hi there okay so this is section two of cellular respiration which is key area seven of unit one of higher human biology and we're gonna be covering the citric acid cycle or as i like to call it the wheel of evil it's not actually that bad it's just once you see the full diagram of it it does look a bit intimidating but overall in terms of what you need to be able to say about it it's not it's not too bad i think uh right so the citric acid cycle is the second stage of aerobic respiration it will only occur if the cell has a supply of oxygen now you already know that from national five the idea is it's moving into the aerobic respiration territory now in the mitochondria now new thing it occurs in the matrix of the mitochondria now we can see we've got a little diagram here you do not need to be able to identify the matrix from a picture of it okay but the idea is we've got our mitochondria and it's this area here now it's only acceptable to say matrix of mitochondria in exams so if they ask you where does this stage occur you have to say matrix of mitochondria only saying mitochondria is not enough detail now the stage is also controlled by enzymes which is nothing new because everything in the world is controlled by enzymes the government is probably controlled by enzymes okay so only if the cell has oxygen it's in the matrix and mitochondria and it's controlled by enzymes and basically it's the first part of aerobic respiration if we're moving away from that first stage that started in the cytoplasm now this is going to end up being the diagram of the citric acid cycle here there are a few compounds in here that you must know the names of and you need to be able to explain what is happening at most stages of this um it's actually once you learn it it's quite nice for an essay question if the the extended response questions come up those 10 mark things at the end but a lot of people do struggle with this so i'm going to try and focus on what the basic things that might crop up to learn are and then we'll get into more complicated territory afterwards so you've got your pyruvate from stage 1 glycolysis okay now the idea is i've got 3c written under pyruvate that's talking about how many carbons there are it actually really helps you to track the number of carbons in these molecules because it kind of starts to make a lot of sense um so the idea is pyruvate is converted into a compound called acetyl okay now acetyl is a two carbon compound so if we've lost that carbon the idea is we've lost it in the form of co2 anytime we're losing carbon we don't just lose carbon by itself we lose it as co2 okay now you notice it's not just called acetyl on that diagram it's called acetyl coa that's because acetyl itself can't exist all by itself so it's picked up by uh an enzyme called basically coenzyme a so acetyl coa means coenzyme a has picked up the acetyl okay now coenzyme a drops the acetyl off in the citric acid cycle okay and what happens here is acetyl is going to combine with a compound called oxaloacetate i've gone too far there okay it's going to combine with a compound called oxaloacetate now a lot of people hate the spelling of this word they hate it but it's a four carbon compound now the idea is we've got a four carbon compound combining with two carbons of acetyl we end up with a six carbon compound called citrate common exam question is um what combines with acetyl to create citrate and the idea is you have to say oxaloacetate or more alternatively what happens to acetyl when it enters the citric acid cycle you have to say it combines with oxaloacetate to form citrate okay so first two important names of things oxaloacetate is the four carbon compound citrate is the six carbon compound okay sometimes in uh internet resources you might see this referred to as citric acid just like you cannot call pyruvate pyruvic acid you must call citrate citrate you can't call it citric acid okay now the idea behind this wheel of evil here is citrate is going to get broken down it's going to start losing its carbons and remember if it's losing carbons it loses them as carbon dioxide so we go from six carbon citrate to a five carbon compound and the idea is we lose more co2 and we are end up with a four carbon compound then we don't lose any co2 and we stay with a four carbon compound and then something happens and it's converted back to oxaloacetate so names of compounds in this that you need to know are two sorry three let's go for three we need to know the name of acetyl and the fact that acetyl coa is acetyl plus coenzyme a okay you need to know the name citrate and you need to know the name of oxaloacetate all these others the five the four or the four you don't need to know anything about them okay you just need to know that they exist you don't need to know that they're five carbons you don't need to know that they're four carbons i have only put that information there because it might help it make a bit of sense to you okay so we've lost our carbon dioxide here we've lost our carbon dioxide here no more carbon dioxide losses occur okay as we're going through this process as it goes through the wheel of evil the idea is we get uh dehydrogenase enzymes doing their jobs now dehydrogenase enzymes the idea is they are cleaving hydrogen and their electrons off of these compounds and passing them to nad to create nadh so we get nadh being formed and on top of that if i just duck my little diagram down we get a little amount of atp formed as a result of these breakdowns the idea is between five and the four carbon compound we have a little bit there of a catabolic reaction occurring the idea of a breakdown from five to four and the creation of atp okay so this diagram it's horrific to look at but in terms of things you need to be able to say right so we've gone from a five carbon to a four carbon which means we've lost a carbon in the form of carbon dioxide that's not the hardest thing to work out nad is being turned into nadh in both stages one and two so hopefully you can just remember the presence of that and then we've got our day of tiny bit of atp oxaloacetate and citrate so i'd say there's quite a lot to learn there but there's also you know it's not too much it's not overwhelming amounts to learn for the citric acid cycle okay but this is everything that's there so i'm going to summarize again very very quickly the idea pyruvate converted to acetyl picked up by coenzyme a which carries it to the citric acid cycle we've lost one carbon dioxide okay acetyl combines with oxaloacetate to form citrate citrate is broken down through a series of reactions to create carbon dioxide nadh and a small amount of atp that's the summary and there you go it's in words i didn't know that side was coming okay so if you are asked in an exam describe the citric acid cycle and it's worth five marks or ten marks or something like that these are the words that you want to be using so okay so pyruvate converted to acetyl carried by coenzyme a to the citric acid cycle acetyl combines with oxaloacetate to form citrate a series of enzyme-controlled reactions breaks down citrate releasing carbon dioxide dehydrogenase enzymes remove hydrogen during the cycle pass hydrogen ions and electrons to nad to create nadh and a small amount of atp is created five facts i think it is possible to learn these five facts about this process okay okay so to summarize pyruvate loses one co2 to become an acetyl group acetyl combines with coenzyme a to form acetyl coenzyme a oxaloacetate combines with acetyl to form citrate citrate is broken down in the citric acid cycle to oxaloacetate over the chain of reactions the wheel of evil carbon dioxide is released and atp is made nad collects hydrogen ions and their electrons to form nadh the only thing that says missing from there is dehydrogenase enzymes because they are of course doing the cutting off of the hydrogen ions and their electrons so that's the citric acid cycle if you couldn't really get that this slide is really going to help you i'd get made up as flash cards and things like that because i actually think that this slide is enormously useful in just stating the rules of the main things that you need to know okay i think you did pretty well if you could say this was an essay question again saying describe the citric acid cycle you did pretty well if you could just recite the things that are on here so say you've got five flashcards together and just made them up and then just remembered your flash cards in the exam you'd probably be good to go so the next stage is the electron transport chain and believe it or not but people hate the next stage more than they hate this one so yeah that's looking forward to that