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Current time:0:00Total duration:12:29

Tissue specific metabolism and the metabolic states

Video transcript

when I look at this image I'm not exactly sure what I see am I am I looking at a diagram that shows all the metabolic pathways in the human body or am i looking at map of the subway in New York City now in order to describe how the body balances the storing and utilization of energy through different nutrients in all the tissues of the body I'm going to just go ahead and get rid of this diagram and I'm going to bring up one that's a little bit more simple now this isn't exactly an anatomically correct diagram but it will work to describe how these different tissues function metabolically first let me bring in some metabolic intermediates and I don't want you to get overwhelmed by all these intermediates as there are quite a few of them because we're going to be going through each one of them individually in order to make sense of this entire diagram now the first metabolic state I'm going to talk about is the absorptive State well the absorptive State is a series of metabolic reactions that your body does when food is in plenty you have food in your intestines and you're absorbing that food and so you don't need to utilize all this energy and said you want to store it so you can use it later so what is this what does this series of metabolic reactions look like in the liver so imagine the carbohydrates are broken down in the intestines and you're absorbing the dye and monosaccharides like glucose so glucose is then transported to the liver and it can have one or two kind of go in one or two pathways the first pathway is that it can be stored as glycogen I circled and filled in glycogen here to remind you that this is the storage form of carbohydrates the other way glucose can go is to be converted into triglycerides which are the storage form of fat now in order to do so the glucose must first be converted into glycerol as well as fatty acids and the process of converting glucose into fatty acids involves going through pyruvate and acetyl co a to form the fatty acid known as palmitate which is the only fatty acid that the body can synthesize on its own and then these glycerol and fatty acids can be combined to form triglycerides now in the liver we don't store triglyceride it's stored mostly in adipose tissues and triglycerides can't be transported in the blood so these triglycerides have to be converted into very low density lipoprotein or VLDL which can then be exported out of the liver and into the blood next we have the amino acids that aren't contained in protein so the meat we ate in our cheeseburger is broken down into proteins that are then broken down to their component amino acids and these of mat amino acids are taken up by the liver the liver will break them down into keto acids and these alpha keto acids when we break them down they're going to give off ammonia which is then excreted is wastes in the form of urea so these the energies that was in the amino acids is now in our alpha keto acids now where can this go and it can also go one of two ways if we want to store the energy that's in these keto acids we're going to convert them to fatty acids and those fatty acids can then be converted into triglycerides and those triglycerides will be once again exported into the blood and then later we'll store them in the adipose tissue but if the liver needs energy during this absorptive state these keto acids can also be broken down into a seed alcoa and that acetyl co a can go through the Krebs cycle and electron transport chain to produce ATP or usable energy now let's move on to adipose tissue so adipose is fat and fat can kind of be thought of as just a vast a warehouse of energy storage so when we eat the carbohydrates are broken down and the glucose is transported in our blood to adipose tissue and it's taken up and in the absorptive state this glucose in adipose tissue is going to be turned into triglycerides but first similar to the liver it needs to be converted to glycerol and fatty acids and then the glycerol and fatty acids are going to be converted into triglycerides however this isn't the only thing that's happening that adipose tissue during the absorptive state remember we exported those very low-density lipoproteins from the liver well those are now going to come down through the blood and they're going to enter the adipose tissue and be turned into fatty acids and these fatty acids can then be combined with glycerol to form triglycerides and similar to above where I circled and filled in like Jhin triglycerides are now circled and filled in because this is now the storage form of our fats and it's stored in the adipose tissue so let's move on to the muscle what's going on in the muscle well Luke once again just like in the liver and in adipose tissue glucose is taken up by the muscle cell during the absorptive state and glucose can now have one of two pathways it can similar to the liver we can convert it to storage form and glycogen but also the muscle may need energy even though in general our body is storing energy in our storage forms it's sometimes during the absorptive site it still needs to utilize some of this energy that we've eaten so that glucose can be converted to pyruvate and through cellular respiration we can create ATP and the muscle will have usable energy and in the absorptive state muscle also takes up the amino acids from proteins that we eat and these amino acids are just stored as protein in our muscle now let's talk about the brain so in the liver fat and muscle cells the primary actions of the metabolic pathways during the absorptive state was to store energy to store energy as glycogen triglycerides and protein but in the brain we don't store energy the brain consumes a vast amount of energy so during the absorptive state even though generally the body is tending to store energy the brain is just going to use it so the glucose from the blood is taken up and it's converted to pyruvate and just like in the muscle we talked about it goes through cellular respiration to produce ATP so that the brain can continue to do work so as you look at all of these metabolic pathways during the absorptive state it's makes sense that when the body is as abundant energy because it's just eaten that we're going to store that energy but what happens when we haven't eaten in a while when our body needs to take the energy that's stored in glycogen and triglycerides and protein and and use it well that's what happens in the post absorptive state so for a moment here I'm going to just remove the absorptive state from our diagram so now our body has absorbed all of its nutrients from the gut see there's no longer any fat protein or carbohydrates here and we've stored it in its respective storage forms in the liver we've stored carbohydrates as glycogen and in the fat tissue we've stored fats as triglycerides and in the muscle we've stored protein so say a little time has gone by and now the body needs to utilize this energy but it can't just use protein or glycogen or triglycerides and so first break those nutrients down into more usable forms of energy and the series of metabolic reactions that occur are known as the post-absorptive state in general during the absorptive state we're building up and storing energy in the post-absorptive state we're going to be breaking down that those molecules and releasing the energy so in general a lot of these reactions are just going to be going in the opposite direction but not all of them so let's go through them here and once again we're going to start in the liver now in the liver glycogen is going to be broken down into glucose and glucose can then be exported out of the liver into the blood and be used for energy all over the body now the amino acids are actually still going to be taken up by the liver and just like during the absorptive State the amino acids are going to be converted to keto acids and give off ammonia which will produce urea as a waste product now remember in the absorptive state the keto acids were turned into fatty acids to make triglycerides well now in the post-absorptive state we're going to use these directly to make glucose and that glucose can be exported to be used by other parts of the body but the liver still requires energy of its own so in part these keto acids are still going to be broken down into a seed alcoa to produce ATP so what about glycerol and fatty acids remember in the absorptive state we were combining them into triglycerides but this is a storage nutrient and in the post-absorptive state we want to create more usable energy so these are actually going to be also converted into glucose now there's one other important step in the liver that's happening now these fatty acids can also undergo a second reaction they can be broken down to form ketones now ketones are an important metabolic intermediate because they're one of the very few types of energy that can be used by the brain now that we have a good idea of what's going on the liver in the absorptive state what's going on in adipose while I mentioned earlier that adipose is the storehouse of energy and that energy is stored in the form of triglycerides now in the post absorptive site we want to use this energy but we can't just use a triglyceride we have to break it down so the triglycerides are broken down into glycerol and fatty acids and they can just be exported into the blood and are brought to the liver to help make more glucose now let's move on to muscle and muscle we have proteins and the protein is going to be broken down into its amino acids and these amino acids are going to be exported into the blood and just like the glycerol and the fatty acids in adipose tissue these amino acids are also going to make their way to the liver so that they can be converted to alpha keto acids and then convert it into glucose which is a more usable form of energy but muscles also contain some glycogen and the glycogen is stored in muscles so that they'll have a little bit more direct access to energy so in the post absorptive state this glycogen is going to be converted into glucose and that glucose can be converted into pyruvate which can then go on to create a c2 loco a and produce energy through the electron transport chain and this is all happening within the muscle cell but I don't have it drawn in here but this these series of reactions right here in order to produce this ATP for energy it requires oxygen but what happens if the muscle cell is low on oxygen well luckily glucose can also be broken down into another product and that's lactate and when it's broken it down into lactate we produce ATP but we don't need the oxygen like we do during cellular respiration now you may be asking well if you don't eat the oxygen and you can still get the ATP why doesn't glucose always be broken down into lactate and improves ATP and there's two reasons for this this process here isn't as efficient it doesn't produce as many molecules of ATP as would be produced if we went through cellular respiration and also this lactate is an acid and it disturbs the pH balance of the blood so lastly let's move back to the brain now I said in the during the absorptive state that the brain just always uses energy and that's true once again we're just taking the brain is taking glucose from the blood converting it into pyruvate and going through cellular respiration to produce ATP but also remember the ketones from the liver are also used by the brain for energy in the post-absorptive state now that we've gone through the post-absorptive state you see how the different tissues are breaking down and using energy let's just bring back in the absorptive metabolic pathways so you can get an idea of all the different types of metabolic reactions that are going on depending on whether your body is trying to store nutrients or utilize the nutrients