in this video we're going to look at lipids as an energy molecule so in general lipids have a greater energy yield over glucose and protein yielding about 9 kilo calories per gram versus 4 kilo calories per gram and that's because there's significantly more chemical bonds so remember the correlation chemical bonds are potential energy and if you have more of them you're going to have more potential energy so more energy that can be harvested from the molecule when you break these chemical bonds now keep in mind in our earlier discussion the majority of the products that we generate from fat digestion is going to be transported via the lymphatic system as kyomicrons and it's in the lymphatic system where your endothelial enzymes are going to break down the triglycerides into fatty acids and glycerol so of all the lipids usually it's going to be the triglycerides that are going to be oxidized for energy and we're going to break these two components down because there's a separate glycerol pathway as well as a separate fatty acid pathway so when we look at glycerol it is a threecarbon molecule it's going to get converted into the intermediate I had mentioned to you uh in a previous video glyceraldahhide 3 phosphate so remember glyceraldahhide 3 phosphate was going to be used sort of in the payoff phase to generate ATP so we see that process right over here where glyceraldahhide 3 is going to be used in glycolysis about midway through and it will generate pyuvic acid so we only have three carbons in glycerol as opposed to the six carbons so in general glyceraldahhide 3 phosphate or the shorthand G3P is going to yield about half the amount of ATP so around 15 overall once it goes through all the process compared to glucose the fatty acids however are where the bulk of the chemical bonds are going to be found so remember fatty acids are going to be very long hydrocarbon chains so you're going to have carbon carbon carbon carbon carbon carbon and associated with each carbon you're going to have hydrogen so in this particular example I've just drawn out a saturated fatty acid and what happens during the pathway to break down fatty acids is that they're going to be broken down sort of two carbons at a time and these two carbon carbons become acidic acid fragments that are going to then be reduced as the product acetal COA so that acetal COA excuse me not the products the co-enzyme is going to be reduced as acetal COA and acetal COA then gets shuttled into the citric acid cycle or the Krebs cycle so we have that process here where we're using water to break those chemical bonds and we've previously calls the called these types of reactions hydraysis reactions and that's to break the chemical bonds down we're breaking chemical bonds therefore we're generating high energy electrons nad+ is there to pick up those high energy electrons and in some cases FAD as well eventually we produce acetal COA and a whole bunch of electron carriers we shuttle acceto COA into the KB cycle and we generate even more NADH and more FADH2s so this process cumulatively the breakdown of fatty acids is known as beta oxidation and it sort of takes a backseat to glucose metabolism because a higher initial energy investment is needed so you'll notice that in this process right here we initially have to invest ATP so usually uh beta oxidation occurs at a slightly elevated body temperature to facilitate this process compared to glucose so a more higher intensity uh is needed to initiate the breakdown of lipids now dietary glycerol and fatty acids are not essential for energy um but anything that is above and beyond what you need in terms of energy requirements we're going to store them as triglycerides okay the body prefers glucose as an energy source but technically glucose is not essential either uh as an energy source because you can use proteins you can use fats as energy sources okay now with that being said can you survive without carbohydrates and the answer is yes you can survive without carbohydrates but the same can't be true or the same isn't true for lipids because you need some of those essential uh fatty acids like lenolic acid we had mentioned before you need amino acids to make your own proteins so of the three what you need to survive are lipids and proteins now the process of triglyceride synthesis or lipogenesis is going to occur when you have a lot of energy available right because fatty acid breakdown generates so much ATP because it has so many chemical bonds it stands to reason that you're going to require a lot of ATP to synthesize your own fatty acids your own triglycerides so this is only going to happen when you have a high amount of uh energy so glucose is very easily converted to fat because acetal coa is one of those intermediate steps and it is the starting point for fatty acid synthesis so not only is it the product of beta oxidation right the reverse process to generate your own fatty acids is going to be dependent on acetal coa and in fact that's going to be a common theme for both fatty acids as well as proteins the intermediate steps of glycolysis of the citric acid cycle can be used to go backwards to do the reverse reactions to generate your own amino acids so what is the reverse of lipogenesis stored fat can be converted into glycerol and fatty acids for fuel and this actually is the preferred fuel source for the liver the cardiac muscle and resting skeletal muscles for this process you need oxallo acidic acid right so this was another compound that I had mentioned oxallo acidic acid with acetal COA forms citrate so that's the first step of the citric acid cycle right oxallo acidic acid or oxalloacetate is a fourcarbon molecule you fuse the twocarbon acetil you get a sixcarbon citrate molecule and then you progress by slowly breaking citrate down in a step-wise manner during the KB cycle so in this case uh when we have those acetal COAs fusing to oxylo acidic acid we can do uh do this process if glucose levels are deficient so we can do sort of the reverse reaction to generate glucose so lipolyis is usually going to be coincident with gluconneogenesis so it can be stimulated through the hormone cortisol as we mentioned earlier cortisol targeting atapose cells to break down their lipids and release the building blocks to ultimately generate glucose to maintain glucose homeostasis now if we don't have oxalloacetate or oxallo acidic acid the acetal coa is going to be converted by ketogenesis in the liver to what we call ketone bodies and this can really impact the pH in the body so let's examine some homeostatic imbalances if there's too many ketones in the blood this is referred to as ketosis so ketones in general are acidic and they can lead to a condition called metabolic acidosis now one way that we help to mitigate pH homeostasis is through the respiratory system in fact that's the other than the buffers that we have in circulation like the bicarbonate ion the respiratory system is a fairly quick way of mitigating pH imbalance issues so if we're metabolically active to prevent acidosis we increase our rate and depth of breathing and so for somebody who is going through ketosis or ketoacidosis which is sort of like the really bad ketosis um they're going to increase the rate and depth of respiration and their breath is actually going to smell very fruity as they are vaporizing acetone and their rapid breathing helps to release some of that CO2 uh to raise your body's pH now this is given actually a name it's called kousal breathing now because of the pH imbalance it can have a negative effect on the nervous system so these individuals might appear drunk uh because of that pH imbalance so we see this common in individuals who are starving who are not uh dieting a well-balanced diet uh and diabetics who are not properly controlling their insulin levels so ketone bodies ultimately are going to be excreted in the urine so what about phospholipids well phospholipids are needed for cell membranes and myelin so they're not really used for energy they're more of a structural component likewise cholesterol is for structural components in the cell membrane and also the building block for various steroid hormones so the synthesis of these materials is going to occur in the liver so remember that a lot of these lipids are pretty much all of them are hydrophobic so they're not going to be transported very readily in the plasma which is dominated by water so the liver synthesizes specialized transport proteins called lipoproteins for things like cholesterol and fats to be able to deliver these compounds in circulation as for cholesterol cholesterol is going to be synthesized using the building block acetal COA so you can see how important acetto COA is both as a reactant in the KB cycle as well as a reactant to reverse engineer things like glucose or triglycerides or in this case cholesterol so cholesterol in addition to be used as a component of the cell membrane it's also a building block to form bile salts which we talked about in an earlier series of videos needed for the emulsification of lipids so here we have sort of a flowchart indicating all of these processes so you have your neutral fats your triglycerides they're going to go through lipolyis to produce glycerol and fatty acids glycerol is going to be converted into glyceraldahhide 3 phosphate ultimately shunted into the glycolytic pathway to generate ATP the fatty acids are slowly going to be broken down through beta oxidation into acetal COA and that acetal COA is then shunted into the KB cycle for more ATP production now acetal COA can also be used for the synthesis of cholesterol which then becomes an important component for steroid hormones as well as bile salts we can do reverse reactions so acetal COA can be used as a building block to make fatty acids the intermediates in glycolysis can be used like glyceraldahhide 3 phosphate to synthesize glycerol uh you could also have ketogenesis right you convert acetal COA into ketone bodies but remember that's only occurring in the absence of oxallo acidic acid