in this video we're going to give a brief overview of cellular respiration specifically the parts of the cellular respiration pathways that don't directly use oxygen So glycolysis is the first step and in this first step glyolysis Glyco for sugar lis for break Let me uh correctly write that down for you So glyco for sugar and specifically that sugar is going to be glucose and then lis to break So basically we're taking a glucose molecule which has six carbons in it and we're going to break it down into two molecules that have three carbons each So we're not losing any carbon And in this pathway we are generating ATP but very inefficiently through substrate level phosphorilation Once we finish and produce those pyrovates there's actually what we call a transitional stage in between glycolysis and the next cycle called the citric acid cycle So citric acid cycle goes by several names Tricirylic acid cycle also KB cycle I'll try to use citric acid or KB cycle uh predominantly So in this process we're taking this transitional phase product which was pyrovate which had three carbons being converted into a twocarbon molecule called an acetil or acetate And that acetil group was associated with a co-enzyme So it is this acetal COA that goes into the citric acid cycle And again you have substrate level phosphorilation So again it's not very efficient We're not making a whole lot of ATP We're going through lots of chemical reactions The differences here is glycolysis is occurring in the cytool while in the citric acid cycle it's occurring within the matrix of the mitochondria So that's the sort of liquid part in here of the mitochondria Now in proaryotic cells for those of you that are planning on taking microbiology both of these reactions are occurring in the cytool Our last series of reactions are known as oxidative phosphorilation and this is where we will repurpose energy into many different forms and we'll talk about this in the next video But there are two major steps here The first step is called the electron transport chain And don't be fooled You're not transporting electrons You're using the energy in those high energy electrons to do transport work The second stage is sort of the payout phase and that's called kiosmosis And this is where we make ATP through this process called oxidative phosphorilation So we're adding phosphate groups from the cytool onto ADP and the energy is coming from oxidation reactions from these reactions where we've broken down chemical bonds Now when we specifically look at glycolysis glycolysis has 10 chemical reactions And I'm not going to hold you responsible for those 10 chemical reactions I want you to know what goes in So what are the reactants and what comes out of the reaction that is the products So a single glucose molecule is going to go in and we're going to go through a series of reactions and we ultimately produce two pyrovate We're going to generate a net of 2 ATP And you'll see in a moment when I dive a little bit deeper and break down glycolysis you know what is actually happening And you're going to generate two of those energy carriers those high energy electron carriers NADH Now on the reactant side I'm not writing out the ADP or the NAD+ Now the ATP the way that it is formed as I'd mentioned before is substrate level phosphorilation where the enzyme is going to take the phosphate group off of one molecule and it creates a new chemical bond on ADP to make ATP So this is your substrate level phosphorilation Now if we actually took a deeper dive into glycolysis there are three major phases in glycolysis and you'll see this in many textbooks The first is what we call the investment phase So there's an enzyme called hexocinase that you'll come across in your reading And what hexocinase does is it will put a phosphate group onto the glucose molecule So you get what's called glucose 6 phosphate And this initial investment so we're stripping a phosphate group from ATP kind of seems counterintuitive but the reason why the cell does this is is that if I have a cell and there's glucose molecules going into the cell I don't want equilibrium to ever be reached I want glucose to consistently go into the cell And so by converting oops by converting the glucose into glucose 6 phosphate it's a different solute So the gradient for glucose is always going to be less than in the cell higher outside So this ensures a continual influx of glucose Now there's a second enzyme that puts another phosphate group and rearranges glucose So you get what's called fructose 16 bis phosphate Now again these two names glucose 6 phosphate fructose 16 bis phosphate you're not responsible for I just want you to sort of understand the context that in the initial activation phase or what many textbooks will refer to as the investment phase you are actually using two ATP molecules So the second phase is known as the cleavage phase the sugar cleavage phase So this is where we break down that sixcarbon sugar into two molecules that have three carbons each And the two molecules that we break it down into are dihydro excuse me dihydroxyacetone phosphate and glyceraldahhide free phosphate And they are interchangeable However I want you to sort of remember this name not in greater detail only because we're going to be mentioning it a lot Glyceraldahhide 3 phosphate or oftentimes we'll just abbreviate it as G3P This is a very important molecule in case you go into uh botany uh you look at microbiology This is a building block that is generated during photosynthesis and basically you go backwards You do the reverse reactions to engineer glucose So very important step in photosynthesis It's also going to be a very important intermediate So I'm going to be using that term a lot An intermediate is a molecule in a series of reactions that's somewhere in the middle So in the case of glucose and pyrovate in glycolysis glucose was the reactant pyrovate was the product Glyceraldahhide 3 phosphate is a molecule in the middle So it's sort of in that transition Uh same thing with fructose 16 bis phosphate or glucose 6 phosphate Now the last phase of glycolysis is sort of the payout phase It's also known as the sugar oxidation or ATP formation phase So this is sort of the crux of glycolysis And in this process we're taking those high energy electrons that we generated by breaking chemical bonds and we're passing it to NADH excuse me passing it to NAD+ to form NADH So this is where those two NADH's is coming from This is also where we generate the two pyrovate Now again I will use the term pyrovate um and pyuvic acid There are interchangeable but I tend to prefer pyrovate Uh and you produce 4 ATP Now remember you used 2 ATP before So 4 minus 2 that's where the two net ATP comes around At this point we have our pyrovate in the cytool and we need to get ultimately the pyrovate into the mitochondrial matrix Now there are two possibilities and this depends upon whether or not oxygen is around The oxygen is not going to be part of the chemical reactions The dependence on oxygen we'll kind of highlight in the next video But what pyrovate if oxygen is around the pyrovate is going to go into the mitochondrial matrix it gets used up So you have a gradient of pyrovate which ensures the influx of pyrovate into the mitochondrial matrix In the case of an anorobic environment or an insufficient amount of oxygen the gradient reaches equilibrium So you're not transporting the pyrovate into the mitochondrial matrix anymore and it starts to accumulate in the cytool and that becomes very problematic because the way you uh should think about it is glucose you have a lot of glucose in the cell you don't have a whole lot of pyrovate because usually that pyrovate under aerobic conditions gets used up So think about it like a little seessaw where you have lots of glucose on one side and very little pyrovate on the other you're going to favor the forward reaction not so much the reverse But if you start to get an equilibrium you might start to favor the reverse reaction and therefore you're not really producing the two net ATP and this becomes problematic So you might shift over to an anorobic process like fermentation shown here And that fermentation is going to lower the amount of pyrovate that you have So it ensures glycolysis proceeds in the forward direction The other significance of this fermentation is notice what's happening to NADH NADH is being oxidized back to NAD+ so that it can be reused in glycolysis because NADH plus excuse me NAD NAD+ is in a limited quantity and so if all of it has been reduced then you don't have anything to use in glycolysis So what happens once we are in the matrix of the mitochondria so this is sort of the go between between glycolysis oops spelled correctly right So glycolysis and the citric acid cycle or the KB cycle So in between here this is not an official stage of uh cellular respiration Uh we call it by many names pyrovate metabolism Uh we also call it a transitional stage but basically we're processing the pyrovate and in that processing we're breaking one chemical bond And of course whenever we break a chemical bond that generates high energy electrons that are then going to be picked up by NAD+ So we're reducing NAD+ to NADH and we're oxidizing pyrovate into acetil groups These acetil groups are fused with a co-enzyme generating a molecule called acetal COA So for every pyrovate we generate one acetil coa we generate one NADH we don't generate any ATP here However notice that this carbon that we broke off when we break the chemical bond well that becomes carbon dioxide So this is the first instance that we generate carbon dioxide as a waste product Now we always want to normalize this to the original number of glucose that we started out with which was one and that one glucose gave rise to two pyrovates So we always multiply this by two so that everything is in proportion to the original glucose that we started out with So during this transitional stage we generate an additional two NADH's and two CO2s as a waste product and two acet acetil coas All right So next we move into the citric acid cycle Again a series of very chem uh complex chemical reactions Each reaction has its own enzyme We're not going to be holding you responsible for that What I want you to know however is acetal COA goes in and remember we have two of them but each acetil coa will go through the cycle once So in essence per glucose we are going through the cycle twice Now the first reaction you're combining the two carbons in the acetil group to a fourcarbon molecule called oxyloacetate This is where you generate citrate and why the pathway is named the citric acid cycle From this sixcarbon molecule we're progressively breaking it down therefore generating those high energy electrons and those high energy electrons are then picked up by NAD+ So that happens once twice and three times So per acetal COA we're producing three NADH's but remember we're multiplying that by two So it's a total of six NADH's that we are generating Now you'll also notice FAD This is an electron uh carrier that transports weaker electrons not as high energy So per cycle we actually produce one of those So a grand total of two of them Lastly we produce one ATP So normalizing it a total of 2 ATP are produced by this acetal COA entering the citric acid cycle Here you'll notice again that no oxygen is being used directly Okay you do produce CO2 once Uh where's the second time oh here we go Twice So you are producing uh two carbon dioxide So 2 * 2 a total of four carbon dioxides But each of these steps are going to be oxidation reduction reactions where we're repurposing the energy in those chemical bonds So why is it that we say this pathway is aerobic but no O2 is used in the reactions well we're going to be addressing that question in the next video but just to give you a teaser we need oxygen for oxidative phosphorilation And one of the reasons for that is to help recycle those co-enzymes If we fail to recycle the co-enzymes then all of these pathways come to a screeching halt Now if we just had glycolysis as an organism fermentation is useful but only up to a point because it's producing things like lactic acid and that leads to pH imbalances So you want to ensure a steady oxygen supply for the efficient recycling of these co-enzymes and ultimately the efficient generation of ATP through oxidative phosphorilation