Q.80 The metabolic pathway which is common to both fermentation and cellular respiration is
(A) the TCA cycle
(B) the electron transport chain
(C) glycolysis
(D) synthesis of acetyl CoA from pyruvate

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In biology, understanding metabolic pathways is key for exams like NEET or college entrance tests. A common question asks: The metabolic pathway which is common to both fermentation and cellular respiration is—with options like TCA cycle, electron transport chain, glycolysis, and synthesis of acetyl CoA from pyruvate. The correct answer is (C) glycolysis. This article breaks it down step-by-step, explaining why glycolysis is the shared pathway and why the others aren’t.

What Are Fermentation and Cellular Respiration?

Both processes generate ATP (energy) from glucose without oxygen in fermentation, but cellular respiration uses oxygen fully.

• Fermentation: Anaerobic (no oxygen). Glucose breaks down to pyruvate via glycolysis, then regenerates NAD⁺ (e.g., lactic acid or ethanol fermentation). Net gain: 2 ATP.

• Cellular respiration: Aerobic. Includes glycolysis, pyruvate oxidation, TCA cycle, and electron transport chain (ETC). Net gain: ~36-38 ATP.

The metabolic pathway common to fermentation and cellular respiration is the one that happens in both.

Correct Answer: Why Glycolysis?

Glycolysis (option C) occurs in the cytoplasm of all cells. It converts one glucose (C₆H₁₂O₆) to two pyruvates, producing 2 ATP and 2 NADH.

Key steps:

1. Glucose → Glucose-6-phosphate (uses 1 ATP).

2. Fructose-1,6-bisphosphate splits into two 3-carbon molecules.

3. Each yields 1,3-bisphosphoglycerate → pyruvate (produces 4 ATP total, 2 NADH).

In fermentation, glycolysis ends here with NAD⁺ regeneration. In respiration, pyruvate continues aerobically. That’s why glycolysis is universal.

(Image: Simplified glycolysis showing 10 enzyme steps.)

Explanation of All Options

(A) The TCA Cycle

The TCA (Krebs) cycle happens in mitochondria, oxidizing acetyl CoA to CO₂, producing NADH/FADH₂. It’s aerobic-only—fermentation skips it entirely. Not common.

(B) The Electron Transport Chain (ETC)

ETC in mitochondrial inner membrane uses O₂ to make ATP from NADH/FADH₂. Fermentation lacks oxygen and ETC. Exclusive to respiration.

(C) Glycolysis

Correct. Anaerobic, shared first step. Powers both processes.

(D) Synthesis of Acetyl CoA from Pyruvate

Pyruvate decarboxylates to acetyl CoA (via pyruvate dehydrogenase) in mitochondria—requires oxygen. Fermentation converts pyruvate differently (e.g., to lactate). Not shared.

Why This Matters for Exams

Mastering this distinguishes anaerobic vs. aerobic metabolism. Glycolysis evolved first, explaining its universality across life forms—from yeast to humans.

Quick Tip: Remember “Glycolysis: Gateway to both paths.”