 For lunch today, you had a turkey sandwich, a bag of sour cream and onion potato chips, an apple, and a cookie for dessert. It tasted good and you know somehow your body will turn it into the energy you need to get through your afternoon. But how exactly does that work? Let's find out. Metabolism is the process your body uses to turn what you eat into the energy your body uses. It's all the chemical reactions carried out by your cells to change food into energy. There are two main types of metabolic pathways, catabolic and anabolic. Catabolic reactions break large complex molecules into simpler ones. This process usually releases energy. Digesting the turkey sandwich into usable sugars, fats and proteins is a catabolic reaction. Anabolic reactions assemble complex molecules from simpler ones. This process requires using energy. Forming new cells, tissues, muscles and revitalizing organs from the food you eat are all anabolic processes. The energy for anabolic reactions is provided by the catabolic reactions. This energy can also be stored in cells as adenosine triphosphate or ATP. Let's practice what we've just covered. Look at each process and determine whether each is a catabolic or anabolic process. Feel free to pause the video if you need more time to decide. Synthesizing protein. Digesting food. Replicating DNA. Creating muscle. And breaking down stored fats. Here are the processes sorted into catabolic and anabolic processes. Humans and nearly all other organisms rely on breaking down glucose to provide energy for cells. An aerobic organism accomplishes this by using glycolysis and aerobic respiration including the Krebs cycle and the electron transport chain. Let's see how this works in the human body. Glycolysis is the first step in digestion to turn food into energy. In this example, let's drag a glucose molecule to the first metabolic pathway, glycolysis, that's used in aerobic organisms. Next is the Krebs cycle. Simply put, this is the major source of energy in all living organisms. What's produced in the Krebs cycle are building blocks for other important body processes. Let's drag the pyruvic acid to the metabolic pathway in the Krebs cycle where it will be processed in an aerob. Then we move on to the electron transport chain. This takes place in the mitochondria, the powerhouse of the cell. It's a cluster of proteins that transfer electrons which drives the creation of ATP. As we've already stated, cells use ATP as energy. Watch as we drag the NADH and FADH2 to the metabolic pathway where they will be processed in an aerob. In the presence of oxygen, aerobic metabolism includes glycolysis, the Krebs cycle, and the electron transport chain. This results in an energy gain of 38 ATP per glucose. When oxygen or other electron acceptors aren't available, an organism can't complete respiration. Humans use fermentation when we need a large amount of energy in a hurry. Some organisms survive by fermentation alone. In this case, watch as the glucose molecule starts down the fermentation path. Now, we'll move the pyruvic acid and NADH to the fermentation pathway. Depending on the fermentation pathway used, it can produce lactic acid, alcohol, or other end products. Fermentation only results in a gain of 2 ATP per glucose. It doesn't require oxygen. The human body uses a number of processes to convert a turkey sandwich into the energy it needs to fuel your body, create new tissue, and repair any damage. This was a brief overview of how these processes work together to turn food into the ATP cells need. You've completed metabolic pathways.