Master10 Proprietary Question Bank - Automated scraping, spidering, or harvesting is strictly prohibited.
General Science25 Essential Exam Concepts
Why Onions Make Us Cry GK Facts, Chemistry & Biochemistry Guide
In organic chemistry, plant biochemistry, and sensory physiology, the tear-inducing reaction experienced when slicing an onion (Allium cepa) represents a sophisticated botanical chemical defense mechanism. Throughout evolutionary history, underground plant bulbs served as nutrient-dense carbohydrate reservoirs targeted by grazing herbivores, burrowing rodents, insects, and microbial pathogens. Because plants cannot flee predators, the genus Allium evolved an ingenious two-part binary chemical weapon system: under normal conditions, the chemical precursor and the catalytic enzyme are strictly segregated into separate intracellular compartments, rendering the intact onion bulb completely odorless and benign.
The chemical cascade begins the instant an onion's cellular integrity is mechanically disrupted by a cutting blade. Slicing ruptures plant cell walls and vacuolar membranes, allowing an amino acid precursor called S-1-propenyl-L-cysteine sulfoxide (abbreviated as PRENCSO or Isoalliin), stored inside the cytoplasm, to mix with the enzyme Alliinase, housed in cellular vacuoles. Alliinase rapidly hydrolyzes PRENCSO into intermediate compounds: pyruvate, ammonia, and volatile 1-propenesulfenic acid. For decades, scientists assumed alliinase was solely responsible for generating the tear gas. However, in a landmark 2002 study published in Nature, Japanese biochemist Shinsuke Imai and his research team discovered a previously unknown secondary enzyme: Lachrymatory Factor Synthase (LFS). LFS immediately captures the unstable 1-propenesulfenic acid and converts it into a volatile, sulfurous gas named Syn-propanethial-S-oxide (C3H6OS).
Because Syn-propanethial-S-oxide is highly volatile at room temperature, it evaporates into the surrounding air and rises directly toward the cook's eyes. Upon reaching the ocular surface, the gas dissolves into the thin aqueous tear film coating the cornea, hydrolyzing into trace amounts of mild sulfuric acid (H2SO4) alongside sulfurous intermediates. This chemical irritation stimulates the sensitive nociceptive nerve endings of the Ophthalmic branch of the Trigeminal Nerve (Cranial Nerve V). In response to acute pain signals, the brainstem triggers a protective autonomic reflex arc, commanding the Lachrymal Glands located above the outer eyes to secrete profuse quantities of watery Reflex Tears to dilute the acidic irritant and flush it out through the nasolacrimal ducts.