Biology Pointers
Overview of the Krebs Cycle
It generates energy by oxidizing acetyl-CoA into CO₂, producing ATP, NADH,
and FADH₂.
Location: Mitochondrial matrix.
Be clear on its connection: Part of cellular respiration, following glycolysis
and leading to the electron transport chain.
Key Steps to Remember
Acetyl-CoA + Oxaloacetate → Citrate.
Isocitrate → α-Ketoglutarate (NADH and CO₂ produced).
Α-Ketoglutarate → Succinyl-CoA (NADH and CO₂ produced).
Succinyl-CoA → Succinate (ATP or GTP produced).
Succinate → Fumarate (FADH₂ produced).
Malate → Oxaloacetate (NADH produced).
Energy Yield per Turn
3 NADH, 1 FADH₂, 1 ATP (or GTP), and 2 CO₂ per acetyl-CoA.
Regulation
Enzymes: Isocitrate dehydrogenase and α-Ketoglutarate dehydrogenase
are key control points.
Inhibitors: High ATP, NADH, and citrate levels.
Activators: High ADP and NAD⁺ levels.
Exam Strategy
Focus on the flow of carbon atoms and energy (ATP, NADH, FADH₂).
Be prepared to answer how and where the Krebs cycle connects to glycolysis
and the electron transport chain.
Part 1: General Concepts
1. Where does the Krebs cycle take place in the cell?
2. What is the main purpose of the Krebs cycle?
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3. Which molecule combines with acetyl-CoA to form citrate in the first
step?
Part 2: Reactions and Intermediates
4. What are the two reactions in the Krebs cycle where carbon dioxide
(CO₂) is released?
5. Which intermediate is formed after citrate is converted during the
second step?
6. What is the molecule that donates high-energy electrons to FAD in the
cycle?
Part 3: Energy Yield
7. How many NADH molecules are produced in one turn of the Krebs
cycle?
8. What is the total number of ATP (or GTP) molecules produced per
acetyl-CoA?
9. How many molecules of FADH₂ are generated in one turn of the cycle?
Part 4: Regulation
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10. What are the key enzymes regulated in the Krebs cycle?
11. What activates the cycle when the cell needs energy?
12. Which molecule inhibits isocitrate dehydrogenase?
Part 1: General Concepts
1. Where does the Krebs cycle take place in the cell?
a) Cytoplasm
b) Nucleus
c) Mitochondrial matrix
d) Ribosome
2. What is the main purpose of the Krebs cycle?
a) To produce glucose
b) To generate energy carriers like NADH and FADH₂
c) To transport oxygen
d) To break down proteins
3. Which molecule combines with acetyl-CoA to form citrate in the first
step?
a) Malate
b) Oxaloacetate
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c) Fumarate
d) Succinate
Part 2: Reactions and Intermediates
4. What are the two reactions in the Krebs cycle where carbon dioxide (CO₂)
is released?
a) Isocitrate → α-Ketoglutarate and α-Ketoglutarate → Succinyl-CoA
b) Citrate → Isocitrate and Succinate → Fumarate
c) Malate → Oxaloacetate and Succinate → Fumarate
d) Fumarate → Malate and Isocitrate → α-Ketoglutarate
5. Which intermediate is formed after citrate is converted during the second
step?
a) Oxaloacetate
b) Isocitrate
c) Succinyl-CoA
d) Fumarate
6. What is the molecule that donates high-energy electrons to FAD in the
cycle?
a) Malate
b) Citrate
c) Succinate
d) Oxaloacetate
Part 3: Energy Yield
7. How many NADH molecules are produced in one turn of the Krebs cycle?
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a) 2
b) 3
c) 4
d) 6
8. What is the total number of ATP (or GTP) molecules produced per acetyl-
CoA?
a) 1
b) 2
c) 3
d) 4
9. How many molecules of FADH₂ are generated in one turn of the cycle?
a) 1
b) 2
c) 3
d) 4
Part 4: Regulation
10. What are the key enzymes regulated in the Krebs cycle?
a) Citrate synthase and succinate dehydrogenase
b) Isocitrate dehydrogenase and α-Ketoglutarate dehydrogenase
c) Malate dehydrogenase and fumarase
d) Oxaloacetate synthase and citrate kinase
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11. What activates the cycle when the cell needs energy?
a) High levels of ATP and NADH
b) High levels of ADP and NAD⁺
c) High levels of FADH₂ and citrate
d) High levels of oxygen
12. Which molecule inhibits isocitrate dehydrogenase?
a) ADP
b) Citrate
c) ATP
d) NAD⁺
Key Concepts of the Krebs Cycle
Purpose
The Krebs cycle is a series of chemical reactions in cells that generate
energy.
It’s part of cellular respiration and occurs in the mitochondria, the
“powerhouse” of the cell.
Inputs
The cycle starts with acetyl-CoA, a molecule derived from carbohydrates,
fats, or proteins.
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Main Products
ATP: Energy currency of the cell.
NADH and FADH₂: Molecules that carry electrons to the next stage of
respiration.
CO₂: A waste product that is exhaled.
Steps
Acetyl-CoA combines with oxaloacetate to form citrate.
Through a series of reactions, citrate is broken down, producing CO₂ and
capturing energy in the form of NADH, FADH₂, and ATP.
The cycle regenerates oxaloacetate to continue the process.
Energy Yield
Each turn of the cycle produces:
3 NADH
1 FADH₂
1 ATP (or GTP)
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These molecules are used in the electron transport chain to produce more
ATP.
Regulation
The cycle speeds up when the cell needs more energy and slows down when
energy levels are sufficient.
Simplified Explanation for Beginners
Imagine the Krebs cycle as a factory:
Input: Acetyl-CoA enters the factory.
Processing: The factory uses a series of machines (enzymes) to extract
useful energy.
Output: The factory produces ATP (energy packets), NADH and FADH₂
(helpers for energy-making), and CO₂ (waste gas).
The factory resets itself (regenerating oxaloacetate) to process more acetyl-
CoA.
