 In this video I will define metabolism, explain metabolic pathways, and describe the two major types of metabolic pathways. Metabolism is the sum of all the chemical reactions that take place in the body. We have two broad subdivisions of metabolism. There's catabolism, which are the chemical reactions that break down large molecules into smaller molecules and releases energy in anabolism, which are the chemical reactions that build large molecules from smaller molecules and requires the input of energy. A metabolic pathway is a series of biochemical reactions that convert a substrate molecule or multiple substrate molecules through a step-by-step process where the substrates are converted to intermediate substrates and then eventually to the final product or products. Each chemical reaction in a metabolic pathway is catalyzed by a specific enzyme, and so a metabolic pathway is a series of enzymes working together to convert a substrate into an end product. In the illustration here we can see that the substrate represented with a green triangle is converted into the end product represented with a yellow square. Enzyme 1 converts the substrate into the intermediate substrate A represented with the purple rectangle, then Enzyme 2 converts intermediate substrate A to intermediate substrate B, and then Enzyme 3 will finish the conversion, converting intermediate enzyme B into the final end product. Now you'll notice that the end product then binds to Enzyme 1 in order to inhibit this metabolic pathway. Once enough of the end product has been made, we don't need to convert more substrate into intermediate substrate A and intermediate substrate B, so the end product will inhibit the first enzyme in the metabolic pathway. Small pathways can be classified as either anabolic or catabolic. An anabolic pathway is where small molecules assemble into large ones and energy is required in order to fuel an anabolic pathway. An example of an anabolic pathway is DNA replication, another example would be transcription or translation. In these processes we're building large molecules, DNA replication, the product is the DNA nucleic acid polymer and nucleotides are being assembled into that polymer, or similarly during transcription it's the RNA nucleotides that are small molecules assembled into a large RNA nucleic acid polymer. In translation the amino acids are small molecules that are assembled into a large polypeptide polymer. In contrast a catabolic pathway is a metabolic pathway where a large molecule is broken down into smaller molecules and energy is released in this process. A variety of organic molecules can be broken down by catabolic pathways in order to release energy that can then be stored as potential energy in the chemical bonds of the molecule ATP. Energy will also be released as heat that can help to maintain the body temperature. An example of a catabolic pathway is glucose catabolism. Glucose contains 6 carbons, 12 hydrogens and 6 oxygens and in glucose catabolism glucose will react with 6 molecules of oxygen and the products at the end of glucose catabolism are 6 molecules of carbon dioxide, 6 molecules of water and also some energy has been released equal to 673 kilocalories. That energy could be converted to heat to help warm the surrounding tissue or energy can be stored as chemical potential energy to form ATP molecules that can then be used to drive anabolic pathways.