Krebs Cycle

: Where does the Krebs cycle take place?

In the mitochondrial matrix

What molecule enters the Krebs cycle?

Acetyl-CoA, made from pyruvate during pyruvate oxidation

What are the products of one turn of the Krebs cycle?

3 NADH, 1 FADH₂, 1 ATP (or GTP), 2 CO₂

: What are the total products per glucose (2 turns)

NADH, 2 FADH₂, 2 ATP, 4 CO₂

: What happens to the CO₂ produced?

It’s released as a waste gas — we breathe it out

How is ATP produced in the Krebs cycle?

: By substrate-level phosphorylation

What is the role of NADH and FADH₂?

They carry high-energy electrons to the Electron Transport Chain

What molecule is regenerated at the end of the cycle?

Oxaloacetate (4C) — it combines with new Acetyl-CoA to restart the cycle

Why does the cycle run twice per glucose molecule?

Because glycolysis makes 2 pyruvate → 2 Acetyl-CoA → 2 turns

What is the main function of the Krebs cycle?

To extract energy from Acetyl-CoA and load NADH/FADH₂ with electrons for ATP production later

How is the Krebs cycle linked to glycolysis and the electron transport chain?

Glycolysis produces pyruvate → converted to Acetyl-CoA → enters Krebs cycle → produces NADH/FADH₂ → go to electron transport chain

Explain the purpose of the Krebs cycle in terms of cellular energy production.

The Krebs cycle plays a central role in aerobic respiration by breaking down Acetyl-CoA into CO₂ and producing electron carriers like NADH and FADH₂. These carriers are packed with energy and are used in the electron transport chain to generate large amounts of ATP. Although the Krebs cycle itself only produces 2 ATP per glucose, it provides the majority of the electrons needed for ATP synthesis. It also supplies metabolic intermediates for biosynthesis. Without it, cells would not be able to efficiently convert nutrients into usable energy.

Molecules

Molecule Name	Why It Matters

Acetyl-CoA

Enters the cycle (from pyruvate oxidation)

Citrate

First molecule formed (6C)

Succinyl-CoA

Where ATP is made

NADH / FADH₂

Energy carriers

CO₂	Waste gas (carbon being removed)

Oxaloacetate

Last molecule — cycle restarts here

Why is it important?

• It finishes breaking down glucose

• Loads up NADH and FADH₂ with energy

• That energy gets used to make a ton of ATP in the next step (Electron Transport Chain)

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Citrate → Isocitrate → α-Ketoglutarate → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate

Krebs Cycle – Steps in Point Form:

.

• 1. Acetyl-CoA (2C) combines with Oxaloacetate (4C)
  → Forms Citrate (6C)

• 2. Citrate is rearranged into Isocitrate (6C)

• 3. Isocitrate → α-Ketoglutarate (5C)
  → 1 CO₂ released, 1 NADH made

• 4. α-Ketoglutarate → Succinyl-CoA (4C)
  → 1 CO₂ released, 1 NADH made

• 5. Succinyl-CoA → Succinate (4C)
  → 1 ATP (or GTP) is made

• 6. Succinate → Fumarate (4C)
  → 1 FADH₂ made

• 7. Fumarate → Malate (4C)
  → Water is added

• 8. Malate → Oxaloacetate (4C)
  → 1 NADH made

• Cycle restarts with new Acetyl-CoA

key reactions in Krebs cycle (2C = Acetyl-CoA):

• Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C)

• Citrate → Isocitrate

• Isocitrate → α-Ketoglutarate + CO₂ + NADH

• α-Ketoglutarate → Succinyl-CoA + CO₂ + NADH

• Succinyl-CoA → Succinate + ATP

• Succinate → Fumarate + FADH₂

• Fumarate → Malate

• Malate → Oxaloacetate + NADH