continuing from the last video we're going to go to the next stage of cellular respiration um at least what the next stage looks like when we are in the presence of oxygen so this would all be considered aerobic respiration that aerobic um term refers to oxygen presence we have our pyruvate molecules left over from glycolysis and these pyruvate molecules um we we'll focus on one time so here's our pyruvate molecule first thing that's going to happen is this carboxy group is going to get removed from the pyate as a result pyruvate is able to make its way past the outer uh mitochondrial membrane and then with the help of a pyruvate carrier protein makes it across the mitochondrial inner membrane um that carboxy group is those pieces are used to make a carbon dioxide molecule and because we are undergoing an exergonic reaction here right we're breaking this Bond we're creating more disorder that energy is used for the endergonic reaction of reducing an NAD plus to make nadh and then that um what's left over of that pyruvate um this carbon attached to the methyl group and double bond to oxygen that's called an acetal group and this acetyl group is going to bind a structure called co-enzyme a forming our uh acetal COA structure and so remember this is going to happen twice so one for each pyruvate molecule so what we end up with is um two molecules of carbon dioxide uh two molecules of acetal cha and then two molecules of nadh so we get one of those from each pyu at and two pyruvates so you multiply all those by two two carbon dioxides two acetyl coas and two nadhs those acetal COA molecules then are what kicks off our next phase of aerobic respiration which is the citric acid cycle sometimes called the Krebs cycle um here's another big uh eight step reaction uh we start with acetal COA that co-enzyme a is removed um and we create a a six carbon molecule called citrate by binding this this um acetal Group after this acetal COA is removed to this fourar carbon molecule oxaloacetate and so for this citric acid cycle it's important to keep in mind how many carbons we're dealing with because each each time we remove a carbon that's an exergonic reaction and and that is used to the energy released from that exonic reaction is used to fuel an endergonic reaction which creates um a uh a um higher potential energy molecule so citrate six carbon molecule um under go a reaction to form an intermediate that also has six carbons this six carbon molecule uh gets oxidized so we end up with a five carbon molecule the carbon that we lost is used to form carbon dioxide and the energy from this exergonic reaction is used to take an nad+ and reduce it to become an nadh same thing happens in this next step this five carbon molecule becomes a four carbon molecule we release carbon dioxide we reduce nad+ uh and then that creates nadh then we have another exonic step here in step five um and here uh energy is released and the energy is going to fuel an endergonic reaction but here we're not making um nadh instead we are taking a GDP molecule and same structure except instead of an Adine base it's a guanine base that's where the G comes from so we create some GTP this is an example of a substrate level phosphorilation then um this four carbon molecule is going to get reduced fur it's going to get excuse me oxidized further in Step six this oxidation reaction is used to fuel the reduction of another electron carrier called fad D fad is very similar to nad+ and it gets reduced to become fadh2 which is very similar to nadh so it's another electron carrier and then in Step eight another exergonic reaction uh or another um uh oxidation of a four carbon molecule is going to lead to the reduction of nad+ to make more nadh this reaction uh regenerates the oxyacetate which can then be used to b a new acetal group and start the cycle all the way over again so for each molecule of glucose we get two acetyl COA molecules which means that we get two cycles of the citric acid cycle so as a result we produce two G GTP which can be like ATP we produce six in total nadh's two fadh2s and um six or excuse me four carbon dioxide molecules so these numbers here are for a single acetyl COA for each single acetyl COA you get one GTP 3 nadh 1 fadh2 two carbon dioxide and so if you want to know the total that we get from a single molecule of glucose you would double all these 2 GTP 6 nadh 2 fadh2 and four carbon dioxides once we move on from the citric acid cycle we kind of stop tracking the you know what's left over from the glucose and we now turn our attention to what those electron carriers we just produced are going to do and and this is where those electron carriers right our cells expending all that energy to reduce all of these NAD pluses and fad molecules to make uh more higher potential energy molecules this is where all of that's going to pay off this is the first step in oxidative phosphorilation which is electron transport there is a group of proteins embedded in the inner mitochondrial membrane called the electron transport chain and what I want you to think about for electron transport is electron transport is about setting up dominoes okay so here we are going to use our electron carriers that we produced nadh and that nadh is going to become oxidized it is going to lose an electron and that electron will get passed through these proteins in the electron transport chain when that electron passes through the electron transport chain we use we harness the energy from this exergonic reaction to move a hydrogen ion from the inner mitochondrial M Matrix to the intermembrane space between the outer membrane and the inner mitochondrial membrane we need energy to do this because we're moving hydrogen across its concentration gradient from low concentration to high concentration and so as the electron is passed through the electron transport chain um that energy is able to fuel the movement of multiple hydrogen ions against their concentration gradient so in this analogy of setting up dominoes these hydrogen ions are our dominoes we've set up a system where we have lots of molecules that want to move in a particular direction and then the next step in oxidative phosphorilation is about harnessing that potential energy we've created through this concentration gradient another thing I want to point out here is here is where we reveal Oxygen's role in here and this is why we need oxygen to live is because when nadh gets oxidized what we know about Redux reactions whenever something is oxidized something else needs to be uh reduced and in this case it's oxygen that is going to get reduced the oxygen will accept this electron become reduced and become water so this is where we produce our water and this is where our o oxygen is used up that's the whole reason why we need oxygen to live is so our cells can do this so This oxygen can be here just to accept that hydrogen excuse me accept that electron that was released as a as a result of nadh and fadh2 oxidation something needs to be reduced so oxygen steps up and does that so here's where we get into part two of oxidative phosphorilation which is chemi osmosis and so we just set up all of our dominoes we we created this concentration gradient of the hydrogen ion and during CH osmosis we're going to knock those dominoes down so all these hydrogen ions want to move from this space where there's lots of hydrogen ions to this base where there are fewer hydrogen ions the only path they really have to do that is through this membrane protein called ATP synthes what ATP synthes does is it kind of has two components there's a um a membrane embedded component that acts as a rotor so it essentially it can spin and so I guess it's spinning the opposite way in this image um so when the hydrogen ion binds to this ATP synthese it causes that rotor to spin or turn and as that rotor spins and turns that movement that kinetic energy is used to take our ADP molecules and phosphorate them to become ATP so all of this setup from um the from pyruvate oxidation all the way through the citric acid cycle into the electron transport chain all all of that was to set up this molecule ATP synthes to phosphorate ADP into ATP using the kinetic energy created by these hydrogen ions that are uh organized into a steep concentration gradient to move down back into the mitochondrial Matrix so it's really important that you understand these steps that I've highlighted and and sort of track how one step leads to the next and what the role of these major players are in electron transport and kind of how they serve ultimately this purpose of phosphor ADP to make ATP so a recap of each phase of cellular respiration we've talked about we start with one glucose molecule and oxygen and through glycolysis we net 2 ATP through substrate level phosphorilation and two nadh molecules two electron carriers PID oxidation generates two more nadh's and two carbon dioxides citric acid cycle we get two atps six nadh's two fadh2 so eight electron carriers and four carbon dioxide molecules through oxidative phosphorilation which is electron transport chain and chem osmosis combined we generate somewhere between 26 and 28 ATP molecules and six molecules of water due to uh oxygen being that final electron acceptor so in total we generate 30 to 32 molecules of ATP um 10 nadh's two fadh2s all 12 of these are used up during the electron transport chain and six molecules of carbon dioxide so each of these components of the original formula that we discussed are accounted for our glucose um which is broken down or oxidized our oxygen which is ultimately the thing that gets reduced um with to produce carbon dioxide and water respectively um so in the next video we're going to wrap up this modu ual by talking about what happens if there is no oxygen to be a final electron acceptor how does our cell continue to make energy when that occurs and ways in which our cell can kind of regulate this process to make sure that it's operating and producing energy at a really efficient level