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Quizzes > High School Quizzes > Science

Krebs Cycle Practice Quiz

Challenge yourself with citric acid and TCA cycles

Difficulty: Moderate
Grade: Grade 12
Study OutcomesCheat Sheet
Paper art promoting Citric Cycle Challenge interactive quiz for high school biology students.

What is the primary purpose of the citric acid cycle?
To store genetic information
To oxidize acetyl-CoA and produce electron carriers like NADH and FADH2
To directly synthesize proteins
To convert glucose into pyruvate
The citric acid cycle oxidizes acetyl-CoA to generate reducing equivalents essential for ATP production through the electron transport chain. This process is a cornerstone of cellular respiration.
Where in the eukaryotic cell does the citric acid cycle take place?
Nucleus
Mitochondrial matrix
Endoplasmic reticulum
Cytoplasm
The citric acid cycle occurs in the mitochondrial matrix, which provides the necessary environment for the enzymes involved. This location also facilitates a direct connection to the electron transport chain.
Which molecule combines with oxaloacetate to begin the citric acid cycle?
CO2
Acetyl-CoA
Glucose
Pyruvate
Acetyl-CoA condenses with oxaloacetate to form citrate, initiating the citric acid cycle. This reaction is essential for oxidative metabolism and energy production.
What is the first product formed when acetyl-CoA reacts with oxaloacetate in the citric acid cycle?
α-Ketoglutarate
Citrate
Isocitrate
Succinate
The enzyme citrate synthase catalyzes the condensation of acetyl-CoA and oxaloacetate to form citrate. This marks the first committed step of the citric acid cycle.
Which step in the citric acid cycle directly produces NADH?
Conversion of citrate to isocitrate
Conversion of succinyl-CoA to succinate
Conversion of isocitrate to α-ketoglutarate
Conversion of malate to acetyl-CoA
The conversion of isocitrate to α-ketoglutarate, catalyzed by isocitrate dehydrogenase, produces NADH and releases CO2. This irreversible, regulated step is crucial for controlling the cycle's rate.
Which enzyme catalyzes the condensation of acetyl-CoA and oxaloacetate to form citrate?
Isocitrate dehydrogenase
Citrate synthase
Succinyl-CoA synthetase
Aconitase
Citrate synthase is the enzyme responsible for catalyzing the first step of the citric acid cycle. It condenses acetyl-CoA with oxaloacetate to form citrate, committing the substrate to further oxidation.
What is the role of aconitase in the citric acid cycle?
It converts citrate into isocitrate
It decarboxylates α-ketoglutarate
It produces ATP directly
It synthesizes succinyl-CoA
Aconitase catalyzes the isomerization of citrate to isocitrate via a cis-aconitate intermediate. This rearrangement is necessary to position the molecule for subsequent oxidation reactions.
How many carbon atoms are in the citrate molecule formed in the citric acid cycle?
Seven
Six
Five
Four
Citrate is formed by combining a 4-carbon oxaloacetate with a 2-carbon acetyl-CoA, resulting in a 6-carbon molecule. This carbon count is a fundamental aspect of the cycle's stoichiometry.
Which compound is released as a byproduct during the conversion of isocitrate to α-ketoglutarate?
H2O
CO2
FADH2
O2
During the reaction catalyzed by isocitrate dehydrogenase, CO2 is released as a decarboxylation byproduct. This step helps drive the reaction forward by removing a product from the equilibrium.
Which step of the citric acid cycle produces ATP or GTP through substrate-level phosphorylation?
Conversion of malate to oxaloacetate
Conversion of isocitrate to α-ketoglutarate
Formation of citrate
Conversion of succinyl-CoA to succinate
The conversion of succinyl-CoA to succinate is the only substrate-level phosphorylation step in the citric acid cycle, generating GTP (or ATP directly in some tissues). This reaction provides a direct means of producing high-energy phosphate bonds.
Which molecule is produced during the oxidation of malate to oxaloacetate?
NADH
CO2
FADH2
ATP
In the reaction catalyzed by malate dehydrogenase, malate is oxidized to form oxaloacetate while reducing NAD+ to NADH. NADH then contributes electrons to the electron transport chain for ATP synthesis.
What are the main electron carriers generated by the citric acid cycle?
ATP and Coenzyme A
NADH and FADH2
FADH2 and Coenzyme Q
NADPH and ATP
The citric acid cycle produces NADH and FADH2, which are crucial for delivering electrons to the electron transport chain. These carriers enable the production of a large amount of ATP during oxidative phosphorylation.
Which molecule is regenerated by the citric acid cycle to allow continuous cycling?
Succinate
Acetyl-CoA
Oxaloacetate
Citrate
Oxaloacetate is both consumed at the start and regenerated at the end of the citric acid cycle, ensuring that the cycle can continue. Its regeneration is essential for the sustained oxidation of acetyl-CoA.
How many molecules of CO2 are released during one complete turn of the citric acid cycle?
Three
One
Two
Four
In one complete turn of the cycle, two decarboxylation reactions release two molecules of CO2. This reflects the removal of carbon atoms originally derived from acetyl-CoA.
How does the citric acid cycle contribute to amino acid metabolism?
It converts amino acids directly into glucose.
It exclusively breaks down amino acids for energy.
It is unrelated to amino acid metabolism.
It produces intermediates that serve as precursors for amino acid synthesis.
Several intermediates of the citric acid cycle are diverted for the synthesis of amino acids. This dual role highlights the cycle's importance in both energy production and biosynthetic pathways.
Which reaction in the citric acid cycle is considered rate-limiting and why?
Conversion of isocitrate to α-ketoglutarate, due to its regulation and irreversible decarboxylation.
Conversion of succinyl-CoA to succinate, because it produces ATP.
Conversion of malate to oxaloacetate, because it is inhibited by high NADH levels.
Conversion of citrate to isocitrate, because it is reversible.
The isocitrate dehydrogenase-catalyzed conversion of isocitrate to α-ketoglutarate is a tightly regulated and irreversible step, making it rate-limiting. Its sensitivity to NADH levels ensures that the cycle adapts to the cell's energy status.
How does high cellular ATP level affect the citric acid cycle?
Flux increases as high ATP levels stimulate enzyme activity.
Flux decreases due to the inhibition of key regulatory enzymes like isocitrate dehydrogenase.
Flux becomes erratic, switching between high and low depending on substrate availability.
Flux remains unchanged because the cycle operates at a constant rate.
High ATP levels indicate that the cell's energy demand is low, leading to feedback inhibition of key enzymes in the citric acid cycle. This reduces the overall flux through the cycle to prevent unnecessary energy production.
How is the citric acid cycle modified during gluconeogenesis?
Intermediates are diverted for gluconeogenesis and replenished by anaplerotic reactions.
The cycle runs in reverse to directly produce glucose.
Gluconeogenesis bypasses the cycle entirely by using fatty acid oxidation.
There is no modification; the cycle operates independently of gluconeogenesis.
During gluconeogenesis, key intermediates of the citric acid cycle are siphoned off for the synthesis of glucose. Anaplerotic reactions compensate for this loss by replenishing the cycle's intermediates.
Which metabolic adaptation involving the citric acid cycle is commonly observed in cancer cells?
Enhanced glutaminolysis, where glutamine is converted to α-ketoglutarate to fuel the cycle.
Increased glycolysis with reduced citric acid cycle activity.
Complete shutdown of the citric acid cycle.
Decreased production of NADH to avoid apoptosis.
Many cancer cells adapt by enhancing glutaminolysis, converting glutamine to α-ketoglutarate to fuel the citric acid cycle despite alterations in their metabolic pathways. This adaptation supports rapid cell proliferation and anabolic growth.
Which of the following best explains the role of anaplerotic reactions in the citric acid cycle?
They convert the cycle from an oxidative to a reductive pathway.
They remove excess intermediates from the cycle to prevent overload.
They replenish cycle intermediates that are drained by biosynthetic processes.
They enhance the production of ATP by bypassing substrate-level phosphorylation.
Anaplerotic reactions are essential for replenishing intermediates that are diverted for various biosynthetic purposes. This ensures that the citric acid cycle can continue operating efficiently even when intermediates are withdrawn.
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Study Outcomes

  1. Analyze the sequence of chemical reactions in the citric acid cycle.
  2. Identify key enzymes and their specific roles within the cycle.
  3. Interpret the energy production steps associated with the cycle.
  4. Apply theoretical knowledge to solve practical problems in cellular respiration.
  5. Synthesize connections between the citric acid cycle and overall metabolic processes.

Krebs & Citric Acid Cycle Cheat Sheet

  1. Citric Acid Cycle Fundamentals - Dive into the powerhouse of the mitochondria where acetyl‑CoA is transformed into ATP, NADH, and FADH₂ through a sequence of enzyme‑driven steps. This cycle is your cell's VIP energy factory, remixing macronutrients into fuel. Explore the full cycle
    2. byjus.com
  2. Central Hub of Metabolism - This cycle elegantly ties together carbohydrates, fats, and proteins by feeding all their breakdown products into a single energy‑producing loop. It's like the metabolic Grand Central Station! Wikipedia overview
    3. en.wikipedia.org
  3. Key Step: Citrate Formation - Watch acetyl‑CoA high‑five oxaloacetate to form citrate, kicking off a cascade of transformations that ultimately regenerates oxaloacetate - rinse and repeat! It's the cycle's signature move. Full step breakdown
    4. byjus.com
  4. Energy Payoff per Turn - Every spin of the cycle nets you one ATP (or GTP), three NADH, one FADH₂, and two CO₂ molecules - powerful currency for your cell's energy budget. Keep track of your gains! Numbers and yields
    5. en.wikipedia.org
  5. Feeding the Electron Transport Chain - NADH and FADH₂ drop off high‑energy electrons at the ETC, driving a proton gradient that cranks out even more ATP via oxidative phosphorylation. It's teamwork at its finest! ETC connection
    6. en.wikipedia.org
  6. Cycle Regulation - Enzymes like citrate synthase, isocitrate dehydrogenase, and α‑ketoglutarate dehydrogenase act as molecular switches, responding to ATP, ADP, NADH, and succinyl‑CoA levels to throttle the cycle up or down. Think of them as gatekeepers keeping energy production in check! Fiveable study guide
    7. fiveable.me
  7. Biosynthesis Superhighway - Beyond energy, CAC intermediates moonlight as building blocks for amino acids, glucose, and fatty acids. This dual role makes the cycle a master multitasker in both breakdown and build‑up pathways! Online Sciences article
    8. online-sciences.com
  8. Metabolic Disorder Insights - Cracking the cycle's secrets helps explain conditions like mitochondrial diseases and metabolic syndromes, giving you the biochemical keys to understanding energy‑related disorders. Knowledge is power! SparkNotes summary
    9. sparknotes.com
  9. Mnemonic Magic - Try "Citrate Is Krebs' Starting Substrate For Making Oxaloacetate" to nail the sequence: Citrate, Isocitrate, α‑Ketoglutarate, Succinyl‑CoA, Succinate, Fumarate, Malate, Oxaloacetate. Mnemonics make memorization a breeze! Byju's mnemonics
    10. byjus.com
  10. See It in Action - Visual learners, rejoice! Animations and interactive diagrams show the cycle's choreography inside the mitochondrial membrane so you can watch each step come to life. BioInteractive animation
    11. biointeractive.org
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