Skip to content Get cutting-edge science videos from JoVE sent straight to your inbox every month. What type of acid do these fruits contain? Citric acid. Citric acid is also the first product formed in the Krebs cycle, and therefore this acid occurs in the metabolism of virtually all living things.
Recall that glycolysis, stage I of cellular respiration, produces two molecules of pyruvate. These molecules enter the matrix of a mitochondrion, where they start the Krebs cycle. The reactions that occur next are shown in Figure below. You can watch an animated version at this link: http://www.youtube.com/watch?v=p-k0biO1DT8. The Krebs cycle starts with pyruvic acid from glycolysis. Each small circle in the diagram represents one carbon atom. For example, citric acid is a six carbon molecule, and OAA (oxaloacetate) is a four carbon molecule. Follow what happens to the carbon atoms as the cycle proceeds. In one turn through the cycle, how many molecules are produced of ATP? How many molecules of NADH and FADH2 are produced? Before the Krebs cycle begins, pyruvic acid, which has three carbon atoms, is split apart and combined with an enzyme known as CoA, which stands for coenzyme A. The product of this reaction is a two-carbon molecule called acetyl-CoA. The third carbon from pyruvic acid combines with oxygen to form carbon dioxide, which is released as a waste product. High-energy electrons are also released and captured in NADH.
The Krebs cycle itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure above). This produces citric acid, which has six carbonatoms. This is why the Krebs cycle is also called the citric acid cycle. After citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2, another energy-carrying compound. Carbon dioxide is also released as a waste product of these reactions. The final step of the Krebs cycle regenerates OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvic acid molecules when it splits glucose. Watch the OSU band present the Krebs cycle: http://www.youtube.com/watch?v=FgXnH087JIk.
After the second turn through the Krebs cycle, the original glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its chemical bonds has been stored in a total of 16 energy-carrier molecules. These molecules are:
The Krebs cycle is reviewed at http://www.youtube.com/watch?v=juM2ROSLWfw.
Use this resource to answer the questions that follow.
This page titled 2.28: Krebs Cycle is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request. LICENSED UNDER
If you're seeing this message, it means we're having trouble loading external resources on our website. If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.
The Krebs Cycle, also called the citric acid cycle, is the second major step in oxidative phosphorylation. After glycolysis breaks glucose into smaller 3-carbon molecules, the Krebs cycle transfers the energy from these molecules to electron carriers, which will be used in the electron transport chain to produce ATP. Most organisms use glucose as a major fuel source, but must break down this glucose and store the energy in ATP and other molecules. The Krebs cycle is contained within mitochondria. Within the mitochondrial matrix, the reactions of the Krebs cycle adds electrons and protons to a number of electron carriers, which are then used by the electron transport chain to produce ATP. The Krebs cycle starts with the products of glycolysis, which are two three-carbon molecules known as pyruvate. This molecule is acidic, which is why the Krebs cycle is also called the tricarboxylic acid cycle (TCA). Throughout a number of reactions, these molecules are further broken down into carbon dioxide. Energy from the molecules is moved to other molecules, called electron carriers. These molecules carry the stored energy to the electron transport chain, which in turn creates ATP. Then, the cell uses this ATP to power various cellular reactions, such as the activation of enzymes or transport proteins. The Krebs cycle is the second of 4 different processes which must happen to extract the energy from glucose. Altogether, the Krebs cycle consists of 9 sequential reactions. The first step of utilizing glucose, glycolysis, produces a few ATP as well as the molecules which will be processed with the Krebs cycle. During glycolysis, a single glucose molecule is split into two smaller, three-carbon molecules called pyruvate. Pyruvate is then converted to acetyl CoA. Acetyl CoA is then utilized within the Krebs cycle to produce several major products. In turn, these products then drive the formation of ATP, the cell’s main energy source. Before the first stages of the Krebs cycle, pyruvate is converted into acetyl CoA. During this process, one molecule of CO2 and one molecule of the electron carrier NADH are produced. The Krebs cycle involves converting this acetyl CoA into carbon dioxide. During the steps of the cycle, two molecules of CO2 are released, in addition to 3 more molecules of NADH, one of FADH2, and one of GTP. So, for every 1 pyruvate molecule added, the Krebs cycle will produce:
A molecule of glucose contains 2 pyruvate molecules, so 1 glucose molecule will produce double the amount of products listed above as it moves through the Krebs cycle. These products will then be converted to ATP in later stages of aerobic respiration. Carbon dioxide is the only “waste” product and must be removed from the cell. Large organisms must remove carbon dioxide from all their cells. In these animals, carbon dioxide is typically exchanged in the gills or lungs for oxygen, which helps drive the final stages of aerobic respiration. The Krebs cycle happens only within the mitochondrial matrix. Pyruvate is formed in the cytosol of the cell, then imported into the mitochondria. Here, it is converted to acetyl CoA and imported into the mitochondrial matrix. The mitochondrial matrix is the innermost part of the mitochondria. The graphic below shows the different parts of mitochondria. The mitochondrial matrix has the required enzymes and environment for the complex reactions of the Krebs cycle to take place. Further, the products of the Krebs cycle drive the electron transport chain and oxidative phosphorylation, both of which occur in the inner mitochondrial membrane. The electron carriers will dump their electrons and protons into the chain, which ultimately drives the production of ATP. This molecule is then exported from the mitochondria as the main energy source for the cell. Mitochondria are found in almost all organisms, especially multicellular organisms. Plants, animals, and fungi all use the Krebs cycle as an indispensable part of aerobic respiration. The Krebs cycle has 9 main reactions, which happen quickly in succession. The image below shows these reactions. Note that citrate is the first molecule created after acetyl CoA is added. This is why the Krebs cycle is also known as the citric acid cycle. The products of the cycle are in the image above. This process is known as a “cycle” because it always ends on oxaloacetate which can be combined with a new acetyl CoA to produce a new molecule of citrate for each cycle. The Krebs cycle is likely the most important part of the process of aerobic respiration because it drives the formation of electron carriers. These carriers are important. They carry the energy used to create a large number of ATP molecules in the final steps of aerobic respiration. The electron carriers produced (NADH and FADH2) cannot provide energy to cellular process directly. Instead, the processes of the electron transport chain and oxidative phosphorylation will use the energy from these molecules to activate the enzyme complex ATP synthase, which produces ATP. |
Two products of the Krebs cycle are
zusammenhängende Posts
Urheberrechte © © 2024 frojeostern Inc.
