there are 13 major metabolic pathways that you need to know for the mcap in the next 15 minutes I'm going to teach you every single one of them which means at the end of this video this cell will look like this and you'll actually understand it so let's get started this is a eukaryotic cell for this video we're going to focus on eukaryotic metabolism as that is what's most likely to be tested on the mcap we're going to identify where each pathway occurs in this cell and in the body what each pathway needs as precursors and what are they going to produce as products then we'll briefly talk about why and when we use each pathway we're going to start with our first of 13 Pathways glycolysis we're going to zoom in on this picture of our cell and go right up into the cytoplasm because that's where glycolysis takes place now I'm going to use green for all of our Pathways just to keep us organized so right up in the upper left corner is where we're going to write glycolysis so glycolysis glycolysis Lis meaning breaking glyco sugars glucose so what we're doing is we're breaking up glucose for fuel so in order to do that our precursor of course we need is glucose this is carbohydrate metabolism so we need glucose we're also going to need some NAD plus because we're going to reduce that to nadh in glycolysis we also need some energy to get us started here so we're going to use two ATP early on in the glycolytic steps I'm going to draw that with little energy around it since ATP is our energy source of our cell so we've got glucose nad+ and two ATP coming into glycolysis and then what we're going to produce is pyate as our major product and we have two pyrovate for every one glucose we're also going to produce nadh and we're going to produce four ATP now that means net is 2 ATP right cuz we we used up to and we produced four so two net ATP they love testing that on the mcap all right so we've got our pyate and where pyate goes next depends on both our energy levels in our cell and our oxygen levels so let's say we have high oxygen levels and low energy levels we need energy we have high oxygen but we have low ATP what we're going to do next is a pathway called the pyate dehydrogenase complex or reaction pdh now pdh is going to produce acetal COA from our pyate and acetyl COA is our big uh molecule that's involved in our mitochondria so I'm going to put it right in the center here so we've got acetal Co a pyruvate dehydrogenase also produces one molecule of nadh and one molecule of CO2 so we'll put that in but our big one that we care about is acetal COA because that's what's going to move on to our next pathway so our next pathway is the citric acid cycle or kreb cycle and it's also in the mitochondria so our citric acid cycle is a cycle so I draw it kind of with arrows instead of a box right so it's going to be a cycle that starts with acetal COA and it produces nadh so it produces nadh it also produces fadh2 which is another electron carrier and CO2 but it's a cycle so it Loops right back around to acetyl COA so acetal COA here is our precursor for the citric acid cycle right as it goes in and we also need nad+ and fad plus all right so that's going to be our non-hydrogenated electron carriers and then we add our electrons to them they become fadh2 and nadh before we get to the electron transport chain please subscribe to this channel for more videos that help you learn MCAT content test taking strategies and mental Fitness so that you can perform your best on test day all right so once we've gone through the citric acid cycle our electron carriers actually go into our electron transport chain our electron transport chain is embedded into the inner membrane of the mitochondria so I always draw it kind of right below the citric acid cycle on the membrane the other interesting thing about the electron transport chain and we're going to get to this when we talk about memorizing intermediates is complex 2 of the electron transport chain is the same enzyme as succinate dehydrogenase in the citric acid cycle so they literally overlap they have an enzyme in common and that enzyme takes fad and turns it into fadh2 and then back to fad and back to fadh2 so fadh2 doesn't have to travel far to drop off its electrons it stays in the same enzyme complex all right so electron transport chain citric acid cycle connected and what comes into the electron transport chain our nadh and what's produced by our electron transport chain I'm going to go ahead and draw it on the outside of the mitochondria but I I want you to know that it's on the inside is we produce water and we produce ATP ATP so coming into the electron transport chain is also oxygen and this is why I said back up here we needed oxygen so here's the rule if we're going to be using the mitochondria for energy production to produce ATP we got to go through the electron transport chain in order to go through the electron transport chain it needs oxygen so we have to have oxygen in our cells in our bodies in order for the mitochondria to do its job so if we want the mitochondria to produce energy we've got to have oxygen all right zooming back out for just a second we can see that we have what is known as aerobic respiration so the process of glycolysis through the PD through the citric acid cycle and through the electron transport chain to produce ATP from glucose is what's commonly known as aerobic resp Iration the aerobic part comes into play when we hit the mitochondria but what if we didn't have oxygen so what would happen to our processes if we still needed energy but we didn't have oxygen well let me show you we're going to zoom back into our glycolytic pathway down to pyruvate because you'll notice here we didn't need oxygen for this part and if we have low oxygen and we still have low ATP we need energy what we essentially want to do is keep running glycolysis right because glycolysis is still producing ATP so we can still get some fuel through our cells via glycolysis in order to do that though we not only need a steady supply of glucose we need a steady supply of NAD plus and that's a limited resource in our cells so we need a way to turn our nadh back into NAD plus and the way we do that is by converting pyruvate to lactic acid and the enzyme pathway that does that is lactate dehydrogenase much like pyate dehydrogenase all right so that lactate dehydrogenase is going to take our pyruvate and turn it into lactate and remember here our lactate is not our goal all right we're not trying to produce more lactate that's not doing anything for ourselves our real goal is the other product which is nad+ all right so we take that nad+ and now that NAD plus can help regenerate the beginning the precursors for glycolysis and allow us to continue doing glycolysis to make energy lactate is not great for our bodies doesn't feel good that's why we get that lactic acid build up in our muscles when we're doing Anor robic work because we're doing lots and lots of glycolysis and producing lactate as a byproduct all right now what if we need more glucose so we you know we've been doing glycolysis doing all these things but actually our energy levels are good we have high ATP we're feeling pretty solid but we don't have enough glucose then what we can do is we can go through gluconeogenesis so I'm going to draw it from pyruvate here just so you know we usually have to go all the way back from the citric acid cycle and turn our oxaloacetate from our citric acid cycle back to pyruvate and then enter glycolysis but for all intents and purposes pyruvate is our precursor all right so this is gluconeogenesis I'm going to just write it as g n g G all right but glucon neo genesis Genesis creation Neo new glucose glucose so we're producing new glucose here so we're going to take our pyate and we're basically just going to go back the other way uh do reverse glycolysis with three different steps and we're going to produce some glucose and our glucose will be produced by these cells now this glucose can go and be a precursor again for glycolysis but if we have high energy and we have high glucose levels we have plenty of glucose to go around we may just want to store that glucose for later and the pathway that allows us to store glucose is known as glyco Genesis Genesis meaning creation glyco glycogen so glycogen is our storage form of glucose it's just a polysaccharide of lots of glucose monomers so glycogen storage form and whenever we're ready we just store it and whenever we need glucose whenever we have low glucose levels we'll go ahead and break that glycogen down through a pathway called glyco Geno liis Lis meaning breaking right so glycogen ois we're breaking glycogen down glycogenesis we are forming glycogen and so that glycogenolysis just breaks down glycogen back to GL glucose for use in glycolysis in the cells an important thing to know here this whole system gluconeogenesis glycogenesis and glycogenolysis this all happens in liver cells specifically so this is specific to liver cells and then liver cells can then go release that glucose to be sent out to other parts of the cell but glycogen storage happens in the liver and gluconeogenesis formation of new glucose also happens in liver cell cells so this is not just all cells this is specific to the liver all right so that was all of carbohydrate metabolism we now have everything that can happen with carbohydrates in terms of metabolic processes in our cell before we move on to lipid metabolism I do want to mention one lower yield pathway called the pentos phosphate pathway this pathway branches off of glycolysis pentos phosphate pathway and this pathway is used when we have have plenty of energy to go around so we have high ATP levels and instead what we need is nucleotides and nadph so our products here from the pentos phosphate pathway are nucleotides like new DNA molecules right nucleotides and nadph which we're going to see in a moment so I just wanted to mention the pentos phosphate pathway produces NAD P so NAD plus comes in np+ and pre uh intermediates from glycolysis there's several points at which it can enter the pentos phosphate pathway via the glycolytic intermediates but our product here is nucleotides or nadph so that is just a low yield but important pathway when we're talking about lipid metabolism which we're going to hit shortly all right let's say we don't have enough glucose and we need to break down fats for our energy so we're going to start with fatty acids fatty acids are going to be our precursors here and again we need to have oxygen because the process of breaking down fatty acids actually happens partially in and partially outside of the mitochondria and that process that pathway is called beta oxidation beta oxidation I'm going to kind of draw it into the mitochondria all right so fatty acids come in and acetal COA is produced and then from there that acetal COA goes right into the citric acid cycle and the electron transport chain as long as there's oxygen so that we can produce ATP so we need to have oxygen here for beta oxidation and this will happen when we need energy low ATP and we also do not have glucose low glucose right because we're going to be using fatty acids instead all right let's say we have plenty of energy to go around we actually want to store some fats we got plenty of sugar plenty of energy we don't need that acetyl COA to come from fats anymore so we're going to go ahead and store it and we're going to store it through a pathway called fatty acid synthesis I'm going to write it as FAS here all right now this happens out in the cytool the issue is our acetal COA is in our mitochondria so the first thing that needs needs to happen here is we need to shuttle it out and we do that via what's known as a citrate shuttle so we turn our acetyl Coy into citrate there's this couple steps here that we're not going to get into in this video but it's going to shuttle itself out of the mitochondria and then be our precursor for fatty acid synthesis fatty acid synthesis is of course going to produce our fatty acids so boom fatty acids becomes our product and from there we can go even further and build what's known as triog glycerides which is our final storage form of fats triag glycerides tags all right so that is it's a relatively simple process but there's one key piece here in addition to the citrate coming into the fatty acid synthesis we also need nadph nadph in order for fatty acid synthesis to happen it's a series of reduction reaction reactions and we need an electron carrier so what's really cool is where this nadph comes from is our pentos phosphate pathway so both the pentos phosphate pathway and the fatty acid synthesis pathway will happen when we have plenty of energy all right our energy is fine we have no energy need and in fact we're in storage mode so that NPH will help for fatty acid synthesis in terms of where in the body this happens fatty acid synthesis is again happening mostly in liver cells so when in doubt if there is a specific cell in which metabolism is happening it's probably the liver or muscle cells all right we have one more type of metabolism to go and it's very low yield on the MCAT so I just want to mention where it is and how it happens and that is our protein metabolism so for protein metabolism we have just two Pathways to think about ketogenesis again Genesis forming Ketone bodies so ketogenesis again happens with the mitochondria kind of in and out so we're going to draw it this way ketogenesis Genesis and it's happening by pulling a cal COA and intermediates from the citric acid cycle and forming what are known as Ketone bodies all right Ketone bodies can be used as a source source of energy for the brain if our sugar levels are really low so our brain can only use carbohydrates as a source of energy if our carb sources are really low like we we haven't eaten any sugar in a while what we'll do instead is we'll produce Ketone bodies and then bring those Ketone bodies to the brain for energy so that they can use Ketone bodies instead of using glucose because we don't have any glucose again the brain cannot use use fat for energy now if we don't need those Ketone bodies we can go ahead and do ketolysis right same kind of deal uh that we've seen in our glycogen formation and breakdown so ketolysis again I'm going to kind of draw it in and out of our mitochondria because it's there it's getting a little messy that's okay and then that's going to eventually form our citric acid cycle intermediates again like Alpha ketoglutarate all right so going out Ketone bodies and then coming back in through ketolysis our final Pathway to produce our citric acid cycle intermediates and again be used for fuel that's what's happening in the brain the brain is doing ketolysis to break down those Ketone bodies for fuel all right we did it here is all 13 metabolic pathways on one page for you to review and go through the precursers and the products so just a review we went through the following metabolic reaction in this video glycolysis the pyrovate dehydrogenase complex reaction the citric acid or crep cycle the electron transport chain lactic acid fermentation gluconeogenesis glycogenolysis glycogenolysis yep saying that nice and slow the pentos phosphate pathway beta oxidation fatty acid synthesis ketogenesis and keto liis you now know when and where these reactions happen and what their precursors and products are next up you'll want to learn how they're turned on and off we talked briefly about this but I want you to step it up by learning the hormonal and homeostatic regulations for each of these metabolic processes in our bodies then finally before test day maybe a couple weeks before memorize those key intermediates and enzymes for each of the pathways we talked about today then you're all set to master metabolism on the MCAT one more thing if you're watching this video but have not yet taken a practice exam exam you need to take one it will help you figure out your strengths and your weaknesses and make an effective efficient study plan I promise you it's worth the time to do it so there's a link in the description below for a free course that I have that will teach you how to take a practice exam how to review it effectively and how to use that info to make your study plan go ahead and check it out and as always happy [Music] studying