If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:17:47

Video transcript

so we already know that if we start off with a glucose molecule we start off with a glucose molecule which is a six carbon molecule six carbon molecule that this essentially gets split in half by glycolysis and we end up with two pyruvic acids or two pyruvate molecules so glycolysis literally splits this in half it Lysa's the glucose and we end up with two pyruvates or pyruvic acids pyruvate and these are three carbon molecules three carbon molecules there's obviously a lot of other stuff going on in the carbons you saw in the past and you can look up their chemical structures on on the internet or Wikipedia and see them in detail but this is kind of important thing is that it was it was lysed it was cut in half and this is what happened in glycolysis glycolysis glycolysis and this happened in the absence of oxygen or not necessarily it can happen in within the presence or in the absence of oxygen it doesn't need oxygen and we got a net payoff of two ATP's two ATP's net and I always say the net there because you remember it used to ATP's in kind of that investment stage and then it generated four so it's on a net basis it generated four used to it gave us two ATP's and it also produced two nadh --is two na DHS that's what we got out of glycolysis and just so you can visualize this a little bit better let me draw a cell right here maybe I'll draw it down here let's say I have a cell that's its outer membrane maybe its nucleus we're dealing with the eukaryotic cell doesn't have to be the case it has this DNA and it's chromatin form all spread around like that and then you have some mitochondria and you know there's a reason why people call it the power centers of the cell we'll we'll look at that in a second so this is a mitochondria has an outer membrane an inner membrane just like that I'll do more detail on the structure of a mitochondria maybe later in this video or maybe I'll do a whole video on them that's another mitochondria right there and then all of this fluidic space out here that's between the organelles or in the organelles you kind of view them as parts of the cell that all right you know they do specific things they do specific things kind of like organs do specific things within our own bodies so this so between all of the organelles you have just this fluidic space this is just fluid of the cell and that's called a cytoplasm cytoplasm and that's where glycolysis occurs so the glycolysis occurs in the cytoplasm glycolysis now we all know I've in the overview video we we know what the next step is it's the Krebs cycle or the citric acid cycle and that actually takes place in the inner membrane there were the inner I shouldn't say the inner space the inner space of these mitochondria let me draw it a little bit bigger let me draw up mitochondria here so if this is a mitochondria it has an outer membrane it has an inner membrane if I have just one inner membrane we call it a crista if we have many we call them cristae these little convoluted inner membrane you give it a label I'll say there cristae plural and then it has two compartments right it's because it's divided by these two membranes this compartment right here is called the outer compartment this whole thing right there that's the outer compartment and then this inner compartment in here this inner compartment in here is called the matrix the inner compartment right there is called the matrix now you have these pyruvates they're not quite just ready for the Krebs cycle but I guess well it's a good it's a good intro into what how do you make them ready for the Krebs cycle they actually get oxidized and I'll just focus on one of these pyruvates we just have to remember that the pyruvate that this happens twice for every molecule of glucose so we have this kind of preparation step step four for the Krebs cycle we call that pyruvate oxidation pyruvate oxidation and essentially what it does is it Cleaves one of these carbons off of the pyruvate and so you end up with a two carbon compound you don't have just two car but this backbone of carbons is just two carbons called acetyl co a acetyl co a and if these names are confusing because you know what's acetyl coenzyme a these are very bizarre you can do a web search on it but I'm just going to use the words right now because it'll keep things simple and look at the big picture so it generates a siedel Kawai which is this two carbon compound it also it also reduces some NAD+ to NADH and this process right here is often a given credit or or the krebs cycle of the citric acid cycle gets credit for this step but it's really a preparation step for the krebs cycle now once you have this two carbon once you have this two carbon chain acetyl co a right here you are ready to jump into the Krebs cycle this long talked-about Krebs cycle and you'll see the second why it's called a cycle the acetyl co a and all of this is catalyzed by enzymes and enzymes are just proteins that bring together the constituent things that need to react in the right way so that they do react so catalyzed by enzymes this acetyl co a merges with some ox a low acetic acid very fancy word but this is a four carbon molecule this is a four carbon molecule right here these two guys are kind of reacted together or merge together depending on how you want to view it draw it like that it's all catalyzed by enzymes it's important you know some tests will say is this an enzyme catalyzed reaction yes everything in the Krebs cycle is an enzyme catalyzed reaction and they form citrate or citric acid citric acid which is the same stuff in your lemonade or your orange juice and this is a six carbon six carbon molecule which makes sense you have a two carbon and a four carbon you get a six carbon molecule and then the citric acid is then oxidized over a bunch of steps and on this is a huge simplification here but it's just oxidized over a bunch of steps together the carbons are cleaved off both two carbons are cleaved off of it to get back to a solo acetic acid and you might be saying hey what these carbons are cleaved off like you know when let's say this carbon is cleaved off what happens to it it gets it becomes co2 if it gets put on to some oxygen and leaves the system so this is where the oxygen or the carbons or the carbon dioxide actually gets formed and similarly when these carbons get cleaved off it forms co2 and actually for every molecule of glucose you have six carbons when you do this whole process once you're generating three molecules of carbon dioxide but you're going to do it twice you're gonna have six carbons six carbon dioxide's produce which accounts for all of the carbons right you get rid of three carbons for every turn of this well two for every turn but really for the steps after glycolysis you get rid of three carbons we're going to do it for each of the pyruvates you're going to get rid of all six carbons which we'll have to exhale eventually but this cycle it doesn't just generate carbons the whole idea is to generate nadh is and fadh2 s and ATP s so you know it'll be right here and this is a huge simplification I'll show you the detailed picture in a second will reduce some nad into NADH nad plus into NADH will do it again and of course these are in separate steps there's intermediate compounds I'll show you those in a second nad plus well another nad plus molecule will be reduced to na NADH maybe will produce some some ATP will turn into or some ADP will turn into ATP ADP turning into ATP maybe we have some and not maybe this is what happens so i'm faad gets let me write it this way some F ad gets oxidized into fadh2 and the whole reason why we even pay attention to these you know you might think hey cellular respiration is all about ATP why do we even pay attention to these nadh --is and these fadh2s that get produced as part of the process the reason why we care is that these are the inputs into the electron transport chain these get oxidized or they lose their hydrogens the electron transport chain and that's where the bulk of the TP is actually produced and then maybe we'll have another nad get reduced or gain and hydrogen right reduction is gaining an electron or gaining a hydrogen whose electron you can hog in a D H and then we end up back statics a low acetic acid and we can perform the whole citric acid cycle over again so now that we've written it all out let's account for what we have so depending on let me draw some dividing line so we know what's what so this right here this right here everything to the left of that line right there is glycolysis glycolysis we learned that already and then most especially introductory textbooks will give the krebs cycle credit for this pyruvate oxidation but that's really a preparatory stage the Krebs cycle is really formally this part where you start with acetyl co a you merge it with oxaloacetic acid and then you go and you form citric acid and you use which essentially gets oxidized and produces all of these things that will need to either directly produce ATP or you do it indirectly in the electron transport chain but let's account for everything that we have let's account for everything that we have so far we already accounted for the glycolysis right there to net ATP's to NADH is now in the citric acid cycle or in the Krebs cycle well first we have our pyruvate oxidation that produced one NADH one NADH but remember if we want to say what are we producing for every glucose this is what we produced for each of the pyruvates right this was from one a th from just this pyruvate but bike lot but glycolysis produced two pyruvates so everything after this we're going to multiply we're going to multiply by two for every molecule of glucose so I'll say four so the pyruvate oxidation times two means that we got two NADH s 2 n a DHS right there and then when we look at this side one of the formal Krebs cycle what do we get we have how many NADH is one two three NADH is so three na dhih H's times two because we're going to perform this cycle for each of the pyruvates produced from glycolysis so that gives us six na DHS we have one ATP per turn of the cycle this is going to happen twice once for each pyruvic acid so we get two ATP's and then we have one fadh2 but this group we're going to do this cycle twice this is per cycle so times two we have to F a DHS now sometimes in a lot of books these two NADH is or per turn of the Krebs cycle or per pyruvate this one NADH they'll give credit to the Krebs cycle for that so sometimes instead of having this intermediate step they'll just write for NADH is right here and you're going to do it twice once for each pyruvate so they'll say eight NADH s get produced from the Krebs cycle but the reality is six from the Krebs cycle two from the preparatory stage now the interesting thing is we can now account whether we get to the 38 ATP's promised by cellular respiration we've directly already produced for every molecule of glucose two ATP's and then two more ATP s so we have four ATP's four ATP's how many NADH is do we have 2 4 and then 4 plus 6 is 10 we have 10 10 NADH is and then we have to fadh2 fadh2s I think in the first video on cellular respiration I said F ADH it should be fadh2 just to be particular about things and these so you might say hey you know where's our 38 ATP's we only have four ATP's right now but these are actually the inputs in the electron transport chain these molecules right here get oxidized in the electron transport chain every NADH in the electron transport chain produces three ATP's so these 10 NADH s are going to produce 30 ATP's in the electron transport chain and each fadh2 when it gets oxidized and gets turned back into F ad in the electron transport chain will produce two ATP so two of them are going to produce four ATP's in the electron transport chain so we now see we get four from just what we've done so far glycolysis the preparatory stage and the Krebs or citric acid cycle and then eventually these outputs from glycolysis and the citric acid cycle when they get into the electron transport chain are going to produce another 34 so 34 plus 4 it does get us to the promised 38 ATP that you would expect that you would expect in a super efficient cell this kind of theoretical maximum in most cells they really don't get quite there but this is a good number to know if you're going to take the AP bio test or in most introductory biology courses there's one other point I want to make here everything we've talked about so far this is carbohydrate metabolism or sugar catabolism we could call it we're breaking down sugars to produce ATP glucose was our starting point but animals including us we can we can catabolized other things we can catabolized proteins we can catabolized proteins we can catabolized we can catabolized fats if you have any fat on your body you have energy you your in theory your body should be able to take that fat and you should be able to do things with that you should be able to generate ATP and the interesting thing the reason why bring it out up here is obviously glycolysis is of no use to these things although fats can be turned into into glucose in the liver but what the interesting thing is that the the Krebs cycle is the entry point for these other catabolic mechanisms proteins can be broken down into amino acids which can be broken down into acetyl co a fats can be turned into into glucose which actually could then go the whole cellular respiration but the big picture here is acetyl co a is the general catabolic kind of intermediary that can then enter the Krebs cycle and generate ATP regardless of whether our fuel is carbohydrates sugars proteins or fats now we have a good sense of how everything works out right now I think now I'm going to show you kind of a diagram that you might see in your in your biology textbook or the I'll actually show the actual diagram from Wikipedia I just want to show you this looks very daunting and very confusing and I think that's why many of us have trouble with cellular respiration initially because there's just so much information it's hard to process what's important but I want to just highlight the important steps here just so you see it's the same thing that we talked about from glycolysis you produce two pyruvates that's the pyruvate right there they actually show its molecular structure this is the pyruvate oxidation step that i talked about the preparatory step NEC we produce a carbon dioxide and we reduce an nad plus into NADH then we're ready to enter the Krebs cycle the acetyl co a and the oxaloacetate or oxaloacetate acid they are reacted together to perform to create citric acid they've actually drawn the molecule there and then the citric acid is oxidized through the Krebs cycle right there this all of these steps each of these steps are facilitated by enzymes and it gets oxidized but I want to highlight the interesting parts here we have an nad get reduced to NADH we have another nad get reduced to NADH and then over here another nad get reduced to NADH so so far if you include the preparatory step we've have two for NADH s formed three directly from the Krebs cycle that's just what I told you now we have in this diagram they say GDP or GTP gets formed from GDP the GTP is just guanosine triphosphate it's another purine that can that can be a source of energy but then that later can be used to form an ATP so this is just the way they happen to draw it but this is the actual ATP that I drew in the diagram on the top and then they have this Q group and I won't go into it and then it gets reduced over here it gets those two hydrogen's but that essentially ends up reducing the fadh2 s so this is where the fa d fadh2 gets produced so as promised we produced for each pyruvate that input it remembers we're going to do it twice for each part of it we produced one two three or NADH s we produced one ATP and one fadh2 and that's exactly what we saw what we saw up here I'll see in the next video
Biology is brought to you with support from the Amgen Foundation