so we already know that if westart off with a glucose molecule, which is a 6-carbonmolecule, that this essentially gets split in halfby glycolysis and we end up 2 pyruvic acids or twopyruvate molecules. so glycolysis literallysplits this in half.
crash diet uric acid, it lyses the glucose. we end up with two pyruvatesor pyruvic acids. ruby and these are 3-carbonmolecules. there's obviously alot of other stuff
going on in the carbons. you saw it in the past. andyou could look up their chemical structures on theinternet or on wikipedia and see them in detail. but this is kind of theimportant thing. is that it was lysed,it was cut in half. and this is what happenedin glycolysis. and this happened in theabsence of oxygen. or not necessarily.
it can happen in the presenceor in the absence of oxygen. it doesn't need oxygen. and we got a net payoffof two atps. and i always say the net there,because remember, it used two atps in that investmentstage, and then it generated four. so on a net basis, it generatedfour, used two, it gave us two atps. and it also producedtwo nadhs.
that's what we got outof glycolysis. and just so you can visualizethis 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'redealing with a eukaryotic cell. that doesn't haveto be the case.
it has its dna and its chromatinform all spread around like that. and then you havemitochondria. and there's a reason why peoplecall it the power centers of the cell. we'll look at thatin a second. so there's a mitochondria. it has an outer membraneand an inner membrane just like that.
i'll do more detail on thestructure of a mitochondria, maybe later in this videoor maybe i'll do a whole video on them. that's another mitochondriaright there. and then all of this fluid,this space out here that's between the organelles-- andthe organelles, you kind of view them as parts of the cellthat do specific things. kind of like organsdo specific things within our own bodies.
so this-- so between all of theorganelles you have this fluidic space. this is just fluidof the cell. and that's calledthe cytoplasm. and that's where glycolysisoccurs. so glycolysis occursin the cytoplasm. now we all know-- in theoverview video-- we know what the next step is. the krebs cycle, or thecitric acid cycle.
and that actually takes placein the inner membrane, or i should say the inner spaceof these mitochondria. let me draw it a littlebit bigger. let me draw a mitochondriahere. so this is a mitochondria. it has an outer membrane. it has an inner membrane. if i have just one innermembrane we call it a crista. if we have many, wecall them cristae.
this little convolutedinner membrane, let me give it a label. so they are cristae, plural. and then it has twocompartments. because it's divided bythese two membranes. this compartment right here iscalled the outer compartment. this whole thing right there,that's the outer compartment. and then this inner compartmentin here, is called the matrix.
now you have these pyruvates,they're not quite just ready for the krebs cycle, but iguess-- well that's a good intro into how do youmake them ready for the krebs cycle? they actually get oxidized. and i'll just focus on oneof these pyruvates. we just have to remember thatthe pyruvate, that this happens twice for everymolecule of glucose. so we have this kindof preparation step
for the krebs cycle. we call that pyruvateoxidation. and essentially what it doesis it cleaves one of these carbons off of the pyruvate. and so you end up witha 2-carbon compound. you don't have just two carbons,but its backbone of carbons is just two carbons. called acetyl-coa. and if these names areconfusing, because what is
acetyl coenzyme a? these are very bizarre. you could do a web search onthem but i'm just going to use the words right now, because itwill keep things simple and we'llget the big picture. so it generates acetyl-coa,which is this 2-carbon compound. and it also reduces somenad plus to nadh. and this process right here isoften given credit-- or the
krebs cycle or the citricacid cycle gets credit for this step. but it's really a preparationstep for the krebs cycle. now once you have this 2-carbonchain, acetyl-co-a right here. you are ready to jump intothe krebs cycle. this long talked-aboutkrebs cycle. and you'll see in a secondwhy it's called a cycle. acetyl-coa, and all of thisis catalyzed by enzymes.
and enzymes are just proteinsthat bring together the constituent things that need toreact in the right way so that they do react. so catalyzed by enzymes. this acetyl-coa merges withsome oxaloacetic acid. a very fancy word. but this is a 4-carbonmolecule. these two guys are kind ofreacted together, or merged together, depending on howyou want to view it.
i'll draw it like that. it's all catalyzed by enzymes. and this is important. some texts will say, is this anenzyme catalyzed reaction? yes. everything in the krebscycle is an enzyme catalyzed reaction. and they form citrate,or citric acid. which is the same stuffin your lemonade
or your orange juice. and this is a 6-carbonmolecule. which makes sense. you have a 2-carbonand a 4-carbon. you get a 6-carbon molecule. and then the citric acidis then oxidized over a bunch of steps. and this is a hugesimplification here. but it's just oxidized overa bunch of steps.
again, the carbonsare cleaved off. both 2-carbons are cleavedoff of it to get back to oxaloacetic acid. and you might be saying, whenthese carbons are cleaved off, like when this carbonis cleaved off, what happens to it? it becomes co2. it gets put onto some oxygenand leaves the system. so this is where the oxygen orthe carbons, or the carbon
dioxide actually gets formed. and similarly, when thesecarbons get cleaved off, it forms co2. and actually, for every moleculeof glucose you have six carbons. when you do this whole processonce, you are generating three molecules of carbon dioxide. but you're goingto do it twice. you're going to have six carbondioxides produced.
which accounts for allof the carbons. you get rid of three carbonsfor every turn of this. well, two for every turn. but really, for the steps afterglycolysis you get rid of three carbons. but you're going to do it foreach of the pyruvates. you're going to get rid of allsix carbons, which will have to exhale eventually. but this cycle, it doesn'tjust generate carbons.
the whole idea is to generatenadhs and fadh2s and atps. so we'll write that here. and this is a hugesimplification. i'll show you the detailedpicture in a second. we'll reduce some nadplus into nadh. we'll do it again. and of course, these arein separate steps. there's intermediatecompounds. i'll show you thosein a second.
another nad plus moleculewill be reduced to nadh. it will produce some atp. some adp will turn into atp. maybe we have some-- and notmaybe, this is what happens-- some fad gets-- let me writeit this way-- some fad gets oxidized into fadh2. and the whole reason why we evenpay attention to these, you might think, hey cellularrespiration is all about atp. why do we even pay attentionto these nadhs and these
fadh2s that get producedas part of the process? the reason why we care is thatthese are the inputs into the electron transport chain. these get oxidized, or they losetheir hydrogens in the electron transport chain, andthat's where the bulk of the atp is actually produced. and then maybe we'll haveanother nad get reduced, or gain in hydrogen. reduction is gainingan electron.
or gaining a hydrogen whoseelectron you can hog. nadh. and then we end up backat oxaloacetic acid. and we can perform the wholecitric acid cycle over again. so now that we've written itall out, let's account for what we have. so depending on--let me draw some dividing lines so we know what's what. so this right here, everythingto the left of that line right there is glycolysis.
we learned that already. and then most-- especiallyintroductory-- textbooks will give the krebs cycle credit forthis pyruvate oxidation, but that's really apreparatory stage. the krebs cycle is reallyformally this part where you start with acetyl-coa,you merge it with oxaloacetic acid. and then you go and you formcitric acid, which essentially gets oxidized and produces allof these things that will need
to either directly produce atpor will do it indirectly in the electron transport chain. but let's account for everythingthat we have. let's account for everythingthat we have so far. we already accounted for theglycolysis right there. two net atps, two nadhs. now, in the citric acid cycle,or in the krebs cycle, well first we have our pyruvateoxidation. that produced one nadh.
but remember, if we want to say,what are we producing for every glucose? this is what we produced foreach of the pyruvates. this nadh was from justthis pyruvate. but glycolysis producedtwo pyruvates. so everything after this, we'regoing to multiply by two for every molecule of glucose. so i'll say, for the pyruvateoxidation times two means that we got two nadhs.
and then when we look at thisside, the formal krebs cycle, what do we get? we have, how many nadhs? one, two, three nadhs. so three nadhs times two,because we're going to perform this cycle for each of thepyruvates produced from glycolysis. so that gives us six nadhs. we have one atp perturn of the cycle.
that's going to happen twice. once for each pyruvic acid. so we get two atps. and then we have one fadh2. but it's good, we're goingto do this cycle twice. this is per cycle. so times two. we have two fadhs. now, sometimes in a lot of booksthese two nadhs, or per
turn of the krebs cycle, or perpyruvate this one nadh, they'll give credit to thekrebs cycle for that. so sometimes instead of havingthis intermediate step, they'll just write fournadhs right here. and you'll do it twice. once for each puruvate. so they'll say eight nadhs getproduced from the krebs cycle. but the reality is, six from thekrebs cycle two from the preparatory stage.
now the interesting thing is wecan account whether we get to the 38 atps promised bycellular respiration. we've directly already produced,for every molecule of glucose, two atps andthen two more atps. so we have four atps. four atps. how many nadhs do we have? 2, 4, and then 4 plus 6 10. we have 10 nadhs.
and then we have 2 fadh2s. i think in the firstvideo on cellular respiration i said fadh. it should be fadh2, just to beparticular about things. and these, so you might say,hey, where are our 38 atps? we only have fouratps right now. but these are actually theinputs in the electron transport chain. these molecules right here getoxidized in the electron
every nadh in the electrontransport chain produces three atps. so these 10 nadhs are goingto produce 30 atps in the and each fadh2, when it getsoxidized and gets turned back into fad in the electrontransport chain, will produce two atps. so two of them are going toproduce four atps in the so we now see, we getfour from just what we've done so far.
glycolysis, the preparatorystage and the krebs or citric acid cycle. and then eventually, theseoutputs from glycolysis and the citric acid cycle, whenthey get into the electron transport chain, are goingto produce another 34. so 34 plus 4, it does get usto the promised 38 atp that you would expect in asuper-efficient cell. this is kind of your theoreticalmaximum. in most cells they reallydon't get quite there.
but this is a good number toknow if you're going to take the ap bio test or in mostintroductory biology courses. there's one other pointi want to make here. everything we've talked aboutso far, this is carbohydrate metabolism. or sugar catabolism,we could call it. we're breaking down sugarsto produce atp. glucose was our startingpoint. but animals, including us, wecan catabolize other things.
we can catabolize proteins. we can catabolize fats. if you have any fat on yourbody, you have energy. in theory, your body should beable to take that fat and you should be able to dothings with that. you should be ableto generate atp. and the interesting thing, thereason why i bring it up here, is obviously glycolysis is ofno use to these things. although fats can be turnedinto glucose in the liver.
but the interesting thing isthat the krebs cycle is the entry point for these othercatabolic mechanisms. proteins can be broken down into aminoacids, which can be broken down into acetyl-coa. fats can be turned into glucose,which actually could then go the whole cellularrespiration. but the big picture here isacetyl-coa is the general catabolic intermediary that canthen enter the krebs cycle and generate atp regardlessof whether our fuel is
carbohydrates, sugars,proteins or fats. now, we have a good sense of howeverything works out right now, i think. now i'm going to show you adiagram that you might see in your biology textbook. or i'll actually show you theactual diagram from wikipedia. i just want to show you,this looks very daunting and very confusing. and i think that's why many ofus have trouble with cellular
respiration initially. because there's just somuch information. it's hard to processwhat's important. but i want to just highlightthe important steps here. just so you see it's the samething that we talked about. from glycolysis you producetwo pyruvates. that's the pyruvateright there. they actually show itsmolecular structure. this is the pyruvate oxidationstep that i talked about.
the preparatory step. and you see we producea carbon dioxide. and we reduce nadplus into nadh. then we're ready to enterthe krebs cycle. the acetyl-coa and theoxaloacetate or oxaloacetic acid, they are reactedtogether to create citric acid. they've actually drawnthe molecule there. and then the citric acid isoxidized through the krebs
cycle right there. all of these steps, eachof these steps are facilitated by enzymes. and it gets oxidized. but i want to highlightthe interesting parts. here we have an nad getreduced to nadh. we have another nad getreduced to nadh. and then over here, anothernad gets reduced to nadh. so, so far, if you include thepreparatory step, we've had
four nadhs formed, threedirectly from the krebs cycle. that's just what i told you. now we have, in this diagramthey say gdp. gtp gets formed from gdp. the gtp is just guanosinetriphosphate. it's another purine that canbe a source of energy. but then that later can beused to form an atp. so this is just the way theyhappen to draw it. but this is the actual atpthat i drew in the
diagram on the top. and then they havethis q group. and i won't go into it. and then it gets reducedover here. it gets those two hydrogens. but that essentially endsup reducing the fadh2s. so this is where the fadh2gets produced. so as promised, we produced,for each pyruvate that inputted-- remember, so we'regoing to do it twice-- for
each pyruvate we produced one,two, three, four nadhs. we produced one atpand one fadh2. that's exactly whatwe saw up here. i'll see you in thenext video.
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