Q.25 In most higher eukaryotes, the fatty acid synthesis occurs in cytosol,
but in plants it occurs in
In plants, fatty acid synthesis occurs in chloroplasts, unlike the cytosol in most higher eukaryotes. Plants use the prokaryotic-type type II fatty acid synthase (FAS II) system in chloroplast stroma, where acetyl-CoA carboxylase and acyl-ACP intermediates produce palmitate (16:0) and stearate (18:0) for membrane lipids, powered by photosynthetic NADPH/ATP.
Option Analysis
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(1) Cytosol: Correct for animals/higher eukaryotes (type I FAS multienzyme); plants compartmentalize de novo synthesis in plastids for light-dependent regulation.
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(2) Endoplasmic reticulum: Site of lipid modification (desaturation, elongation), acyl editing, phospholipid synthesis—not primary FA chain elongation.
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(3) Mitochondria: Limited β-oxidation, lipoic acid synthesis; imports FAs, lacks FAS enzymes for de novo synthesis.
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(4) Chloroplast: Correct for plants—stromal ACCase + KAS/TE enzymes with ACP; exports C16/C18 acyl-ACPs to ER for complex lipids.
Answer: (4) Chloroplast.
Introduction to Plant Fatty Acid Synthesis Location
In most higher eukaryotes, fatty acid synthesis occurs in cytosol, but in plants it occurs in chloroplasts. This plastid localization couples FA biosynthesis to photosynthesis, providing reducing power (NADPH) and carbon (from Calvin cycle) for membrane lipid production.
Chloroplast FAS II Pathway
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Enzymes: Heterodimeric ACCase → malonyl-CoA → malonyl-ACP
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Cycles: 7 condensations (KAS I-III) yield 16:0-ACP/18:0-ACP
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Export: Thioesterases (FATB/A) release free FAs or acyl-CoA to ER
Organelle Comparison
| Organelle | Animals | Plants | Function |
|---|---|---|---|
| Cytosol | FAS I | Minor | Multienzyme complex |
| ER | – | Elongation | Desaturation |
| Mito | – | β-ox | Degradation |
| Chloroplast | Absent | FAS II | De novo synthesis |
GATE Plant Physiology Relevance
Tests compartmentation: Plants = chloroplast FAS, animals = cytosolic. Key for lipid biosynthesis, photosynthesis coupling, herbicide targets (e.g., ACCase inhibitors).
