M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
The citric acid cycle, also known as the Krebs cycle, is involved in cell respiration and produces NADH and FADH2 for the electron transport chain. The Krebs cycle also produces two ATP, but much more ATP is produced later, in the electron transport chain, so that is not its main purpose.
The second step of aerobic respiration goes by many different names. Some text books will call it the citric acid cycle others will name it after the guy who discovered it the Krebs cycle or I've even seen it refereed to as the TCA cycle which stands for tricarboxylic acid cycle. No matter what name you go by, it's all the same thing it's the final breakdown of glucose after the glucose was initially split in half like glycolosis. Now it happens in the matrix part of the mitochondria and the ultimate yield of the Krebs cycle for every one glucose that enters the cell it gives you a pair of ATPs, 8NADHs, and a couple of FADH2 now I'll discuss what goes on a little bit more in just a moment.
But first I want help and make sure that you understand what's going on, now if we take a look at any living organism that's a eukaryote i.e. it has a nuclears it'll have somewhere in the cell mitochondria like this plant cell you can see it has mitochondria and that's a trick question that a lot of teachers like to stick with it. They'll ask you which cells had mitochondria and they want you to say just animal cells but remember plants do aerobic respiration to break down the sugar that they themselves are making for their own purposes. So if we take a look at the structure of the mitochondria you can see it has an outer membrane and then it has this folded inner membrane called the "Cristae," within the cristae's folds you'll have an inner space where compartment called the matrix. And this is why I tend to prefer talking about the Krebs cycle as opposed to using one of the other names because let's see the matrix ever seen that movie? Who stared in it Keanu Reeves KR Krebs cycle, Krebs cycle is using respiration.
It's one of those little memory tricks in my mind that will help you recall which of all the different cycles happens in the matrix of the mitochondria versus what happens in the chloroplast or in photosynthesis? We take a look at the whole process of aerobic respiration every step generates some ATP the glycolsis takes in the glucose splits it in half to form some pyruvate and spits out a little bit of ATP as well as a couple of these NADH molecules. The Krebs cycle takes in those pyruvates breaks it up spits out a little bit more ATP and then sends off the NADH and FADH2 which are high energy electron carriers off to electron transport system.
You'll see that the Krebs cycle is what's generating the carbon dioxide that you breathe out every time you break down your sugar and then the electron transport system is what generates the load of ATP that is the purpose of the aerobic respiratory process using the energy of this high energy electrons. So let's take a closer look at the Krebs cycle now I've simplified a lot of the steps because most teachers don't have you memorize each and every molecules' name as you go through the process. But in general what happens is that pyruvate from glycolysis out in the cytoplasm as it starts to enter into the mitochondria enzymes will rip off a couple of high energy electrons and put them onto an electron carrier called NAD positive. You add 2 electrons it becomes negatively charged and then grabs a nearby positively charged hydrogen ion, and becomes NADH. So this NADH is a full high energy electron carrier full of high energy electrons and it goes off to electron transport system where those high energy electrons can be used to generate a lot of ATPs.
You have 2 carbons left because one of the carbons falls off when you pull off those high energy electrons. A 2 carbon group is called an acetyl group you put on to it this holder or helper molecule called co-enzyme A, it's a co-enzyme which means it's not an enzyme but it helps enzymes do their job so we call this combination of 2 carbons plus co-enzyme acetly co-A that 2 carbon acetyl group joins to a 4 carbon molecule called oxaloacetate right at the beginning of the Krebs cycle forming a 6 carbon molecule called citric acid. Which is why this is sometimes called the citric acid cycle, as that happens as co-enzyme A goes back to pick up another 2 carbon group from a broken down pyruvate and it'll just keeps going back and forth shuttling in the 2 carbon groups that came ultimately from the glucose that entered the cell.
You pluck off a couple of more carbon dioxides and every time you do that you pull out some high energy electrons putting them on to those high energy carries called NAD positive turning them to NADH. Ultimately in one of these steps you can make a little bit of ATP every pyruvate that enters generates one ATP so because every glucose 6 carbon molecule creates 2 pyruvates, 1 ATP another ATP. Ultimately you'll also wind up in this last part where you regenerate in that 4 carbon molecule that you began with right here called oxaloacetate during this regeneratory process you actually do make some FADH2 from an electron carrier called FAD now you'll wind up making a lot more NADH than the FADH2 but don't worry it's just a fad alright so that's the Krebs cycle. Pyruvate comes in gets ripped apart you put the high energy electrons that are helping hold the pyruvate together onto our NAD positive making NADH a couple of them go onto FADH2 and you make a little bit of ATP.
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