As you eat food every day, you can easily feel the digestive process and the renewal of energy that results. What you can’t feel, however, are the multiple chemical processes that take nutrition from food to energy. By now, you’re likely already aware that the process of glycolysis breaks down sugars into usable products that can be chemically converted into energy. In this AP Biology Crash Course, we’ll talk about the Krebs Cycle, the process by which those products produce ATP.
The Krebs Cycle (also known as the Citric Acid Cycle or Tricarboxylic Acid Cycle) is the metabolic process by which aerobic organisms produce energy in the form of ATP (adenosine triphosphate). The process takes place within the mitochondrial matrix—the liquid found inside the mitochondrion of the cell—and comprises multiple sequential chemical reactions that release products like carbon dioxide, FADH2,ATP,and NADH.
We’ll discuss these reactions and products as they pertain to AP Biology in more detail later. Before we get into the nitty-gritty of the chemistry involved, however, let’s go back in time—way back—to prehistoric Earth.
From Prehistoric Origins
Imagine life on Earth in its earliest stages: simple, single-celled organisms, like bacteria, respiring and creating changes in the planet’s atmosphere. But how did these organisms function and create energy for themselves? We know that today’s aerobic life relies on the Krebs Cycle for metabolism, so it’s hypothesized that this cycle developed far, far back—perhaps in the so-called “primordial soup” of Earth’s past.
The Krebs Cycle is hypothesized to have potentially originated from anaerobic bacteria. Scientists speculate that these bacteria have been using glycolysis to create energy from as early as 3.5 billion years ago, when there was insufficient oxygen for what we imagine to be “normal” respiration. Once oxygen became more readily available in the atmosphere (approximately 2.7 billion years ago), processes like photosynthesis began to develop. The Krebs Cycle itself is somewhat new to organic life—it’s thought to have appeared around 1.5 billion years ago, when eukaryotes finally developed cellular respiration.
We won’t go too far into the details of life’s beginnings in this AP Biology Crash Course, but it’s important to know that the Krebs Cycle is thought to have come from the random, regular reactions of chemical monomers in the early stages of organic matter. Once this process had developed and begun producing ATP, living organisms would have been capable of forming and producing energy.
To learn more about Earth’s early environment and the beginnings of organic life, be sure to check out our Abiogenesis AP Biology Crash Coursewhen you’re finished with this one!
Chemistry of the Krebs Cycle
As you know by now, cellular respiration isn’t as simple as “sugar goes in, energy comes out.” Since this is AP Biology and not AP Chemistry, you won’t need to know the exact processes of chemical synthesis and breakdown involved in each stage of the Krebs Cycle, but you will need to have an understanding of how inputs change into outputs while catalyzing other processes along the way.
Look at the following diagram for the overall process of the Krebs Cycle (and stay calm)! We’ll break it down step-by-step after the graphic.
Image Source: Wikipedia
It may look overwhelming at first, but if we break the Krebs Cycle down into its component steps, it’s not so difficult. Let’s take it one reaction at a time, and feel free to refer to the diagram again to get a visual idea of each step.
1. Acetyl CoA reacts with oxaloacetic acid (OAA) to form citrate.
2. Citrate loses two carboxyl groups, releasing CO2and becoming
3. Isocitrate is oxidized, releasing CO2. At this stage, NAD+ is converted to NADH.
4. α-ketoglutarate is oxidized, again converting NAD+ to NADH and releasing CO2. The molecule that remains then becomes succinyl CoA.
5. Succinyl CoA loses its CoA and picks up a phosphate group. This phosphate group transfers to ADP to produce ATP.
6. Succinate is oxidized and becomes fumarate. Two hydrogen atoms then transfer to FAD to convert it to FADH2.
7. H2O attaches to fumarate, converting it into malate.
8. At the end of the cycle, OAA has been reformed, and one more molecule of NAD+ is converted to NADH. The OAA is then fed back into the beginning step to continue the cycle.
Note that these steps comprise one “turn” of the Krebs cycle. To break down the two acetyl CoA molecules produced from a single glucose molecule, two turns of the cycle are required.
While it might sound abstract when described in this way, the result of the Krebs Cycle is easy to see in daily life; the CO2 that is released from the process is the same CO2 that we exhale when breathing.
Now, we have an idea of the overall steps of the Krebs Cycle. This isn’t an AP Chemistry crash course, however, so it may seem somewhat obscure to say that “ADP becomes ATP.” In the next section, let’s take a closer look at the process by which energy is actually created from this cycle.
The steps of the cycle aren’t all you’ll need to know for the AP Biology test.Chemiosis—or chemiosmosis—is the primary mechanism of energy coupling and is, therefore, responsible for producing the actual ATP in the respiration process.
Chemiosis takes place within the membrane of the mitochondrion, where protons are pumped through channels to produce what is known as a proton gradient. Loose protons (or hydrogen atoms) diffuse down this gradient via ATP synthase, a transport protein that helps them cross through the mitochondrial membrane. This flow of hydrogen ions is what catalyzes the pairing of phosphate with ADP to form the ATP produced in cellular respiration.
AP Bio Exam Review &Practice
• The Krebs Cycle, also known as the Citric Acid Cycle or Tricarboxylic Acid Cycle, is a series of chemical reactions that produce ATP as part of the metabolism of aerobic organisms. It takes place after glycolysis and is a key element of cellular respiration.
• The Krebs Cycle is believed to have developed during the earliest stages of organic matter (around 1.5 billion years ago), possibly as part of the basis for life on Earth.
• The steps of the Krebs Cycle are as follows:
1. Acetyl CoA + oxaloacetic acid (OAA) formcitrate.
2. Citrate becomes isocitrate.
3. Isocitrate is oxidized. CO2is released, and NAD+ is converted to NADH.
4. α-ketoglutarate is oxidized. CO2 is released, and NAD+ is converted to NADH. Succinyl CoA remains.
5. Succinyl CoA loses CoA and gains a phosphate group, which transfers and converts ADP to ATP.
6. Succinate becomes fumarate. FAD is converted to FADH2.
7. H2O converts fumarate into malate.
8. NAD+ is converted to NADH and OAA reforms to be fed back into the cycle.
• Each glucose molecule produces two molecules of acetyl CoA; therefore, two turns of the Krebs Cycle are required to break down a single molecule of glucose (per-glucose yield is twice the amount of products from a single turn of the cycle).
• Chemiosis is the process by which energy is coupled, and ATP is produced from ADP. The transport protein ATP Synthase allows the transport of protons/hydrogen atoms across the mitochondrial membrane, which creates a proton gradient. The flow of hydrogen ions down this gradient is what catalyzes the attachment of phosphate to ADP to produce ATP.
Well, that’s it. The Krebs Cycle might have seemed daunting at first, but it wasn’t as bad as you expected, was it? If you haven’t quite got it down yet, don’t give up! This AP Biology Crash Course isn’t going anywhere, and you can always continue to review the steps of the cycle until you know it.
If you think you’re ready to take on the AP Bio Exam, however, give this practice question a try:
Q: Describe the purpose of the Krebs Cycle and name its inputs and products.
A: The Krebs Cycle serves as a major metabolic process in aerobic organisms and produces chemical energy from sugars. Inputs include two Acetyl CoA molecules (from a single glucose molecule), and outputs include ATP, CO2, FADH2, and NADH.
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.