What is the Krebs Cycle?
The Krebs cycle (citric acid cycle) is a cyclic series of reactions in the mitochondrial matrix that fully oxidizes acetyl-CoA, harvesting electron carriers. Oxidative phosphorylation then uses those carriers to generate most of a cell's ATP.
The Krebs cycle oxidizes acetyl-CoA to CO2, producing NADH and FADH2 per turn; oxidative phosphorylation then uses these carriers in the electron transport chain to synthesize the bulk of a cell's ATP via chemiosmosis.
- 1.Acetyl-CoA entry — Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).
- 2.Isocitrate → α-Ketoglutarate — Oxidative decarboxylation releases CO2 and produces 1 NADH.
- 3.α-Ketoglutarate → Succinyl-CoA — A second oxidative decarboxylation releases CO2 and produces 1 NADH.
- 4.Succinyl-CoA → Succinate — Substrate-level phosphorylation produces 1 ATP (or GTP).
- 5.Succinate → Fumarate — Oxidation produces 1 FADH2.
- 6.Malate → Oxaloacetate — Final oxidation produces 1 NADH, regenerating oxaloacetate to start the cycle again.
Try it: interactive calculator
Step-by-step worked examples
One turn of the Krebs cycle produces 3 NADH and 1 FADH2. Using ATP yields of 2.5 per NADH and 1.5 per FADH2, how much ATP comes from oxidative phosphorylation for one turn?
ATP = NADH×2.5 + FADH2×1.5 ATP = 3×2.5 + 1×1.5 ATP = 7.5 + 1.5 = 9 ATP
One glucose molecule yields 2 acetyl-CoA, so the Krebs cycle turns twice. How much total NADH does the Krebs cycle alone produce per glucose?
NADH per turn = 3 Total NADH = 3 × 2 turns = 6 NADH
Using 10 NADH and 2 FADH2 from a full glucose breakdown (glycolysis + Krebs + pyruvate oxidation combined), estimate the total oxidative phosphorylation ATP.
ATP = 10×2.5 + 2×1.5 ATP = 25 + 3 = 28 ATP
Flashcards
Quick quiz
Q1.Where does the Krebs cycle take place?
Q2.How many NADH are produced per turn of the Krebs cycle?
Q3.What is regenerated at the end of each Krebs cycle turn?
Q4.Where does oxidative phosphorylation occur?
The full card deck, worked steps and AI-tutor support for “What is the Krebs Cycle?” are in Notek — study by hand before your exam.
Common mistakes
The Krebs cycle directly makes most of a cell's ATP. — Correct: The Krebs cycle makes only 1 ATP per turn directly; most ATP comes later from oxidative phosphorylation using its NADH and FADH2.
The Krebs cycle needs oxygen directly. — Correct: The Krebs cycle itself doesn't use oxygen directly, but it depends on oxidative phosphorylation (which needs O2) to keep recycling NAD+ and FAD.
Oxidative phosphorylation happens in the mitochondrial matrix. — Correct: It happens at the inner mitochondrial membrane, where the electron transport chain and ATP synthase are located.
Each glucose molecule only turns the Krebs cycle once. — Correct: Each glucose produces 2 acetyl-CoA molecules, so the Krebs cycle turns twice per glucose.
FAQ
What is the Krebs cycle?
A cyclic set of reactions in the mitochondrial matrix that oxidizes acetyl-CoA, producing NADH, FADH2, ATP and CO2.
What is the formula for the Krebs cycle?
Acetyl-CoA + 3NAD+ + FAD + ADP + Pi → 2CO2 + 3NADH + FADH2 + ATP + CoA per turn.
How is ATP calculated from oxidative phosphorylation?
Multiply the NADH count by about 2.5 ATP and the FADH2 count by about 1.5 ATP, then sum them.
What are examples of the Krebs cycle in cellular respiration?
Each glucose molecule drives two turns of the Krebs cycle after glycolysis and pyruvate oxidation, feeding electron carriers into oxidative phosphorylation.