58. A process in which fatty acids are shortened by two carbons at a time resulting in release of
acetylCoA is known as
(A) photophosphorylation
(B) carboxylation
(C) 𝛽-oxidation
(D) oxidative phosphorylation
β-oxidation is a crucial metabolic pathway in cellular respiration, breaking down fatty acids to generate energy. If you’re studying biochemistry or preparing for exams like NEET, CSIR NET, or university MCQs, understanding this process is essential—especially for questions like: “A process in which fatty acids are shortened by two carbons at a time resulting in release of acetyl-CoA is known as…”
Let’s dive into the correct answer, why it fits, and clear explanations for every option.
The Correct Answer: (C) β-Oxidation
β-oxidation (beta-oxidation) is the primary catabolic process for fatty acids in mitochondria. It systematically shortens fatty acyl-CoA chains by removing two-carbon units from the carboxyl end (beta position), releasing acetyl-CoA each cycle. This acetyl-CoA then enters the citric acid cycle (Krebs cycle) for ATP production.
How β-Oxidation Works: Step-by-Step
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Activation: Fatty acids combine with CoA to form fatty acyl-CoA.
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Transport: Carnitine shuttle moves it into mitochondria.
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Core Cycle (four steps per round):
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Dehydrogenation (FAD → FADH₂).
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Hydration.
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Oxidation (NAD⁺ → NADH).
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Thiolysis: Cleaves acetyl-CoA, leaving a shortened acyl-CoA.
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Repeats until the chain becomes acetyl-CoA.
For a 16-carbon fatty acid like palmitate, it yields 8 acetyl-CoA molecules, powering oxidative phosphorylation. This matches the MCQ perfectly: “shortened by two carbons at a time resulting in release of acetyl-CoA.”
Why Not the Other Options? Full Breakdown
Each option tests metabolic pathways—here’s why they don’t fit:
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(A) Photophosphorylation: This is light-driven ATP synthesis in photosynthesis (chloroplasts of plants/algae). It uses chlorophyll to create a proton gradient for ATP, not fatty acid breakdown. No carbon shortening or acetyl-CoA involved—it’s energy capture from sunlight, irrelevant to animal lipid metabolism.
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(B) Carboxylation: An anabolic reaction adding CO₂ (carboxyl group) to molecules, like in gluconeogenesis (pyruvate carboxylase) or Calvin cycle (RuBisCO). It builds carbon chains (e.g., oxaloacetate from pyruvate), opposite of shortening fatty acids. No acetyl-CoA release.
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(D) Oxidative Phosphorylation: The final ATP-generating stage of respiration in mitochondria. Electrons from NADH/FADH₂ (including from β-oxidation) drive the electron transport chain and ATP synthase. It consumes reducing equivalents from β-oxidation but doesn’t shorten fatty acids or release acetyl-CoA directly.
Why β-Oxidation Matters in Biology and Medicine
In fasting or high-fat diets, β-oxidation fuels ketone body production, preventing hypoglycemia. Defects (e.g., MCAD deficiency) cause metabolic disorders. For plant biology enthusiasts, similar peroxisomal β-oxidation aids seed germination by mobilizing storage lipids.
This pathway links lipids to energy metabolism, producing more ATP per carbon than glucose oxidation—key for endurance.
Quick MCQ Recap Table
| Option | Process Description | Matches Query? | Why/Why Not |
|---|---|---|---|
| (A) Photophosphorylation | Light-based ATP production | No | Photosynthesis, no fatty acids |
| (B) Carboxylation | CO₂ addition to substrates | No | Builds chains, not breakdown |
| (C) β-Oxidation | 2-carbon fatty acid shortening | Yes | Releases acetyl-CoA |
| (D) Oxidative Phosphorylation | Electron transport ATP synthesis | No | Uses products, doesn’t break fats |
Master β-oxidation for exams—it’s a high-yield topic in genetics, microbiology, and biochemistry!
