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Quizzes > High School Quizzes > Science

AP Biology Krebs Cycle Practice Quiz

Boost quiz performance and clarify energy pathways

Difficulty: Moderate
Grade: Grade 12
Study OutcomesCheat Sheet
Colorful paper art promoting a Krebs cycle trivia quiz for high school and early college students.

What is the primary function of the Krebs cycle?
To break down fatty acids exclusively
To replicate DNA
To produce high-energy electron carriers and ATP
To synthesize proteins
The Krebs cycle produces high-energy electron carriers like NADH and FADH2 along with a small amount of ATP. These products later fuel the electron transport chain for further ATP production.
What is the location of the Krebs cycle in eukaryotic cells?
Nucleus
Cytoplasm
Endoplasmic reticulum
Mitochondrial matrix
The Krebs cycle takes place in the mitochondrial matrix where critical enzymatic reactions occur. Other cellular compartments are involved in different metabolic processes.
Which molecule is the starting substrate that combines with oxaloacetate to form citrate in the Krebs cycle?
Pyruvate
Lactate
Acetyl-CoA
Glucose
Acetyl-CoA combines with oxaloacetate to begin the Krebs cycle by forming citrate. This reaction is fundamental in linking glycolysis to the cycle.
During one turn of the Krebs cycle, how many molecules of carbon dioxide are released?
Two molecules
Three molecules
One molecule
Four molecules
In one complete turn of the Krebs cycle, two molecules of CO2 are produced during the decarboxylation reactions. This release of CO2 is essential for regenerating oxaloacetate.
What is the role of NADH produced in the Krebs cycle?
It serves as a building block for proteins
It transports oxygen within the cell
It carries electrons to the electron transport chain
It directly produces large amounts of ATP
NADH acts as a carrier for high-energy electrons to the electron transport chain, where these electrons are used to generate ATP. The other options do not correctly define its role in cellular respiration.
Which enzyme catalyzes the first step of the Krebs cycle, the formation of citrate from acetyl-CoA and oxaloacetate?
Aconitase
Citrate synthase
Malate dehydrogenase
Isocitrate dehydrogenase
Citrate synthase catalyzes the condensation of acetyl-CoA and oxaloacetate to form citrate. This irreversible step is the first committed step in the cycle.
During the Krebs cycle, which steps are involved in the decarboxylation process that releases CO2?
Citrate synthase and aconitase reactions
Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase reactions
Succinate dehydrogenase and fumarase reactions
Malate dehydrogenase and aconitase reactions
The decarboxylation steps that release CO2 occur during the reactions catalyzed by isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. These steps are key for reducing NAD+ to NADH.
How many NADH molecules are produced per turn of the Krebs cycle?
One NADH molecule
Three NADH molecules
Four NADH molecules
Two NADH molecules
Each turn of the Krebs cycle produces three NADH molecules, which are essential for carrying electrons to the electron transport chain. These carriers help maximize ATP production through oxidative phosphorylation.
Which enzyme is responsible for the formation of FADH2 in the Krebs cycle?
Malate dehydrogenase
Succinate dehydrogenase
Isocitrate dehydrogenase
Aconitase
Succinate dehydrogenase catalyzes the conversion of succinate to fumarate, during which FAD is reduced to FADH2. This step is unique as it is part of both the Krebs cycle and the electron transport chain.
What happens to the acetyl-CoA's two carbon atoms during the Krebs cycle?
They remain in the cycle as citrate
They form part of NADH
They are released as CO2
They are used to capture oxygen molecules
The two carbon atoms from acetyl-CoA are eventually released as carbon dioxide during the decarboxylation steps of the Krebs cycle. This release is essential for regenerating the oxaloacetate needed for another cycle.
How do the products of the Krebs cycle contribute to oxidative phosphorylation?
ATP directly fuels electron transport chain complexes
CO2 enters the electron transport chain to drive ATP synthesis
Citrate activates ATP synthase
NADH and FADH2 donate electrons to the electron transport chain
NADH and FADH2 produced in the Krebs cycle carry electrons to the electron transport chain where these electrons help generate a proton gradient for ATP synthesis. This is a critical step in cellular energy production.
During the Krebs cycle, what is regenerated to combine with acetyl-CoA for another cycle?
Oxaloacetate
Malate
Isocitrate
Citrate
Oxaloacetate is regenerated at the end of the Krebs cycle and is essential for the condensation reaction with acetyl-CoA in the subsequent cycle. This regeneration keeps the cycle continuously running.
Which intermediate of the Krebs cycle is commonly utilized in amino acid synthesis?
Citrate
α-Ketoglutarate
Succinate
Fumarate
α-Ketoglutarate serves as a precursor for the synthesis of several amino acids, linking the Krebs cycle with biosynthetic processes within the cell. This connection illustrates the cycle's important role in both energy production and cellular building block formation.
The activity of the Krebs cycle is primarily regulated by which of the following mechanisms?
Direct activation by CO2
A constant rate unaffected by the cell's energy state
Inhibition by glucose levels
Feedback inhibition by high concentrations of ATP and NADH
High levels of ATP and NADH signal that the cell has sufficient energy, which inhibits key enzymes in the Krebs cycle through feedback inhibition. This regulation ensures that the cycle matches the cell's metabolic needs.
Which process provides the acetyl-CoA that enters the Krebs cycle?
Glycolysis
Beta-oxidation of fats exclusively
Photosynthesis
DNA replication
Glycolysis produces pyruvate, which is then converted into acetyl-CoA by the pyruvate dehydrogenase complex. While fatty acid oxidation also produces acetyl-CoA, glycolysis is the primary metabolic pathway connecting to the Krebs cycle.
How might inhibition of isocitrate dehydrogenase affect the Krebs cycle?
It would speed up the cycle by bypassing a key regulatory step
It would slow down the cycle by reducing NADH production and CO2 release
It would have no effect on the cycle since other enzymes compensate
It would increase the production of ATP in the cycle
Inhibition of isocitrate dehydrogenase reduces the conversion of isocitrate to α-ketoglutarate, which in turn decreases NADH production and CO2 release. This slowdown affects the overall efficiency of the Krebs cycle and diminishes ATP production indirectly.
What metabolic pathway is typically upregulated in response to impaired Krebs cycle activity to help meet a cell's energy needs?
Anaerobic glycolysis
Pentose phosphate pathway
Urea cycle
Fatty acid synthesis
When the Krebs cycle is compromised, cells often increase anaerobic glycolysis to generate ATP without relying on mitochondrial processes. Although this pathway yields less ATP per glucose, it provides a quick solution to energy shortages.
How do anaplerotic reactions support the Krebs cycle when intermediates are drawn off for biosynthesis?
They permanently remove intermediates from the cycle
They convert ATP directly to NADH
They replenish cycle intermediates, ensuring the cycle continues
They initiate the electron transport chain independently
Anaplerotic reactions refill the depleted intermediates of the Krebs cycle that are diverted for biosynthesis. This replenishment is crucial for maintaining continuous cycle function and overall metabolic balance.
What is a characteristic feature of the pyruvate dehydrogenase complex that links glycolysis to the Krebs cycle?
It directly synthesizes ATP from pyruvate without producing NADH
It bypasses the release of carbon dioxide entirely
It consists of multiple enzyme subunits that work together to convert pyruvate into acetyl-CoA
It operates independently as a single enzyme molecule
The pyruvate dehydrogenase complex is a multifunctional enzyme made up of several subunits that together convert pyruvate into acetyl-CoA, producing NADH and releasing CO2 in the process. This complex serves as a critical junction between glycolysis and the Krebs cycle.
A deficiency in which Krebs cycle enzyme is most likely to be associated with a buildup of α-ketoglutarate and subsequent metabolic disorders?
α-Ketoglutarate dehydrogenase
Succinate dehydrogenase
Malate dehydrogenase
Citrate synthase
A deficiency in α-ketoglutarate dehydrogenase prevents the conversion of α-ketoglutarate to succinyl-CoA, leading to its accumulation. This buildup can disrupt normal cellular metabolism and is associated with metabolic disorders.
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Study Outcomes

  1. Understand the sequence of reactions and key enzymes involved in the Krebs cycle.
  2. Describe the role of the Krebs cycle in cellular respiration and energy production.
  3. Analyze the biochemical transformations and energy transfer processes within the cycle.
  4. Evaluate the significance of metabolic byproducts generated during the cycle.

AP Biology Krebs Cycle Cheat Sheet

  1. Energy Factory of the Cell - The Krebs cycle, also known as the citric acid cycle, is the powerhouse that turns acetyl-CoA into usable energy right inside your mitochondria. It's the heart of cellular respiration, producing the fuel cells need to keep you moving. Dive into BYJU's guide
  2. Citrate Kickoff - Every cycle starts when acetyl-CoA teams up with oxaloacetate to form citrate, kicking off a chain of enzyme-driven transformations. This clever regeneration of oxaloacetate means the cycle can run again and again, churning out energy. Explore more on Wikipedia
  3. Powerhouse Products - From one glucose molecule, the cycle gifts you 2 ATP, 6 NADH, 2 FADH₂, and 4 CO₂ - molecules essential for the next energy-harvesting steps. Think of NADH and FADH₂ as the VIP passes to the electron transport chain. Check Pearson's breakdown
  4. Eight-Step Adventure - Eight distinct stages, each guided by unique enzymes like citrate synthase and isocitrate dehydrogenase, map out the full journey from citrate back to oxaloacetate. Mastering these steps is like having the cycle's recipe memorized. Read BYJU's detailed steps
  5. Mnemonic Magic - Remember "Can I Keep Selling Substances For Money, Officer?" to lock down citrate, isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate. A catchy phrase can turn a daunting list into a memory breeze. Unlock the mnemonic
  6. Amphibolic Switch - The Krebs cycle moonlights as both a breakdown (catabolism) and building (anabolism) tool, feeding metabolic pathways in both directions. It's the ultimate metabolic multitasker. Discover BYJU's insights
  7. Enzyme Gatekeepers - Enzymes like isocitrate dehydrogenase and α-ketoglutarate dehydrogenase act as traffic lights, speeding up or slowing down the cycle based on your cell's energy needs. Understanding their regulation is key to grasping metabolic control. Learn enzyme regulation
  8. Electron Donation Heroes - The NADH and FADH₂ you collect here donate electrons to the electron transport chain, supercharging ATP production through oxidative phosphorylation. Think of them as the ultimate energy couriers. See Pearson's explanation
  9. Building Block Precursor - Intermediates from the cycle double as raw materials for making amino acids, nucleotides, and more - showing just how central the Krebs cycle is to all of metabolism. It's not just an energy loop; it's a biochemical hub. Explore metabolic pathways
  10. Metabolic Mastery - Grasping the Krebs cycle is your ticket to understanding how cells generate energy and interconnect metabolic routes, laying the foundation for advanced biology and medical studies. Master this, and you've unlocked a core secret of life's chemistry. Master the cycle with BYJU's
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