so for this part of the chapter we have to talk about something called respiratory substrates and what that means is we are going to be focusing on organic molecules that can be broken down to generate ATP so you might be thinking wait a second we talked about that much earlier where we said glucose is the thing that is broken down to get ATP and glucose is a type of carbohydrate but you see glucose is not the only only thing that our cells can break down in order to generate ATP molecules we can also break down lipids and we can also break down proteins so the question is if your cell actually has access to carbohydrates like glucose lipids such as fatty acids and proteins in the form of um well you can say specific amino acids Which substrate or which molecule would your cell actually break down in order to to generate some ATP molecule you see all three of them can be broken down by the cells that's not the problem here the problem is which one will be the most efficient in order to generate the highest amount of ATP because logically the more ATP you get the better correct so between carbohydrates and lipids we will talk about that one later but let's focus on proteins and you see proteins are composed of amino acids and while amino acids can be broken down to be a source of fuel amino acids actually have a more important function in your cells in which they have to be combined together to make proteins and these proteins can then be used uh to form antibodies enzymes hemoglobin collagen you know all these kinds of things that your cells actually need so while your cell can break down amino acids to generate ATP the cell would prefer not to do such things because because the amino acids are used to build or to produce other things as well so the question over here is between carbohydrates and lipids however examples of carbohydrates glucose and an example of lipid here I'm showing is fatty acid uh you can see one molecule of glucose there and a molecule of fatty acid uh which one is able to generate more ATP when it's broken down Okay so let's look at a chemical formula for two of them glucose is C6 h126 the fatty acid I'm showing you here is an example it's not not all fatty acids have this chemical formula but this particular fatty acid has c18 h342 so right off the bat sometimes I like to ask my students this question if both these molecules are broken down separately which molecule is able to yield or produce or give you more ATP in the end so some students stud will say well glucose and some students will say well no it's fatty acids but you must be able to justify your answer so what you have to do is you have to focus on the hydrogen in this case and the glucose actually has less carbon hydrogen bonds which means less carbon attached to hydrogens and the fatty acids obviously have more carbon hydrogen bonds because they have more carbon and they also have more hydrogen as well you have to say it like that the glucose has less carbon hydrogen bonds and the fatty acids have more carbon hydrogen bonds so when it is broken down the glucose will give you some hydrogen atoms and obviously when the fatty acids are broken down from the carbon hydrogen bonds more hydrogen atoms are released and based on what we've studied the hydrogen should be accepted by carriers such as Neds and feds so because the glucose gives out less hydrogen atoms it will produce less reduced NAD and less reduced fed and conversely for fatty acids because it gives out more hydrogen atoms it will produce more reduced Neds and more reduced fads so far so good so what's the thing because these reduced nads and reduced fads will move towards oxidative phosphorilation and in glucose because it produces less reduced nads and less reduced feds oxidative phosphorilation will take place that is true but you'll get less ATP out of it simple as that and because fatty acids when they broken down will give more reduced nads and reduced feds higher rates of oxidative phosphorilation happens and obviously more ATP are produced in this place so in the exam all you just have to say is fatty acids have more carbon hydrogen bonds and when they are broken down they will release out more hydrogen atoms which will give you more reduced Neds and reduced feds which in turn has a higher rate of oxidative phosphorilation and therefore produces more ATP in the process but the question then is if our cell has access to glucose and fatty acids and if fatty acids actually provide us with more ATP why then does the cell actually use glucose as the main source of energy because it's kind of weird isn't it you have fatty acid right there which is a fantastic source of energy it can provide you with more ATP for the cells yet our main source of energy is still glucose molecules the answer to that is a little bit complicated so but let's look at it now I'm drawing out a cytoplasm here and you can see part of the mitochondria the smooth outer membrane the fold the inner membrane you can see the chist which is those finger like structures and also I've labeled the mitochondrial Matrix under normal circumstances if you remember in the previous videos when glucose is used during respiration assuming this is aerobic respiration by the way the glucose molecule which is six carbon will be broken down into two pyate molecules uh through the process of glycolysis and then the pyrovate will Enter the Matrix and then the pyate which which are three carbon molecules uh they will be broken down through link reaction and they will form something known as acety co-enzyme a and acety COA if you remember will then enter the CRA cycle where they will be completely broken down we know this alternatively if you want to break down fatty acids I'm throwing out fatty acids over here don't have to memorize that um structure but I'm just showing you that the fatty acid here is made out of 14 carbons each of the circle represents a carbon okay and the fatty acids do not undergo a process known as glycolysis no if the cell wants to break down fatty acids the fatty acids will have to first enter the mitochondrial Matrix directly where in The Matrix they will then be broken down into two carbon molecules and because fatty acids are made out of a long hydrocarbon tail you would be able to get a lot of these two carbon molecules once they broken down into these particular two carbon molecules they will then be converted into acety cor a and then they will be shunted or it will enter the crap cycle as well so there are some similarities when you break down glucose or fatty acids but of course it's quite different too because glucose will have to undergo glycolysis and Link reaction which happens in the cytoplasm and then the mitochondrial Matrix whereas fatty acids are broken down directly in the Matrix so you're like okay but that still doesn't answer my question as to why the cell still prefers glucose over fatty acids one possible answer which is quite simple is as follows the pathway which I've highlighted in yellow over here I'm highlighting it in yellow look at the arrow there that pathway can only happen when the cells have enough oxygen which means to say that this pathway is an aerobic pathway all right because why do the cells need oxygen the cells need oxygen for oxidative phosphorilation and it needs to generate uh it needs to regenerate the Neds and feds right we talked about this in the previous video but the important thing that we have to understand here is lipids are only able to be broken down when the cells have sufficient oxygen that is the caveat when it comes to breaking down lipids however glucose is a bit more interesting yes glucose requires oxygen to be broken down in the aerobic pathway as I've highlighted there in yellow as well at the top however glycolysis does not necessarily need oxygen if there is oxygen glycolysis can happen but if there was not enough oxygen in the cell glycolysis can still continue to take place in the in Aerobic pathway we talked about this in the previous video where in animal cells the glucose is broken down into pyate and then it's converted into lactate or in plants and yeast the pyate is converted into carbon dioxide and ethanol so that's the advantage that glucose has over fatty acids during aerobic respiration both glucose and fatty acids can be used but during anerobic respiration only glucose can be broken down that is why glucose is the main source of energy for the cell because it's the easiest source of energy to be broken down as well right because it's only six carbon and six carbon is easier to break down than anything that is containing 14 carbon I guess our cells are a little bit lazy too but you know can you blame them here's the weird thing a lot of people assume that when you're running as fast as you can people assume that you are breaking down fats because they're like oh I'm running quite fast and I need more energy and therefore you know I think my body is breaking down fats no it's actually breaking down glucose because only glucose can be broken down in the anerobic pathway so these are the differences that you have to be aware of between glucose and fatty acid as respiratory substrates