Welcome, back everyone. Today we're going to talk about aerobic respiration. We're going to go over an overview, and we're going to talk about something called glycolysis, and also a part called the prep steps. The first thing that we should know about respiration is that it's actually pretty similar to photosynthesis, only run in reverse. In photosynthesis, we're converting water, carbon dioxide, and ATP into a form of sugar called glucose, and we're also producing oxygen as a by-product. Respiration is almost exactly the opposite. Instead of converting ATP into glucose, we're starting with glucose and converting it into ATP. We're also using oxygen and converting it into carbon dioxide, and along the way we're going to release some heat and some water vapor as well. So, you're going to want to keep these two equations in mind as we go through respiration. Respiration is also different from photosynthesis in that it occurs mostly in the mitochondria of the cell rather than in the chloroplasts. Both plant and animal cells have mitochondria, which is the major site of respiration. So, here we can see that we put in glucose and oxygen, and we export carbon dioxide, ATP, water, and some heat as a by-product. There are two main categories of cellular respiration, depending on whether or not oxygen is present. We have aerobic respiration which is more efficient, makes more ATP, and is also sustainable for a long period of time, and we also have in aerobic respiration, where there is no oxygen present. This process is much less efficient, makes much less ATP, and can only be sustained for short periods of time. Here is a slightly more detailed view of the two kinds of respiration. On the right, we have anaerobic respiration which occurs when there is no oxygen present. On the left, we have aerobic respiration which occurs when there is oxygen present. Both of these two processes begin with something called glycolysis, which we're going to go into detail about in a couple of minutes. Glycolysis produces two molecules of ATP, and then we go into either aerobic respiration or anaerobic respiration depending on whether or not there is oxygen present. If there is no oxygen, you will go into anaerobic respiration. Depending on what kind of organism is respirating, it might produce lactic acid or it might produce alcohol, but in either case it won't produce any more ATP. In anaerobic respiration, you only produce two ATP molecules per molecule of glucose; all of the production happens during glycolysis. Neither making lactic acid nor making alcohol will generate any more ATP molecules. So, you can see that anaerobic respiration is not very good at generating a lot of ATP. Aerobic respiration on the other hand is very good at generating large amounts of ATP, although it does have more steps. Just like anaerobic respiration, it begins with glycolysis, where it generates two ATP molecules. Then it goes into what we call the prep steps, and then the Krebs cycle, where it generates two more ATP. Finally, it goes into something called the electron transport chain where it generates a whopping 34 ATP per molecule of glucose. So the total amount of molecules of ATP for aerobic respiration is something like 38 molecules of ATP per molecule of glucose, whereas anaerobic respiration only makes two. So, you can see that aerobic respiration is significantly better at generating ATP than anaerobic respiration is. In this screencast, we're going to talk about the first two portions of aerobic respiration. We're going to talk about glycolysis, and then we're going to discuss the prep steps. We're not going to talk about anaerobic respiration any more today. Something to be on the lookout for as we discuss respiration is going to be the various kinds of stored energy that are present. You should already be familiar with ATP; adenosine triphosphate which holds energy stored in its bonds. In this class, we choose to liken ATP to a battery because it can be recharged and reused over and over and over again. There are other kinds of molecules that hold energy in their bonds and form storable energy sources. These include NADH and FADH2. These are all rechargeable storable forms of energy that can be cashed in later on, just like ATP. I've drawn these two look like batteries but different sizes. When NADH and FADH2 react with other molecules later on during the respiration process, they will generate ATP molecules which we will learn about later on. For now, just remember that anything shaped like a battery is a stored energy source that will be used later on. Just to make sure we don't get confused, don't forget that NADH is NOT the same as NADPH from photosynthesis. A helpful way to remember this might be to remember that NADPH has the letter P for photosynthesis. So, remember, these are two totally different things. Okay, so now let's get started with glycolysis. We're going to start off by breaking down one molecule of glucose, and remember that glucose has six carbons in it. So, one two three four five six. This six carbon molecule can be broken down into other molecules that will total up to six carbons. It does take some energy to break apart glucose molecules, though, which is why we're going to spend two molecules of ATP to break apart this green molecule. When we've broken apart the glucose molecule, instead of one large molecule with six carbons, we now have two smaller molecules each that have three carbons. This three carbon molecule is called Pyruvate. This reaction also has enough energy left to recharge two molecules of NAD+. We recharged NAD+ and we get molecules of NADH, which is the energy-rich form. In addition to producing two pyruvate molecules and two molecules of NADH, this set of reactions called glycolysis will also produce four molecules of ATP. However, because we already spent two of those molecules to get the reaction going, the total number that we've produced after glycolysis is only 2 ATP. You see a similar concept all the time in business. In order to figure out your profit, you subtract your expenses from your income. You might spend two dollars on things like wages or on filling your stock, however if your customers buy lots of your product you still will make a profit of two dollars. This is exactly what we do during the set of reactions that we call glycolysis. We spend two ATP in expenses, but we make back four ATP in income. This gives us a profit of two ATP for every molecule of glucose. Okay so, so far we have taken a molecule of glucose and we have gone through the process of glycolysis and so now we're going to take the aerobic pathway and we're going to move on to the next part which is called the prep steps. Now, remember whenever you do the prep steps, this means that you have plenty of oxygen. You're going to undergo aerobic respiration, you don't do the prep steps in anaerobic respiration. We're going to take some of the products of glycolysis and rearrange them in order to be ready for the next step, which is the kreb's cycle. One of the most important things we made during the glycolysis process was two molecules of pyruvate, which each have three carbons. We're now going to add that pyruvate to something called Co-enzyme A, and also to two more molecules of energy poor NAD+. A co-enzyme also stores energy and its bonds it can help enzymes to work more effectively. When we mix all of these things together we're going to get new molecules, but look out for how we keep the same number of carbons. Once all of these molecules have been rearranged, we come up with two molecules of something called Acetyl Co-enzyme A or "Acetyl Co A", which each have two carbons. We also generate two molecules of carbon dioxide, which each have one carbon, and we generate two molecules of NADH which again is packed with potential energy. So, we haven't lost any carbons during this process. In the first place with pyruvate, we had two sets of three carbons and then when we rearrange them, we had two sets of two and two sets of one, again for a total of six. We've also now discovered where some of the carbon dioxide comes from during aerobic respiration. We'll be making more later on but here is where part of it comes from. The last thing that we're going to do before we end the screencast is we're going to tally up the score of how many of each kind of molecule we've created. At this point in respiration, so far we made a total of two molecules of ATP. (We actually made for but remember we spent two of them to get Glycolysis going). We made four molecules of NADH, and we haven't made any FADH2 yet. So, that's where we're at. Next time, we'll talk about the Kreb's cycle and about the electron transport chain which is where we really make a lot of ATP.