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Example Questions
Example Question #2 : Reactants And Products Of The Citric Acid Cycle
Which of the following molecules involved in the citric acid cycle initiates the cycle by combining with acetyl-CoA?
Fumarate
Succinate
Malate
Citrate
Oxaloacetate
Oxaloacetate
The first step in the citric acid cycle is for acetyl-CoA to react with oxaloacetate. This forms citrate, which then continues through the cycle, ultimately reforming the oxaloacetate molecule to redo the cycle.
Example Question #231 : Catabolic Pathways And Metabolism
The pyruvate dehydrogenase complex creates acetyl-CoA from pyruvate. What other molecule is a product of this reaction?
Pyruvate is converted to acetyl-CoA by the pyruvate dehydrogenase complex. Carbon dioxide is released during this reaction, and in addition to this, is reduced to .
Example Question #232 : Catabolic Pathways And Metabolism
In what part of the cell does the pyruvate dehydrogenase complex function?
The mitochondrial matrix
Cytoplasm
The mitochondrial intermembrane
In the nucleus
Within the inner membrane of the mitochondria
The mitochondrial matrix
The pyruvate dehydrogenase complex (PDC) is preparing pyruvate for the Krebs cycle by converting it to acetyl-CoA. Because the Krebs cycle functions within the mitochondrial matrix, the PDC is also taking place there. This ensures quick and easy movement from the PDC into the Krebs cycle.
Example Question #233 : Catabolic Pathways And Metabolism
Which process involved in cellular respiration produces the largest quantity of high energy electron carriers?
Pyruvate dehydrogenase complex
ATP synthase
Glycolysis
Electron transport chain
Krebs cycle
Krebs cycle
The Krebs cycle produces the most high energy electron carriers of any process involved in cellular respiration. Per glucose molecule, the Krebs cycle produces and .
Example Question #41 : Citric Acid Cycle
Which of the following molecules enter the Krebs cycle directly, following glycolysis?
Acetyl-CoA
Pyruvate
Ubiquinone
Glucose
Pyruvate dehydrogenase
Acetyl-CoA
Ubiquinone is a part of the electron transport chain, and has little to do with the Krebs cycle. Glucose is broken down during glycolysis, and does not enter the Krebs cycle directly. Many students make the mistake of thinking that pyruvate enters the Krebs cycle, since it is produced in glycolysis, and the Krebs cycle follows glycolysis. However, pyruvate is first converted to acetyl-CoA by the pyruvate dehydrogenase complex in the mitochondrial matrix, and acetyl-CoA enters the Krebs cycle.
Example Question #15 : Reactants And Products Of The Citric Acid Cycle
What is the role of isocitrate dehydrogenase in the citric acid cycle of the mitochondria?
Isocitrate dehydrogenase converts citrate to alpha-ketoglutarate and is inhibited by
Isocitrate dehydrogenase converts alpha-ketoglutarate to isocitrate and is activated by
Isocitrate dehydrogenase converts isocitrate to alpha-ketoglutarate and is inhibited by
Isocitrate dehydrogenase converts citrate to isocitrate and is inhibited by
Isocitrate dehydrogenase converts citrate to isocitrate and is activated by
Isocitrate dehydrogenase converts isocitrate to alpha-ketoglutarate and is inhibited by
Isocitrate dehydrogenase activation leads to oxidative decarboxylation of isocitrate in a two step process producing alpha-ketoglutarate and . In the mitochondria, the reaction produces also a charged electron carrier molecule, , from . Isocitrate dehydrogenase, inhibited by and activated by , is a major regulator enzyme of the citric cycle.
Example Question #16 : Reactants And Products Of The Citric Acid Cycle
Which of the following steps within the citric acid cycle results in the production of a molecule of carbon dioxide ?
The conversion of malate to oxaloacetate
The conversion of succinyl-CoA to succinate
The conversion of isocitrate to alpha-ketoglutarate
The conversion of succinate to fumarate
The conversion of fumarate to malate
The conversion of isocitrate to alpha-ketoglutarate
The only step of the citric acid cycle listed that results in the production of as a side product is the conversion of isocitrate to alpha-ketoglutarate. In this step, the enzyme, isocitrate dehydrogenase catalyzes the conversion of isocitrate to alpha-ketoglutarate, while also converting to and as side products, and generating a molecule of in the process (i.e. reducing the carbon count from 5 in isocitrate to 4 in alpha-ketoglutarate).
The conversion of alpha-ketoglutarate to succinyl-CoA also produces a molecule of as a side product. However, this step is not listed as an answer choice.
None of the other answer choices listed produce as side products.
Example Question #241 : Catabolic Pathways And Metabolism
In which of the following cases would the citric acid cycle be downregulated?
Lower levels of NADH
High levels of ATP
High levels of ADP
Increased amounts of
High levels of ATP
The purpose of the citric acid cycle is to produce energy (both directly via GTP, and indirectly via NADH and . As such, energy can be though of to be on the products side of the sum of the reactions of the Krebs cycle. From Le Chatelier's principle, we know that if we want to inhibit a forward reaction, we can increase the concentration of the products. This will inhibit the forward reaction, and push the equilibrium to the left. Thus, in a high energy state, the ratio of ATP:ADP, like that of NADH: is high since both ATP and NADH are products of metabolism.
Example Question #242 : Catabolic Pathways And Metabolism
Why can't acetyl-CoA produced from beta-oxidation enter into the Krebs Cycle without carbohydrates present?
Acetyl-CoA can enter into the Krebs cycle without carbohydrates present
Acetyl-CoA combines with oxaloacetate to enter into the Krebs cycle which is a carbohydrate
Downstream of the Krebs cycle, the electron transport chain can not function without carbohydrates present, which in turn causes the Krebs cycle to stop functioning
None of these
Acetyl-CoA must first be activated by a carbohydrate before entering into the Krebs cycle
Acetyl-CoA combines with oxaloacetate to enter into the Krebs cycle which is a carbohydrate
The entry point for acetyl CoA in the Krebs cycle is oxaloacetate. Acetyl-CoA and oxaloacetate combine to form citrate, which then continues through the cycle. Oxaloacetate is a carbohydrate, and so without carbs the acetyl-CoA can not enter into the Krebs cycle.
Example Question #243 : Catabolic Pathways And Metabolism
Which of the following is not a regulated step of the citric acid cycle?
Citrate synthase
Isocitrate dehydrogenase
Succinate dehydrogenase
None of these
Alpha-ketoglutarate dehydrogenase
Succinate dehydrogenase
The regulated steps of the citric acid cycle are citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. These steps are inhibited and stimulated by various products and reactants within the citric acid cycle. Succinate dehydrogenase is not regulated by products or reactants, and is therefore not rate limiting.
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