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Current time:0:00Total duration:9:31

Gluconeogenesis: the big picture

Video transcript

what I think it's pretty fascinating is that our body is able to maintain a very narrow and constant range of blood glucose in our body so notably about 60 to 150 milligrams of glucose per deciliter of blood and it's not important that you know this exact number but what I think is significant is that in contrast is something like free fatty acids for example which we'll talk about in fatty acid metabolism fatty acids can range almost tenfold depending on the needs of the body so they can either be really really high or really really low but glucose always stays within a very constant range I would say blood glucose level here and it's important that this is a very constant range of course because there are some tissues in our body such as our brain and some of the cells in our eyes and our kidneys and even our red blood cells that rely on glucose nearly exclusively to produce ATP so remember once glucose is in the blood it can be used by any of the cells in our body through process process of cellular respiration to produce ATP so remember the three big steps of cellular respiration our first glycolysis the breakdown of glucose and then the glucose goes to the Krebs cycle where it undergoes some more oxidation to release all that energy in the glucose molecule and finally the byproducts of glycolysis and Krebs cycle go to the electron transport chain which is able to produce ATP in bulk amount so how does our body keep this blood glucose in such a narrow range and constant range in our bodies so our body is able to do this differently depending on kind of what state the body is in so the body can be either in the fed State or our body can be in something that we call a fasted State so you can imagine kind of the fed state as being just after you've eaten a meals let's say you've eaten a chocolate chip cookie the glucose that has been broken up in your GI tract can then be used to directly contribute to this blood level and then of course the glucose can be used by our cells now in the fasted State so imagine that this is basically all the times you're not eating the body has come up with kind of two different ways to regulate blood glucose levels so now remember that in a fasting State our body needs a way to essentially pump glucose into the blood to keep it at this level essentially to replace the glucose that's being used up by our cells because we don't have this constant intake from you know our chocolate chip cookie so in this case our body has glycogen which is a polymer or kind of like such a string of glucose molecules that it essentially stores away and our body actually kind of ingeniously makes this glycogen by using some of that glucose that is dumped into our body during the fed state so kind of an anticipation of knowing that it's not always going to get glucose from eating it kind of preserved some of it in ass glycogen molecule and most of this glycogen molecule is actually just as a fun fact located in your liver which is why your liver is very very important for carbohydrate metabolism and so in times of fasting our body can actually go ahead and break down this glycogen into the individual glucose molecules which then can be used to keep our blood glucose levels constant so unfortunately it turns out that this mechanism of breaking down glycogen only lasts for about 10 to 18 hours in our body that is to say that after 10 to 18 hours we've essentially used up all of our glycogen stores and we need to eat another meal to kind of build those glycogen stores back up so you can imagine during an overnight fast for example so you know when you go to sleep and you know it's usually about you know hopefully eight to ten hours and so you can imagine there's a point during the day when your body needs another way of producing glucose and so our body has come up with a second 'we called gluconeogenesis which is indeed the topic of this video and gluconeogenesis is exactly what its name implies it is the Genesis or creation of neo new glucose now it's actually quite fascinating just to kind of think about this for a moment what what we're saying in gluconeogenesis is essentially our body is taking precursor molecules that are from a non carbohydrate source so essentially looks at what it has lying around and notably most commonly it uses amino acids in our body as well as a molecule called lactate which is produced as kind of a byproduct oftentimes in exercising muscle cells and it takes kind of these precursor molecules and reconfigures them to produce glucose and it's this glucose that can then be used to be dumped into our blood to maintain this constant blood glucose concentration and a constant supply of ATP for our tissues so now you kind of have a big picture of carbohydrate metabolism and where gluconeogenesis fits in let's go ahead and talk about as promised in this video about this metabolic pathway gluconeogenesis so in order to do this I think most effectively it's actually important to revisit glycolysis briefly so I'm going to go ahead and bring up the reaction diagram that was used to explain glycolysis in a previous video so just to orient you remember that glycolysis begins with glucose up here and glucose is broken down in a series of steps you know most notably at one point it's broken down into this three carbon molecule glyceraldehyde 3-phosphate and then it is broken down even further and reconfigured releasing some ATP and NADH along the way and ultimately forming this molecule pyruvate and pyruvate is a very important molecule because it can continue to the krebs cycle where it can be further ox to produce more nadh that can be used by the electron transport chain to produce ATP alright so that was kind of a big mouthful but just remember big-picture glycolysis breaking down glucose into pyruvate turns out the way I like to think about gluconeogenesis is that the goal of gluconeogenesis is to produce glucose and so it's actually the way I like to think about it is that gluconeogenesis is almost the exact reverse pathway of glycolysis that is to say we start at this end of the reaction pathway we start with pyruvate and we essentially go funnel back the opposite direction through all of these reactions to produce glucose now the key word is that it's almost the exact reverse of glycolysis and it's almost the reverse because I want to call attention to these orange arrows so note that there are kind of three orange arrows so one from glucose to this molecule glucose 6-phosphate another one here and then one at the very end which converts the last molecule to pyruvate what's important to note about these reactions in glycolysis is that unlike the other bi-directional black arrows that are used in most of the reactions these orange arrows are unidirectional what they're trying to indicate is that these three reactions are irreversible in other words they have a negative if we pull out a fancy term from chemistry they have a negative Delta G value or a negative Gibbs free energy which means that if we were to reverse these particular reactions we would have to flip the sign right and so these negative Delta G values would become positive and that's problematic right because we know that in order for any biological reaction to occur we must have a negative Delta G value so our body has come up with a bit of a compromise our body has essentially said okay look we have one two three four five six seven reactions with these kind of bi-directional black arrows that are essentially reversible that is to say they have a delta G value that's near zero and so they can go either direction so our body says okay we'll keep those seven reactions but in going from pyruvate back to glucose we have to come up with a different reaction pathway for three steps that are irreversible so that's exactly what our body did and in fact that's what I'll go ahead and review in the remainder of this video but essentially you can say with those three steps in mind we're just performing the reverse of glycolysis