hi everyone and welcome to learn a level biology for free with miss a stroke in this video we're going through the link reaction and the Krebs cycle so these two are part of aerobic respiration glycolysis is actually the first stage of aerobic respiration and that's an anaerobic and aerobic and I'll link just up here the previous video which explains this process but this one is just looking at these two middle stages which happen in the mitochondrial matrix so the first thing is to the products from glycolysis which we can see a summary of hair have to be actively transported from the cytoplasm which is where glycolysis occurs into the mitochondrial matrix and that is pyruvate and reduced nad or NADH now NADH won't be used until the final step oxidative phosphorylation but pyruvate is used immediately in the Linc reaction so let's have a look at the link reaction at first the key things then are the pyruvate which was made by colossus is oxidized into acetate and in doing that the hydrogen which is lost from pyruvate to oxidize it is picked up by the coenzyme nad and reduces it to form NADH pyruvate is also a three carbon molecule acetate is only two carbons and this is how carbon dioxide or one of the stages where carbon dioxide is formed so the last stage in the link reaction is how acetate is converted into acetyl coenzyme a and that's simply by acetate combining with a coenzyme called coenzyme a and the purpose of this step is so acetyl co a can enter the next stage which is the Krebs cycle but just summarize then the three things that are made in this stage are acetyl coenzyme a comm dioxide reduce coenzyme and reduced nad you don't have any a teepee be made a ttle and from one round of the link reaction you only get one of each of these molecules but for one glucose molecule because you would create two pyruvates the link reaction would happen twice so that's why if you asked what would the products be for every glucose molecule it's two times each of those molecules so then the Krebs cycle I'm going to be looking at how the acetyl coenzyme a enters the cycle so the first step is the acetyl coenzyme a is reacting with a four carbon molecule to create a six carbon molecule because we're adding four and two and in doing that the coenzyme a whose purpose was purely to help bring the acetate into the cycle that coenzyme a is then released and it can be reused in the link reaction so the next stage is simplified quite a lot at a level and you just need to notice a series of redox reactions bringing that six carbon molecule to form a four carbon molecule so first of all if we're going from six to four we must be losing carbon and that is in the form of two carbon dioxide this stage produces ATP and then the redox reactions is linking to these reduced coenzymes so nad is reduced to form reduced nad and we have FA D forming reduced FA D and when these coenzymes become reduced they have picked up a hydrogen atom and therefore a proton and an electron so they are gaining those electrons from the six carbon molecule so because they are gaining electrons they are reduced and because the carbon molecules are losing electrons those are oxidized so just looking at the product zone of the Krebs cycle for one round of this cycle we have three reduced nad one reduced F ad one ATP and two carbon dioxide molecules however if you asked how many of these products would you have glucose because we have two pyruvate we have to acetyl coenzyme a so the cycle happens twice so we multiply all of those by two and this is our subtotal then of all the products in this stage at the moment so just to recap this top bit is the minke reaction and that happens in the mitochondrial matrix this lower part is the Krebs cycle also known as the citric acid cycle and this is also happening in the mitochondrial matrix so I hope you have found that helpful today if you have please give it a thumbs up and if you are new here hit the subscribe button to keep up-to-date on all of the latest videos [Music]