12-7 Krebs Cycle (Cambridge AS A Level Biology, 9700)

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And of course after the link reaction we have to talk about one of the most complicated processes in aerobic respiration this is the well it’s not the final part of aerobic respiration but this is the part where the organic molecule is going to be completely broken down remember we started off with

Glucose molecule and then after glycolysis it was broken down into pyro and then from pyro it enters the link reaction and it’s broken down slightly further to become acety and then the acety is added with the co-enzyme a to become actil COA uh and now the aetl COA

Can enter the next stage which is known as the clap cycle now for clap cycle what you have to know is it happens in the mitochondrial Matrix or the Matrix of the mitochondrian and the steps over here are as follows so you see this is a memorizing chapter so I’m just going to

Dive right into it as you can see the aetl group which which is a two carbon molecule has to be first added with another four carbon molecule in The Matrix and that four carbon molecule is called oxaloacetate don’t worry we’re going to look at that in detail the next step is

Decarbox and dehydrogenation which is oxidation the third step is also the same thing decarbox and dehydrogenation again the fourth step is substrate link reaction the fifth and sixth step are dehydrogenation or oxidation twice okay so these are the steps in the crap cycle that you should

Be aware of and no matter what when you start off with that oxaloacetate which is a four carbon molecule you must end up with that oxaloacetate again now what do I mean by that so let’s look at this so you see you have the aetl COA over

Here which was from the link reaction let’s just look at one of the ACL COA at first as you can see the aetl COA is made up of two carbon and you also see the oxaloacetate molecule which is a four carbon molecule over there like I

Said because it’s a cycle if you use up the oxaloacetate you need to regenerate that oxaloacetate no matter what that is the clux of the kpse cycle so the first step is acle group is joined together with oxyacetate notice I did not say acety COA joins with oxaloacetate I said

The acety group only because the coenzyme a has now done its job and it detaches from the aetl group because remember I told you earlier the function of the co-enzyme a is just to transport the aetl group into the crap cycle now once actil is added together with oxaloacetate it becomes something

Known as citrate you must know the name of that six carbon molecule citate they can ask that in paper for by the way they they may just put like a oxaloacetate is added together with acety group it forms a six carbon molecule they may ask you name the six

Carbon molecule and you must remember the name of the six carbon molecule which is citrate all right now remember the second step decarbox silation and dehydrogenation what is decarbox silation decarbox relation is just removal of carbon in the form of carbon dioxide dehydrogenation is oxidation and the hydrogen is removed and accept by

NAD to become reduced NAD what happens to the citrate molecule you must know that from six carbon it becomes five carbon now do you have to remember the name of the five carbon molecule no you do not have to uh some of my students come crying to class well not crying but

You know they they had that look of Despair on their faces they go oh my God I heard the word cinate fumerate malate uh Alpha ketoglutarate do I need to know all the names of that molecule in the CP cycle no you do not have to and that

Five carbon molecule under goes the third step which is decarbox silation and dehydrogenation again so carbon dioxide is removed hydrogen is removed accepted by Ned which becomes reduced Ned and now it becomes a four carbon molecule never lose sight we are slowly breaking down that aetl group by the way

All right and then after that step four substrate link reaction where a little bit of energy is released and ADP and phosphate is synthesized into ATP all right and then step five dehydrogenation step six dehydrogenation or oxidation the hydrogen in this case okay here’s where you have to be a bit careful uh

One of the hydrogen in step five will be accepted not by NAD but it’s accepted by another type of carrier called fad uh some of my students will ask me why is it fad don’t ask okay just sometimes you just got to accept it for

What it is okay we are not going to go into the detail of that so you just have to know that it’s step five uh fad accepts the hydrogen and becomes reduced fad step six another NAD molecule will accept the hydrogen to become reduced any now remember CP cycle involves two

Acety COA so every product will be two as I’ve put that so in summary for the crap cycle all you have to know is as follows the oxaloacetate is joined together with acety group it becomes citrate and the citrate over here under goes decarbox and dehydrogenation or oxidation and it

Forms two carbon dioxide molecules and two reduced NAD the reason why it’s two it’s because we started out with two atil COA again decarbox and dehydrogenation happens again to carbon dioxide to to reduce nid substrate link reaction which produces 2 ATP step five and step six which is dehydrogenation

Which produces two reduced NAD and two reduced fads um now so in total in the clap cycle when the acety group is completely broken down it produces four carbon dioxide molecules a six reduced Neds two reduced feds and two ATP molecules and this is the summary of the

Crap cycle that you have to know