In this lesson, we'll explore anabolic and catabolic reactions. Many kinds of chemical reactions occur in the human body. Two of the more common types that are important to maintaining optimal conditions in the body, or homeostatis, are anabolic reactions and catabolic reactions. These two types of reactions work opposite of each other. Let's see if you can figure out what their names mean. Have you every heard the term "anabolic" before? You probably have. Anabolic steroids, right? We unfortunately sometimes hear about individuals, especially competitive athletes, misusing anabolic steroids. But why do they use them? To improve performance, right? And this usually occurs by building up more muscle. When you hear the term "anabolic", think of building something up or making it bigger. Anabolic reactions produce something bigger than the reaction begins with. These reactions are also referred to as synthesis reactions. An example of an anabolic reaction would be when relatively small amino acids link together to build a much larger protein. Catabolic reactions work the opposite of anabolic reactions. So if the anabolic reactions build something bigger, what would catabolic reactions do? They break things down into smaller parts, right? These reactions are also referred to as decomposition reactions. Think about when something decomposes. It breaks down, doesn't it? In your cells, the most common type of anabolic and catabolic reactions involve water. During anabolic or synthesis reactions involving organic molecules, the molecules typically align so that a hydroxyl group, which is an oxygen and hydrogen, from one molecule is closely aligned with a hydroxyl group on the next molecule. For the two molecules to bond, one molecule loses a hydrogen from its hydroxyl and the other molecule loses its whole hydroxyl. The last time I checked, two hydrogens and one oxygen make water. Correct? So water is lost so these molecules can combine. What do we say when we have lost water? We are dehydrated, right? So dehydrate means to lose water. And remember that another name for anabolic reactions is synthesis. When water is released during an anabolic or synthesis reaction, the process is referred to as dehydration synthesis. If we take water out to put two molecules together and build something bigger, what might we do to break that product down again? Put the water back in, right? A common type of catabolic reaction is called hydrolysis. "Hydro" refers to water and "Lyse" means to separate. So in hydrolysis, water is used to separate a large molecule into smaller molecules. Notice that in the anabolic reaction, dehydration synthesis, water is taken out, dehydration, to make something bigger, synthesis. And in the opposite reaction, the catabolic reaction, water, hydro, is put in to separate, lysis, or break down the molecule. When we eat, the large molecules in our foods are broken down by hydrolysis during digestion. Digestion thus involves a series of catabolic reactions and, once they are absorbed into the body, the small molecules produced by hydrolysis can be linked together by dehydration synthesis to build bigger molecules that your cells need. For example, anabolic reactions allow for growth, such as growth of your muscles and your bones, and also for repair when you have an injury or cell parts get worn out or an organ gets damaged, for example.