole-cules. Here, energy is generated in three stages: stage 1 -hydrolysis of complex molecules to their building blocks (e.g. proteins are degraded to amino acids, polysaccharides to monosaccharides and fats to fatty acids and glycerol); stage 2 -conversion of these building blocks to simple intermediates (e.g. further degradation of these inter-mediate molecules to acetyl-CoA and other simple molecules); stage 3 -oxidation of acetyl-CoA. Reduced co-enzymes produced from the oxidation of acetyl-CoA participate in the electron transport chain and produce energy. Catabolic processes are convergent. The tricar-boxylic acid (TCA) cycle is an amphibolic pathway, and it has the essence of both anabolism and catabolism.