the first step of cell respiration is glycolysis glycolysis is basically the transformation of glucose into two pyate molecules now if there's oxygen present in the cell then the cell can undergo a reaction known as aerobic cellular respiration in which the two pyruvate molecules formed in the cytoplasm of the cell via the process of glycolysis are then transported into the mitochondria Matrix of the mitochondria of that cell and once within the mitochondrial Matrix the two pyate molecules are transformed into two acetal coenzyme a molecules via process known as pyruvate decarbox now once we form the two acetal co-enzyme a molecules in the mitochondrial Matrix they will then enter a cycle known as the citric acid cycle also Al known as the crep cycle also known as the tricarboxylic acid cycle or simply TCA now the citric acid cycle is basically a series of eight steps a series of eight reactions as shown in the following diagram we have 1 2 3 4 5 6 7 and 8 now the entire purpose of the citric acid cycle is to basically take the energy that is stored within our C to coenzyme a and transfer that energy in the form of high energy electrons to our electron carrying molecules our nadh and fadh2 so the citric acid cycle doesn't actually directly form ATP molecules what it actually does is it forms these nadh and fadh two molecules the high energy electron carriers that will will then be used by the electron transport chain to synthesize the ATP molecules now the way that we described our citric acid cycle on the diagram is for a single acetyl coenzyme a but because glucose produces two acety coenzyme a ultimately that means that this cycle will actually take place twice for any single glucose molec so a single glucose molecule breaks down into two pyruvates and since and since each pyruvate creates that acetyl coenzyme a we have two of these acetyl coenzymes a so this citric acid cycle takes place twice for any given glucose molecule and this citric acid cycle takes place within the mitochondrial Matrix of the mitochondri found within the cell so let's take a look at each one of these eight steps and we're not going to focus too much on the detail because each one of these steps is actually pretty complicated it contains many enzymes and many intermediates so we're going to Simply paint the general picture of this cycle let's begin with step number one now in Step number one we take our acety coenzyme a and we mix it with a four carbon molecule known as oxaloacetate so the oxal acetate is a four carbon molecule that when we mix it with acetyl coenzyme a we form a molecule known as cyal coenzyme a and then that cital co-enzyme a reacts with an H2O molecule to release the co-enzyme A co-actor and to produce a sixc carbon citrate molecule now the fact that this contains six carbons makes sense because our oxaloacetate contained four carbons and the acetyl portion of the acetyl coenzyme contains two carbons so we when we combine these we form the six carbon citrate molecule now the question is what exactly is the point of forming our citrate molecule well basically the reason we form a six carbon molecule is because this citric acid cycle involves two decarbox processes and so we have to add two carbon atoms and that's exactly what we do in citrate so step one Oxo oxaloacetate and acetyl coenzyme a react to form cital coenzyme a which then reacts with water to form the citrate the purpose of this step is to create a molecule that will ultimately undergo the decarbox reactions now let's move on to step number two in Step number two we see that the citrate cannot actually undergo the decarbox directly because the hydroxy group on cital is not positioned on the correct location and so what we do is we use a special type of enzyme that transforms the citrate into isocitrate which basically is an isomer of this molecule so now the isocitrate is able to undergo the decarbox ation reaction so basically I group step one and step two into the same stage let's call that stage one because these two steps involve preparing our molecule for decarbox so let's call these two steps stage one now let's move on to stage two which basically involves step three step four and step five in these steps as we'll see in just a moment we have the carboxy reactions taking place and we also form the only molecule that has the high energy phosphate group our GTP in this stage so let's begin with step three so in step three once we form our six carbon isoc citrate that undergoes a decarbox reaction that means we release our carbon dioxide in step three and we also use our nad+ and we reduce it into the nadh and we also form an H+ ion now we also form because we lose a single carbon that leaves as the carbon dioxide we go from a six carbon molecule to a five carbon molecule known as Alpha keto glutarate now once we form the alpha ketoglutarate that further under goes a decar carboxy reaction so we release yet another carbon dioxide so that means we go from a five carbon to a four a four carbon molecule known as suil co-enzyme a now the co-enzyme a released in Step One reacts with this molecule and goes into the process that's why we have the sual co-enzyme A and we also once again reduce an NAD into our nadh we also release the H+ ion as in Step number three now let's move on to step number five in Step number five we basically transfer a phosphate group from the suil coenzyme a onto the GDP where GDP is basically guanosine diphosphate and we form our GTP Guan guanosine triphosphate so we see that this is the only only step in the citric acid cycle in which we actually produce a high phosphor transfer potential molecule basically our GTP and that GTP is ultimately transformed into an ATP molecule via an enzymatic reaction so I label these three steps as stage number two so stage number one is the preparation step we create that m Ule that is capable of undergoing the two decarbox reactions in step in stage two we basically have the two decarbox reactions and we also have the only reaction that produces the hyos foral transfer potential molecule the GTP now we also use the phosphate to form that GTP and we basically released the co-enzyme a that went into the reaction in Step number four now stage number three are basically steps 6 7 and 8 and the entire purpose of these steps is to basically regenerate our molecule the oxaloacetate the four carbon molecule that we'll need to use again to basically undergo the reaction this process the cycle a second time because we have that second acetal coenzyme a so the final three steps 6 seven and 8 of the citric cycle involve the Regeneration of our oxaloacetate molecule the question is how exactly does this take place so let's begin with our suade the salinate looks something like this we have the carbon 1 2 3 4 and these two H atoms shown in purple will basically react with a single a um fad molecule and that fad molecule will be reduced it will take these two H atoms as well as a single electron from each one of these atoms which will end up on this molecule known as our fadh2 high energy electron carrier and once these two leave we form a pi bond between carbon one and carbon 2 as shown and this is known as the fumerate molecule next we take the fum we basically input a water molecule and that water molecule basically goes onto these two carbon so an H goes onto this carbon and then the hydroxy goes onto this carbon as shown and this is our malate molecule so malate is shown here and finally to form the final product to regenerate our oxaloacetate we have an nad+ that is reduced so it Tak takes our H which basically is this H here once we take that H we form our nadh and we also remove this H here to basically form a pie bond between the oxygen and our carbon and this is the final molecule the four carbon oxaloacetate so in the process of regenerating our oxaloacetate in steps six 7 and 8 we also actually produce the high energy electron carrying molecules we produce a single fadh2 in Step six and a single nadh in Step eight and we also use a water molecule in Step number seven now this step this Cycle takes place twice because we have two acetyl coenzyme a molecules coming from a single glucose and that means the final product will be all these products multiplied by two so the final products of the citric the citric acid cycle when one glucose produces the two ACL co-enzyme a molecules are four carbon dioxides so one two that's two multiplied by two so four we have six nadh so we have one nadh formed here one formed here and one formed here that's 3 multiplied by two we have two fad dh's so we have one coming in Step six multiplied by two so we have two of those we have two GTP one formed in our uh step number six uh we have four H+ ions so we have one two we multiply that by two we form four and we also have our two co-enzyme a molecules so these are the final products of our citric acid cycle when basically we have have one glucose that produces two acetyl coenzymes a that go into this cycle inside the mitochondrial Matrix so once again the entire purpose of the citric acid cycle is not to actually directly synthesize our ATP molecules but it's to transfer the energy from the Cil coenzyme a in the form of the high energy electron so we produce these electron carriers that are then used by the electron transport chain which we'll discuss in the next lecture to synthesize our ATP molecules