{"id":682,"date":"2017-04-16T02:54:38","date_gmt":"2017-04-16T02:54:38","guid":{"rendered":"https:\/\/pressbooks.hccfl.edu\/bio1\/chapter\/7-9-metabolism-of-molecules-other-than-glucose\/"},"modified":"2025-08-29T18:15:15","modified_gmt":"2025-08-29T18:15:15","slug":"7-9-metabolism-of-molecules-other-than-glucose","status":"publish","type":"chapter","link":"https:\/\/pressbooks.hccfl.edu\/bio1\/chapter\/7-9-metabolism-of-molecules-other-than-glucose\/","title":{"raw":"Metabolism of molecules other than glucose","rendered":"Metabolism of molecules other than glucose"},"content":{"raw":"You have learned about the catabolism of glucose, which provides energy to living cells. But living things consume more than just glucose for food. How does a turkey sandwich, which contains various carbohydrates, lipids, and protein, provide energy to your cells?\n\nBasically, all of these molecules from food are converted into molecules that can enter the cellular respiration pathway somewhere. Some molecules enter at glycolysis, while others enter at the citric acid cycle. This means that all\u00a0of the catabolic pathways for carbohydrates, proteins, and lipids eventually connect into glycolysis and the citric acid cycle pathways. Metabolic pathways should be thought of as porous\u2014that is, substances enter from other pathways, and other substances leave for other pathways. These pathways are not closed systems. Many of the products in a particular pathway are reactants in other pathways.\n

Carbohydrates<\/h1>\nSo far, we have discussed the carbohydrate from which\u00a0organisms derive the majority of their energy: glucose. Many carbohydrate molecules can be broken down into glucose or otherwise processed into glucose by the body. Glycogen,<\/strong> a polymer of glucose, is a short-term energy storage molecule in animals (Figure 1<\/strong>). When there is plenty of\u00a0ATP present, the extra\u00a0glucose is converted into glycogen for storage. Glycogen is made and stored in the liver and muscle. Glycogen will be taken out of storage if blood sugar levels drop. The presence of glycogen in muscle cells as a source of glucose allows\u00a0ATP to be produced for a longer time during exercise.\n\n[caption id=\"attachment_680\" align=\"alignnone\" width=\"234\"]\"glycogen\" Figure 1<\/strong> Glycogen is made of many molecules of glucose attached together into branching chains. Each of the balls in the bottom diagram represents one molecule of glucose. (Credit: Glycogen<\/a> by BorisTM<\/a>. This work has been released into the public domain)[\/caption]\n\nMost other carbohydrates enter the cellular respiration pathway during glycolysis. For example, sucrose<\/strong> is a disaccharide made from glucose and fructose bonded together. Sucrose is broken down in the small intestine. The glucose enters the beginning of glycolysis as previously discussed, while fructose can be slightly modified and enter glycolysis at the third step.\u00a0Lactose,<\/strong> the disaccharide sugar found in milk, can be broken down by lactase enzyme into two smaller sugars: galactose and glucose. Like fructose, galactose can be slightly modified to enter glycolysis.\n\nBecause these carbohydrates enter near the beginning of glycolysis, their\u00a0catabolism (breakdown) produces the same number of ATP molecules as glucose.\n

Proteins<\/h1>\nProteins are broken down by a variety of enzymes in cells. Most of the time, amino acids are recycled into new proteins and not used as a source of energy. This is because it is more energy efficient to reuse amino acids rather than making new ones from scratch.\u00a0The body will use protein as a source of energy\u00a0if:\n