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Bioenergetics: The transformation of free energy in living systems

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- [Narrator] I want you to be silent for like three seconds. Now, chances are in those three seconds, your heart beat somewhere between three and five times. Now, your heart's a pretty amazing muscle. I mean, it beats all day, every day for your entire life. And in order to do so, it requires a ton of energy. But where does all this energy come from? So I drew a picture of a sun, here, to describe that all of the energy that our body uses for work is originally derived from the sun. But humans can't just stand outside in the sun and get energy that way. Energy has to be transformed into a usable form of energy for our body that will allow our muscles and things like our heart to beat. Now, where does this energy come from? I'll just erase some of my work here. Well, our story starts with plants. Now, plants take the energy from the sunlight and they convert inorganic compounds into glucose, and the energy from the sunlight is added to this living system in the form of the chemical bonds of glucose. Animals and humans then eat the glucose from these plants, and that glucose is then converted into a usable form of energy, which is known as ATP. And this process of adding energy to the system and creating glucose is known as photosynthesis. And this process of breaking down glucose into a usable form of energy is known as cellular respiration. So let's find out how this all happens. This process starts in the chloroplast of the plant cells. And then this step, known as the light reaction, the energy from light disrupts the H20, or water, causing it to kick off a hydrogen ion, and this hydrogen ion is then bound to NADP to form NADPH. And in the process of all this, ETP is formed. So you can see that we already have an energy form from the light, but this is a relatively smaller amount of energy, and it's this NADPH that's created that is a high-energy electron carrier that can be used to produce a lot more energy. And the next step in this process is known as the Calvin Cycle. And in this part of photosynthesis, we start with carbon dioxide. And NADPH and ATP are added, and there's a series of reactions that occur. And the specifics of these reactions are less important than the outcome, which is glucose. So in these two steps, in the light reaction and the Calvin Cycle, we take the energy that's in sunlight, we store it into the bonds of NADPH and ATP and we use those to run the Calvin Cycle to store all of our energy in the chemical bonds of glucose. Now, what happens when glucose is eaten by an animal and that animal then wants to use the energy? I mentioned earlier that this process is celled cellular respiration, and cellular respiration is very similar to photosynthesis, just in a backwards direction. Now the first step, we have to break glucose down into a molecule known as pyruvate. And this step gives off ATP. And ATP is the usable form of energy, but we're not giving off a whole lot yet. We still have a lot of energy forms contained in the bonds of pyruvate. So pyruvate is then broken down into acetyl-CoA. And then acetyl-CoA then enters the TCA cycle. And TCA stands for tricarboxylic acid. And this cycle is also known by a couple other names like the Kreb cycle and the citric acid cycle. Once again, there's a series of reactions that occur in this cycle, but the specifics of these reactions are less important than the outcome, which is production of C02 as well as NADH and FADH2. Now these molecules are similar to the NADPH in photosynthesis, in that they're high-energy election carriers. Now they enter a series of reactions known as the electron transport chain. And in this reaction, the hydrogen from these high-energy election carriers is bumped off. And we have oxygen over here, which is combined with the hydrogen to form water, or H20. And these hydrogens here drive an enzymatic pump that produces ATP. And you can see here that photosynthesis and cellular respiration are very similar reactions, just in the opposite direction. And although they may have different intermediates, actually the products of one are the reactants of the other, and vice versa, and so let me demonstrate that. In photosynthesis, our reactants are H20 and C02, and our products are oxygen and glucose, whereas in cellular respiration we have glucose as a reactant, as well as oxygen. We're now producing C02 and water, and you can actually see this if I write out the equations for photosynthesis and for cellular respiration. In photosynthesis, the equation is 6CO2 + 6H2O, which produces glucose or C6 H12 06 + 602. And cellular respiration is really just the opposite of that where we take glucose, which is C6 H12 06 + 6 oxygen, which will end up producing 6 carbon dioxide and 6 waters. Now there's an important reactant in product that isn't added in these chemical equations, and that is energy. And in photosynthesis, the energy is a reactant, putting this energy into the chemical bonds of glucose. Whereas in cellular respiration, the energy is a product. And we're taking that energy from the chemical bonds of glucose. So let me just show you one more way to demonstrate how energy changes in these two reactions. Now, to do this I'm gonna draw a reaction diagram. And on the x-axis, here, we have the reaction progress, and on the y-axis we have free energy. Which is also known as G. I'm gonna just dim down the reaction a little bit so that we can work over the top of it. So if you look at the free energy level for where the reactants of photosynthesis start, to where they end with glucose, you're adding energy to this. So you have a low free energy level and you're going to a higher free energy level. So energy is added, and that's from the sunlight, whereas in cellular respiration, you go from the reactants of glucose with lots of free energy, to low free energy with ATP, you are releasing energy. And it's this release in free energy that allows our body to do work, like pump the heart.