Introduction to Citric Acid Cycle

thus far in our discussion on glucose metabolism we focus primarily on anaerobic processes such as glycolysis gluconeogenesis and lactic acid fermentation now glycolysis as we discussed is basically a ten step process that takes place within a silent plasm of our cell and the glucose is transformed into two pyruvate molecules now to simplify things i've only included a single pyruvate molecule but if you want you can multiply all these values by two to get the net result now in the process of glycolysis we also produce two ATP molecules and these two ATP molecules can be used by other processes of the cell that actually require energy now even though glycolysis actually does produce a net amount of ATP molecules glycolysis only collects a very small portion of that usable energy that is stored in a chemical bonds of the glucose molecule so it only harvests a very small amount of potential energy that is stored in a bonds of glucose and to actually be able to collect the remaining energy stored in the chemical bonds of glucose we have to undergo aerobic cell respiration and this only takes place in the presence of oxygen and if we have mitochondria in the cells of our body because remember certain cells don't actually have mitochondria for instance red blood cells don't have mitochondrion so they cannot actually undergo aerobic cell respiration so the majority of the high-energy ATP molecules generated in breaking down glucose are produced via aerobic respiration which must take place in the presence of oxygen and inside the mitochondria of our cell so let's take a look at the following diagram so notice that when glucose is broken down two pyruvates we produce not only ATP molecules but we also actually collect electrons so we abstract four electrons when a single glucose molecule is broken down to pyruvate molecules now those two electrons aren't actually by themselves they're collected by special call enzyme nad Plus remember nad plus so nicotinamide adenine dinucleotide forgot the a so the nicotinamide adenine dinucleotide is a molecule that collects the electrons which are basically abstracted from that glucose because remember glucose is oxidized into pyruvate what that means is it loses electrons because it gains positive charge and so those two electrons don't exist by themselves and they are actually found on an H+ ion so when these electrons combine with that age we form a hydride and so what happens is this hydride is picked up and these electrons are picked up by this molecule here nicotinamide adenine dinucleotide and so two of these electrons are found on a single nad and so this is the same thing as saying we form two NADH molecules because when these combine we form an a D H okay so that's what we mean by these four electrons and we'll come back to these electrons in just a moment we'll see why they're actually so important now under aerobic conditions when we have oxygen inside ourselves what will happen to that pyruvate is it will enter the mitochondrion of that cell and once the pyruvate enters the mitochondrion as we'll see in the next set of Electra's that pyruvate undergoes a decarboxylation reaction in which we oxidize the pyruvate to form acetyl coenzyme a and we release co2 molecule and we collect two electrons again those two electrons are picked up by an nad plus to form an A eh so here we have one NADH here we have two and a D H molecules and instead of using the NADH I simply represent them as electrons we'll see why again in just a moment now this acetyl coenzyme a is a relatively large molecule and only a small portion of that molecule will actually be used in the next process so acetyl coenzyme a looks like this so we essentially have this component and then the remaining portion which is a big portion I'm not going to drawn on the board but this is basically the portion that will go into this next process known as the citric acid cycle so acetyl coenzyme a donates a portion of itself to the next process which consists of a series of oxidation reduction reactions that we collect we know as the citric acid cycle now we also sometimes call it the Krebs cycle or the TCA cycle where TCA stands for try carboxylic acid because as we'll see in just a moment try carboxylic acid TCA is actually an intermediate of the citric acid cycle now every time one cycle of the citric acid cycle actually takes place what happens is two carbon dioxide molecules are produced a GTP molecule is produced and eight electrons are collected via the oxidation and reduction reactions now six of these electrons are picked up by three of these nad plus molecules so we have three and a d-plus pick up three of these hydride ions to form three NADH s but two of these electrons are picked up by another important carrier found our body known as flavin adenine dinucleotide so we have F a D that picks up two of these H ions that each contain one electron each and so we form when this happens we form n fadh2 because each of these H's contains an electrons and that gives us total of two electrons and so three of these are NADH is and one of them is fadh2 as well see in this discussion here now this is what we form when a single pyruvate is broken down but actually we have two of these so we have to multiply all this by two so the net result of the citric acid cycle is we form two or four co2 s 16 electrons and two gtp molecules now what exactly is the function of the state racast cycle why does it actually take place so what is it well the citric acid cycle is the center of glucose metabolism this is where all these fuel molecules actually meet up to then go on and form the ATP molecule so things like amino acids and fatty acids they ultimately enter the citric acid cycle or more specifically aerobic cellular respiration as acetyl coenzyme a some amino acids can actually enter as intermediates of the citric acid cycle and the citric acid cycle has other important functions as well it also actually gives us those intermediate molecules the building block molecules such as oxaloacetate molecules that we use to form many building blocks or things like nitrogenous nucleotide bases amino acids glucose molecules because we have the oxaloacetate in the citric acid cycle that we can use to form glucose via gluconeogenesis we also form things like porphyrin molecules remember that porphyrin is the organic carbon based component of heme groups used by things like hemoglobin proteins and myoglobin protein so the citric acid cycle is a very important cycle it's the center of aerobic cellular respiration of glucose metabolism so the citric acid cycle functions to provide a means by which any fuel molecules so what is a fuel molecule well a fuel molecule is a molecule that contains carbon atoms and can be oxidized to obtain these electrons which then can be used as we'll see in just a moment to form ATP molecules now the citric acid cycle also functions to provide the building blocks needed to to form biological molecules such as nucleotide bases amino acids glucose molecules porphyrin molecules and so forth and we'll discuss all this in much more detail in lectures to come but let's actually generalize generalize what the steps of the citric acid cycle actually are so the c2 coenzyme a donates a small component of that molecule a two carbon component acetyl group into the citric acid cycle and this is what is shown here now once this goes into the citric acid cycle it combines with a four carbon molecule called oxaloacetate and this is the same axial acetate that is formed in the process of gluconeogenesis so we can see that the oxaloacetate form in a citric acid cycle can be used by gluconeogenesis to actually form glucose molecules so in step number one the two carbon acetyl group of acetyl coenzyme a is combined with the four-carbon oxaloacetate to form this six carbon molecule known as the tricarboxylic acid and that's why sometimes we call it the tricarboxylic acid cycle TCA cycle now in the next two steps in step two and three we undergo a decarboxylation reaction and oxidation reduction reaction so in step two the six carbon molecule becomes a five carbon molecule in the process we release a co2 and we also generate we release we abstract two electrons from the carbon six molecule and those two electrons are picked up by the NAD+ molecule to form NADH in the next step we have the five carbon component that again undergoes an oxidation reduction reaction in which we generate a co2 molecule and an NADH molecule so again two electrons are abstracted and collected by the nad plus molecule to form these two molecules and we form the four carbon intermediate C four now in the next series of steps we essentially transform this four carbon molecule back into oxaloacetate in the process we generate a high-energy GTP molecule and nadh molecule as well as the fadh2 molecule so essentially four electrons are released here two of these electrons are picked up by nad and the other two electrons are picked up by the fa D molecule so again energy is nucleus of energy is nicotine mi dinucleotide nicotinamide adenine dinucleotide and f ad is flavin adenine dinucleotide and so because this takes place twice because we have two pyruvate molecules entering the mitochondria we produce two four six NADH s to fadh2 s and to gtp molecules and we also form two and two so four co2 molecules now notice that we said that the citric acid cycle actually used to generate the high-energy electrons notice that the cycle doesn't actually itself generate any ATP molecules although we do generate a total of two GTP molecules every time two pyruvate molecules go into the citric acid cycle we don't actually generate any ATP molecule so why is that well because the citric acid cycle is like the Gateway for forming ATP molecules because once the citric acid cycle takes place and once we abstract all these electrons from these molecules the carbon fuel molecules these electrons which are carried by NADH molecules and fadh2 molecules can now can now move on unto a series of proteins on the inner mitochondrial membrane we call the electron transport chain and these series of proteins shown here basically allow the movement of these electrons down their potential gradient and as the electrons flow along these protein membranes on the membrane of the mitochondria that generates a proton gradient across the two sides of the membrane and as the proton gradient is generated that is then used to basically create ATP molecules so it's not the citric acid cycle that creates the ATP but it's the oxidative phosphorylation processes that take place across electron transport chain proteins that ultimately forms these ATP molecules so notice that no high-energy ATP molecule actually formed in the citric acid cycle and this is simply because the citric acid cycle is actually the Gateway to forming the ATP it doesn't actually form the ATP what it does is it harvest these high-energy electrons we call them high-energy electrons because they're used to form the high-energy ATP molecules across the electron transport chain found on the inner membrane of the mitochondria so essentially these electrons produced here flow across these membranes so we have 1 2 3 & 4 and then we have this ATPase pump that generates ATP molecules and as electrons flow H+ ions essentially are pulled this way because every time let's say this molecule this molecule and this molecule gives off those H ions they are pulled from the matrix into the intermembrane space that creates a proton gradient where we have a high concentration of h+ ions in this space and then this final ATP ATP based protein can use that gradient to drive the formation of those ATP molecules and this takes place entirely on this electron transport chain

Channel: Andrey K

Views: 247,179

Keywords: citric acid cycle, introduction to citric acid cycle, introduction to Krebs cycle, introduction to TCA cycle, Krebs cycle, tricarboxylic acid cycle, TCA cycle, pyruvate breakdown, pyruvate metabolism, electron transport chain, ETC, high energy electrons, coenzyme A, biochemistry

Id: Ge_uJ3iRDCE

Length: 15min 49sec (949 seconds)

Published: Fri May 22 2015