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Explore some fundamental activities of a cell, which include both constructing and taking apart important molecules that store energy. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Stanford School of Medicine.
Video transcript
Voiceover: Let's talk about this thing called metabolism. I remember being told in medical school that metabolism was the sum total of all of the chemical reactions in the human body, but I never actually knnew what that meant. I like to think of metabolism as kind of like the balance in the body between the reactions that build things up, and another way of saying that is anabolism. On the other side of the scale, the reactions that break things down. The medical way of saying that is catabolism, or catabolism. So, metabolism is kind of the balance between building things up, or repairing or storing inside the body and breaking things down, usually for energy needs. How are the things that we eat broken down for energy or stored within the body? Let's start by looking at carbohydrates. So, we eat our carbohydrates, and they are in the form of starches or sugars, and we saw in our last video that those could get broken down into their kind of component parts. The major component, or the major single unit currency of carbohydrates is called glucose. That glucose, and this is very big-picture, can get broken down further into a compound called pyruvate. The pyruvate then gets broken down into a very important molecule that's kind of the center of making energy in the body, and that molecule is called acetyl-CoA. Now, I think of acetyl-CoA kind of like one of those little trampolines you see in the gym, and basically acetyl-CoA can be moved around to different parts. We'll put the trampoline there, and then we'll put it over there, and then we'll put it over there. Basically, different chemcals bounce on and off of that little portable trampoline as they're going around this cycle. This cycle is called the TCA cycle. That stands for tricarboxylic acid cycle, or it's sometimes called the Krebs cycle. Essentially, what you need to know about this cycle is that as the acetyl-CoA moves around the cycle to form all of these slightly different compounds, things are bounced off, and the things that are bounced off are these high-energy carriers. I'll just write high-enegy carriers. What happens to the high-energy carriers next is that they basically bounce down a staircase called the electron transport chain. As they go down that staircase, they give off a chemical called ATP. ATP is the currency of energy in the human body. ATP is the same as energy in the human body. Now, just to add some detail here, water is also given off, and I'll write the chemical formula for water, which is H2O, and in order for this cycle, this TCA cycle to move around, oxygen is required, and carbon dioxide, CO2, and water, again, are given off. Oxygen is also necessary for this step to happen, and this is really neat because this is essentially the reason why we need to breathe. Now let's look at what happens to the protein that's in our diet. Protein was one of the 3 macro nutrients that we looked at in the last video, and the proteins in our diet are broken down by our digestive tract as we saw into unit parts called amino acids. Depending on what kind of amino acid you're looking at, the amino acid can either enter the system here, at this level, or it can enter here, depending on which kind of amino acid it is. When the amino acid makes this transition, something is given off, and that something is the amino group. So the amino group, or the amine, is given off, and that amine becomes one of the waste products in our urine. That's how we get rid of those amine groups. Let's look now at what happens to the fats that we eat in our diet. Fats, basically, are made up of a little glycerol backbone, so I'm gonna write here "glycerol", and long chains of what we call fatty acids. There are three of them attached to each glycerol backbone. What happens when the body needs to use those for energy is that it breaks them down into units of 2 carbons each, and I'll write here fatty acids, and those 2 carbon units can only enter the system over here, at the level of acetyl-CoA. Whereas this small glycerol backbone goes in over here. Those are the breaking down reactions. That's this side of the equation of metabolism, but what about the building up reactions? Well, the amino acids that come from the protein in our diet can be used to make things like muscle and other lean tissue in our bodies. They can be used for repair. They can be used as protein carriers in our blood, and the carbohydrate that we eat, and the glucose in our body, can also be stored as something called glycogen, which is the storage form of carbohydrates in our bodies. The fats that we've eaten, if there are too many of those in our diet, they can also be stored as adipose tissue, so they're put back together and stored in our body. The balance between the breaking down reactions, or the catabolic reactions, and the building up reactions, the balance is controlled by hormones in our body. There are several hormones that lead to breaking down of tissues. Things like glucagon leads to the breakdown of the stores of glycogen in our body. Other hormones are the kind of fight or flight hormones. Epinephrine is one of them and glucocorticoids. These are sort of the stress hormones in our body. These guys all favor the breaking down, or catabolism, of the tissues. The hormones that lead to the building up, or the anabolism, in our bodies, the major one is insulin. I'm sure you've all heard of it. Insulin leads to storage, or things being put away. There's also a part to be played by the sex hormones, or the sex steroids, and then there are some hormones that are anabolic, or favor the building up of proteins, and those are the hormones thyroxin and growth hormone, I'll just write GH. Those guys favor the anabolism or the building up of protein stores, so protein synthesis is favored by these guys. But, they also work on this side. they favor the breakdown of fats and carbohydrate stores. Thyroxin and GH, growth hormone, those guys favor the breaking down of our fat and carbohydrate stores. The last thing I want to add here is that some of these reactions are reversible, so glucose going to pyruvate can be reversed. It can go back up in the other direction, and that's how we can make glycogen from, let's say, these amino acids that make pyruvate. This kind of amino acid can be used to make glycogen. The backbone of the fatty acids called glycerol could be used to make glycogen or to make glucose to feed the brain, but this reaction, this particular one here, is an irreversible, one-way reaction. That means that fatty acids can never go up to make pyruvate and be used as glucose for the brain, or they can never be used to replenish glycogen stores in the body. Another important thing that we have to understand is that this cycle will only go around if ATP is needed by the body. If we're using energy, we're doing exercise, we're demanding ATP and energy from our system, then this cycle will continue going around. But if we don't, let's say we're sedentary and we don't use energy, then acetyl-CoA is going to build up, and it's gonna favor the return in this direction towards adipose tissue. This is how excess carbohydrate, if it's taken in in the diet, can lead to excess adipose tissue, excess fat, if we eat too much fat in our diet, that can lead to adipose tissue, and even if we eat too much protein, it'll just be deaminated and converted to acetyl-CoA, which can be converted to adipose tissue.