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IntroductionThe Krebs cycle or TCA cycle (tricarboxylic acid cycle) or Citric acid cycle is a series of enzyme catalysed reactions occurring in the mitochondrial matrix, where acetyl-CoA is oxidised to form carbon dioxide and coenzymes are reduced, which generate ATP in the electron transport chain. Also see: NEET Key Answer 2022Krebs cycle was named after Hans Krebs, who postulated the detailed cycle. He was awarded the Nobel prize in 1953 for his contribution. It is a series of eight-step processes, where the acetyl group of acetyl-CoA is oxidised to form two molecules of CO2 and in the process, one ATP is produced. Reduced high energy compounds, NADH and FADH2 are also produced. Two molecules of acetyl-CoA are produced from each glucose molecule so two turns of the Krebs cycle are required which yields four CO2, six NADH, two FADH2 and two ATPs. Krebs Cycle is a part of Cellular RespirationCellular respiration is a catabolic reaction taking place in the cells. It is a biochemical process by which nutrients are broken down to release energy, which gets stored in the form of ATP and waste products are released. In aerobic respiration, oxygen is required. Also see: Biochemical Pathways Cellular respiration is a four-stage process. In the process, glucose is oxidised to carbon dioxide and oxygen is reduced to water. The energy released in the process is stored in the form of ATPs. 36 to 38 ATPs are formed from each glucose molecule. The four stages are: 1. Glycolysis: Partial oxidation of a glucose molecule to form 2 molecules of pyruvate. This process takes place in the cytosol. Further reading: Significance of Glycolysis 2. Formation of Acetyl CoA: Pyruvate formed in glycolysis enters the mitochondrial matrix. It undergoes oxidative decarboxylation to form two molecules of Acetyl CoA. The reaction is catalysed by the pyruvate dehydrogenase enzyme. 3. Krebs cycle (TCA cycle or Citric Acid Cycle): It is the common pathway for complete oxidation of carbohydrates, proteins and lipids as they are metabolised to acetyl coenzyme A or other intermediates of the cycle. The Acetyl CoA produced enters the Tricarboxylic acid cycle or Citric acid cycle. Glucose is fully oxidized in this process. The acetyl CoA combines with 4-carbon compound oxaloacetate to form 6C citrate. In this process, 2 molecules of CO2 are released and oxaloacetate is recycled. Energy is stored in ATP and other high energy compounds like NADH and FADH2. 4. Electron Transport System and Oxidative Phosphorylation: ATP is generated when electrons are transferred from the energy-rich molecules like NADH and FADH2, produced in glycolysis, citric acid cycle and fatty acid oxidation to molecular O2 by a series of electron carriers. O2 is reduced to H2O. It takes place in the inner membrane of mitochondria. Also Check: MCQs on Krebs Cycle It is an eight-step process. Krebs cycle or TCA cycle takes place in the matrix of mitochondria under aerobic condition. Step 1: The first step is the condensation of acetyl CoA with 4-carbon compound oxaloacetate to form 6C citrate, coenzyme A is released. The reaction is catalysed by citrate synthase. Step 2: Citrate is converted to its isomer, isocitrate. The enzyme aconitase catalyses this reaction. Step 3: Isocitrate undergoes dehydrogenation and decarboxylation to form 5C 𝝰-ketoglutarate. A molecular form of CO2 is released. Isocitrate dehydrogenase catalyses the reaction. It is an NAD+ dependent enzyme. NAD+ is converted to NADH. Step 4: 𝝰-ketoglutarate undergoes oxidative decarboxylation to form succinyl CoA, a 4C compound. The reaction is catalyzed by the 𝝰-ketoglutarate dehydrogenase enzyme complex. One molecule of CO2 is released and NAD+ is converted to NADH. Step 5: Succinyl CoA forms succinate. The enzyme succinyl CoA synthetase catalyses the reaction. This is coupled with substrate-level phosphorylation of GDP to get GTP. GTP transfers its phosphate to ADP forming ATP. Step 6: Succinate is oxidised by the enzyme succinate dehydrogenase to fumarate. In the process, FAD is converted to FADH2. Step 7: Fumarate gets converted to malate by the addition of one H2O. The enzyme catalysing this reaction is fumarase. Step 8: Malate is dehydrogenated to form oxaloacetate, which combines with another molecule of acetyl CoA and starts the new cycle. Hydrogens removed, get transferred to NAD+ forming NADH. Malate dehydrogenase catalyses the reaction. Krebs Cycle SummaryLocation: Krebs cycle occurs in the mitochondrial matrix Krebs cycle reactants: Acetyl CoA, which is produced from the end product of glycolysis, i.e. pyruvate and it condenses with 4 carbon oxaloacetate, which is generated back in the Krebs cycle Krebs cycle products Each citric acid cycle forms the following products:
Note that 2 molecules of Acetyl CoA are produced from oxidative decarboxylation of 2 pyruvates so two cycles are required per glucose molecule. To summarize, for complete oxidation of a glucose molecule, Krebs cycle yields 4 CO2, 6NADH, 2 FADH2 and 2 ATPs. Each molecule of NADH can form 2-3 ATPs and each FADH2 gives 2 ATPs on oxidation in the electron transport chain. Krebs cycle equation To Sum up Significance of Krebs Cycle
Frequently Asked Questions on Krebs CycleAlso known as the citric acid cycle, the
Krebs cycle or TCA cycle is a chain of reactions occurring in the mitochondria, through which almost all living cells produce energy in aerobic respiration. It uses oxygen and gives out water and carbon dioxide as products. Here, ADP is converted into ATP. This cycle renders electrons and hydrogen required for electron chain transport. 2 ATPs are produced in one Krebs Cycle. Mitochondrial matrix. It is an eight-step process It is called amphibolic as in the Krebs cycle both catabolism and anabolism take place. The amphibolic pathway indicates the one involving both catabolic and anabolic procedures. 3 NADH molecules In one turn of the Krebs cycle, 3 molecules of NADH are produced. Krebs cycle is also known as Citric acid cycle (CAC) or TCA cycle (tricarboxylic acid cycle) Krebs cycle is also referred to as the Citric Acid Cycle. Citric
acid is the first product formed in the cycle. Also Check: NEET Flashcards: Respiration In Plants NEET Flashcards: Breathing And Exchange Of Gases NEET Flashcards: Body Fluids And Circulation NEET Flashcards: Neural Control And Coordination NEET Flashcards: Chemical Coordination And Integration Recommended Video:What is the Krebs cycle and where does it occur?Also known as the citric acid cycle, the Krebs cycle or TCA cycle is a chain of reactions occurring in the mitochondria, through which almost all living cells produce energy in aerobic respiration. It uses oxygen and gives out water and carbon dioxide as products.
What does Krebs cycle occur in the cell?The tricarboxylic acid (TCA) cycle, also known as the Krebs or citric acid cycle, is the main source of energy for cells and an important part of aerobic respiration. The cycle harnesses the available chemical energy of acetyl coenzyme A (acetyl CoA) into the reducing power of nicotinamide adenine dinucleotide (NADH).
Where is Krebs located in the cell?The Krebs cycle uses the two molecules of pyruvic acid formed in glycolysis and yields high-energy molecules of NADH and flavin adenine dinucleotide (FADH2), as well as some ATP. The Krebs cycle occurs in the mitochondrion of a cell (see Figure 6-1).
Where Krebs citric acid cycle occur in a cell?The TCA cycle, also known as the citric acid cycle or Krebs cycle, occurs in the mitochondria and provides large amounts of energy in aerobic conditions by donating electrons to three NADH and one FADH (flavin adenine dinucleotide), which donate electrons to the electron transport chain, creating the proton gradient ...
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