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The Krebs 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. Krebs 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 can be defined as an eight-step process occurring in the mitochondrial matrix. Acetyl CoA, derived from carbohydrates, proteins and fats is completely oxidised to release carbon dioxide. In the form of ATP, the energy released is stored. The eight steps involved are – Step 1: First step is the condensation of acetyl CoA with oxaloacetate (4C) to form citrate (6C), coenzyme A is released. The reaction is catalysed by citrate synthase. Step 2: Citrate is turned to its isomer, isocitrate. The enzyme aconitase catalyses this reaction. Step 3: Isocitrate undergoes dehydrogenation and decarboxylation to form 𝝰-ketoglutarate (5C). A molecular of CO2 is released. Isocitrate dehydrogenase catalyses the reaction. It is an NAD+ dependent enzyme. NAD+ is converted to NADH. Step 4: 𝝰-ketoglutarate (5C) undergoes oxidative decarboxylation to form succinyl CoA (4C). The reaction is catalyzed by 𝝰-ketoglutarate dehydrogenase enzyme complex. One molecule of CO2 is released and NAD+ is converted to NADH. Step 5: Succinyl CoA is converted to succinate by the enzyme succinyl CoA synthetase. This is coupled with substrate-level phosphorylation of GDP to form GTP. GTP transfers its phosphate to ADP forming ATP. Step 6: Succinate is oxidised to fumarate by the enzyme succinate dehydrogenase. In the process, FAD is converted to FADH2. Step 7: Fumarate gets converted to malate by 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. Q1. The Product Essential for Oxidative Phosphorylation in the Krebs Cycle is _______ (a) NADPH and ATP (b) Acetyl CoA (c) CO2 and oxaloacetate (d) NADH and FADH2 Answer: (d) Q2. A Single Molecule of Glucose Generates _________ Molecules of Acetyl CoA, Which Enters the Krebs Cycle. (a) 4 (b) 3 (c) 2 (d) 1 Answer: (c) Q3. ___________ Accepts Hydrogen from Malate (a) FAD (b) NAD (c) NADP (d) FMN Answer: (b) Q4. Which of the Intermediate of the Krebs Cycle Is Utilised in the Formation of Amino Acids (a) Citric acid (b) Malic acid (c) Isocitric acid (d) 𝛼-ketoglutaric acid Answer: (d) Q5. Krebs Cycle Takes Place in Aerobic Respiration Due To (a) Electron transport chain requires aerobic conditions to operate (b) Oxygen is a reactant (c) Oxygen has a catalytic function (d) All of the above Answer: (a) Q6. Acetyl CoA is Formed from Pyruvate By__________ Reaction (a) Dehydration (b) Reduction (c) Oxidative decarboxylation (d) Dephosphorylation Answer: (c) Q7. Which of the Following is Not Formed During the Krebs Cycle (a) Lactate (b) Isocitrate (c) Succinate (d) Both (a) and (b) Answer: (a) Q8. The Arrival of Pyruvate Into the TCA Cycle Is Inhibited by the Presence of a High Cellular Concentration Of (a) Pyruvate (b) NADH (c) Coenzyme A (d) AMP Answer: (b) Q9. ATP Synthase is Powered By (a) Coenzyme motive force (b) CAMP (c) Proton gradient (d) GTP hydrolysis Answer: (c) Q10. FAD is Reduced in Which of the Reactions of the Kreb’s Cycle? (a) Isocitrate to oxaloacetate (b) Succinyl CoA to Succinate (c) Fumarate to malate (d) Succinate to fumarate Answer: (d)

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