Stated clearly presents: What is metabolism? Many people have this vague idea in their minds that the word metabolism has something to do with weight loss. This is partly true; the study of metabolism does include research about how cells build, store, and break down fat, but metabolism is far more interesting than the latest weight loss trends in Hollywood. Cellular metabolism can be defined as the sum total of all controlled chemical reactions that occur inside a cell. Metabolism is so important that several of the chemists working with NASA at the National Science Foundation's Center for Chemical Evolution believe that a better understanding of metabolism could help us finally understand the origin of life itself, and better help us know where to search for life elsewhere in the cosmos. We often think of individual cells as things, but in the eyes of a chemist, a cell is a highly active, highly sophisticated chemical system. The cells in our bodies, and, as a consequence, our bodies themselves, are better thought of not as things, but as whirlpools. You are a continuous flow of matter and energy. Food, water, and oxygen are constantly consumed by your cells. Waste products like CO2 are constantly expelled when you breathe. Other waste products, of course, exit through different routes. [toilet flushing] The molecules in your skin are constantly renewed, fully replaced roughly every 48 days. Your bones, though they seem so solid, so permanent, cycle through a full refresh as often as every 10 years. Again, you are a whirlpool. Your cellular metabolism consists of hundreds of different reactions. To make sense of them all, researchers often separate them into two main categories: anabolic metabolism, reactions that join molecules together to build new ones; and catabolic metabolism, reactions that break molecules apart. To remember the difference, think catabolic as in catastrophe, because catabolic reactions destroy things. When you eat an apple, your digestive tract breaks it down into individual molecules: sugars, fats, and amino acids. These are absorbed by your intestines, enter the bloodstream, and are then distributed to the cells of your body. Once inside the cell, metabolic reactions take over, transforming those molecules into little bits of you. While we often take this for granted, this concept is so important and so bizarre that it's worth repeating. A cell's metabolism, its internal network of chemical reactions, takes molecules from the environment-- the molecules of an apple in this case--breaks them down, and transforms them into pieces of the cell itself. Sometimes the cell builds replacement parts for its own molecules that have been worn out. Other times, it builds energy molecules. These act like fuel, and will power the cell's future activities. Muscle cells use energy molecules to contract, brain cells use them to help form thoughts, and so on. This chart shows just some of the many networks, cycles, and series of chemical reactions happening inside your cells. As you can see, cellular metabolism is fairly complex. Different species have evolved different metabolic pathways, which is one reason they often use different things as food. Hydrogen sulfide gas, for example, is very toxic to you and me, but certain species of bacteria actually eat the stuff. That said, scientists studying the origin of life have discovered that one very special series of metabolic reactions, the citric acid cycle, exists in one form or another in almost all species studied so far. The fact that it's nearly universal suggests that the citric acid cycle is both extremely old and extremely important The citric acid cycle, sometimes called the Krebs cycle, named after the researcher that discovered it, starts when food particles-- sugars, fats, and amino acids that came from your lunch--are transformed into citric acid. By binding citric acid together with other molecules and breaking them apart again in a variety of different ways, citric acid is transformed into multiple types of molecules, and then back to citric acid again. Now, this might seem counterproductive, simply spinning in a circle like that, but many of the side molecules it reacts with along the way are transformed in the process, modified into parts, or at least the precursors of parts, that your cell needs to repair damaged molecules, grow, reproduce, and power its movements. The citric acid cycle is the engine of life, the core whirlpool inside your cells. In our case, fats, sugars, and amino acids from our food are broken down and eventually fed to the citric acid cycle. Carbon dioxide and water are produced as waste products that we then breathe out. In certain species of bacteria, however, the citric acid cycle runs in reverse. Water and carbon dioxide are used as food sources, and react together to eventually produce large carbon molecules that bacteria use to build fats, sugars, and amino acids. Because the reverse citric acid cycle uses such simple starting molecules, water and carbon dioxide, things that would have been superabundant on the early earth, some researchers believe that the reverse citric acid cycle has been misnamed. It likely predates the "normal" citric acid cycle by millions of years. In fact, some argue that the reverse citric acid cycle or something similar to it might predate life itself, and may have kick-started the first living cells on our planet. We'll talk more about that in a future video So in summary, what is cellular metabolism? Cellular metabolism is the sum total of all the controlled chemical reactions that happen inside a living cell Anabolic reactions take small molecules and put them back together to build new ones. catabolic reactions--remember catastrophe--break molecules apart. At the core of metabolism is a special cycle of catabolic and anabolic reactions called the citric acid cycle. You are a whirlpool I'm Jon Perry, and that is metabolism stated clearly. This animation is part of a series on the origin and chemistry of life funded in part by the National Science Foundation, NASA, and their Center for Chemical Evolution. The animation was also funded by people like you who support my work on patreon.com/statedclearly. Thank you so much. I really really could not do this without your help
