Why we need metabolism?

- [Instructor] When I think about metabolism, I think about a pawn shop. What do I mean by this? Well, let's say you have a few objects, an old cell phone, maybe your grandfather's pocket watch, maybe some precious metals, and even a diamond or two. Now, all of these objects have value, but try going down to your local Starbucks and using any one of 'em to purchase a cup of coffee. It doesn't work because in everyday interactions, these objects are not a usable form of currency in order to purchase things. You need cash. Now this is where the idea of the pawn shop comes in. The purpose of a pawn shop is to convert items into a usable form of currency. Now, if you understand the basic principle of a pawn shop, that it's there to convert value into a useful currency. Then you also understand the purpose of metabolism. And let me show you that. So metabolism's kind of like the pawn shop of the body. Take macronutrients and protein and carbohydrates and fats, which have vast stores of energy, but our body can't directly use that energy. In order to use it, it has to be converted into usable energy in the form of ATP. Just like at a pawn shop, you had all these items. They had value, but the value wasn't usable, you had to first convert 'em into cash. Let's go into some of the details of how metabolism works, how do you transfer the energy between these different macronutrients? Let's start with carbohydrates. The main carbohydrate that you want to remember is glucose. Glucose can be broken down into pyruvate or it can be stored in a more efficient manner in the form of glycogen. Pyruvate is then broken down into a acetyl coA. Now in order to see how the different macronutrients can be converted between one and another, let's move on to protein. Now, protein is composed of numerous amino acids. And when protein is metabolized, it is broken down into these individual amino acids. These amino acids can then subsequently be converted to pyruvate or acetyl CoA. Notice how in many of these reactions, the arrows point both directions. So and that demonstrates that the reaction can go either direction. So a protein can be converted, can be broken down into its amino acids, that are then turned into pyruvate. And then it can be converted to glucose and stored as glycogen. And this reaction can also go the other direction. Glycogen can be broken down into glucose and then through pyruvate, it can become amino acids and then stored as protein. So let's see how this works for fats. The basic form of a fat is a triglyceride. And triglycerides are broken down into glycerol and fatty acids. It's these glycerol and fatty acids that can then be converted into other metabolic intermediates. So just like with proteins, fats can also be broken down into glycerol and glycerol can be converted to pyruvate and then that pyruvate can either eventually become a protein or go up and become a carbohydrate. So just like in a pawn shop, where you can trade different items for items, in metabolism you can kind of trade in the different macronutrients for one another. But they don't all have the same value. And by value, I mean energy. So glucose is not as efficient at storing the energy in chemical bonds as triglycerides. And because of this, when the body needs to store energy, it takes the glucose it has, and it converts it into triglycerides because that's a more efficient manner of storage. But what happens when the body needs to utilize the energy stored in triglycerides or in glucose or in protein? Well, to answer that question, we have to go through another series of reactions known as cellular respiration. In order to do so, I'm gonna remove these metabolism lines here for a second. And I'm gonna use another color to describe the reactions of cellular respiration and how we produce usable energy in the form of ATP. So let's start with glucose. Just like before, glucose is converted to pyruvate. And this is the same reaction that I mentioned before in metabolism, but I want to note one additional part of this reaction. And it's that this reaction where you break glucose down into pyruvate, the process known as glycolysis, produce a couple molecules. And these molecules are ATP and NADH. So you can already see that we're starting to produce some energy. But it's really not that much. The next step is to convert that pyruvate into acetyl CoA. And notice here that I only put, I put a one directional arrow. And that's because pyruvate can only be converted to acetyl CoA, acetyl CoA can't go backwards. I put acetyl CoA here in red, because it's a very important intermediate in the process of producing energy. And, it's important because it's our entry molecule into the TCA cycle. TCA stands for tricarboxylic acid. And this cycle is also known by a couple other names, such as the Krebs cycle or the citric acid cycle. But it's a series of reactions. And there's only one reaction that I want you to remember. And that's the conversion of oxaloacetate to citrate. And I want you to remember it because it's the first reaction in this TCA cycle. And in this reaction, we take oxaloacetate, and it's combined with the acetyl CoA to produce citrate. Now this cycle includes a few more reactions. But the particulars of these reactions are less important. And, as the cycle progresses, it produces a few byproducts. And these byproducts are CO2 and NADH and FADH2. Now note that this, that in glycolysis we also produce an NADH. And this NADH will also come into play down here. So NADH and FADH2 are known as high energy electron carriers. And they're important because they can be thought of as the fuel that runs the electron transport chain. And the electron transport chain is just a stair step of reactions that produces a whole bunch of ATP. And in the process, you have oxygen converted to water. So we use the energy that is contained in these high energy electron carriers to produce ATP. So let's go through a quick review here. Remember we started with a pawn shop, with the idea that different items of value are convertible, but they're not necessarily usable unless they're converted to a usable form of currency like cash. Now this is the same idea in metabolism, where our different macronutrients can be converted between one another in order to store energy in different forms. But if we want to use that energy, we have to go through the process of cellular respiration to produce ATP, which can be thought of as the energy currency of the body.