Cutting and peeling of onions brings tears to the eyes because of the presence of

1. Essential Macronutrients and Sulfur in Plants (basic)

Welcome to your first step in mastering applied chemistry! To understand how the world around us works—from why plants grow to why some vegetables have such pungent smells—we must first understand plant nutrition. Unlike animals, which consume organic matter for energy, plants are "food factories." They utilize sunlight, water, and inorganic minerals to synthesize their own food through photosynthesis Science-Class VII, Life Processes in Plants, p.143.

While carbon, hydrogen, and oxygen are obtained from air and water, plants rely on the soil for several essential minerals. These are broadly categorized into Macronutrients (needed in large quantities) and Micronutrients (needed in trace amounts). The most critical macronutrients are often remembered by the acronym NPK, but others like Magnesium and Sulfur play indispensable roles in the plant's internal machinery.

Nutrient	Primary Role in Plants
Nitrogen (N)	Constituent of proteins and chlorophyll; promotes green, leafy growth Environment, Shankar IAS Academy, Agriculture, p.363.
Phosphorus (P)	Essential for energy transfer (ATP) and enzyme regulation Environment, Shankar IAS Academy, Agriculture, p.363.
Potassium (K)	Helps in water regulation and provides resistance to drought and disease.
Sulfur (S)	A vital component of essential amino acids, which are the building blocks of all plant proteins Environment, Shankar IAS Academy, Agriculture, p.363.

Specifically, Sulfur is absorbed from the soil by the roots in the form of sulfate ions. Once inside, the plant uses it to build complex molecules. It is this sulfur that eventually becomes part of the unique chemical compounds found in certain vegetables. These nutrients are transported throughout the plant via specialized tissues: Xylem carries water and minerals up from the roots, while Phloem translocates the finished products, like amino acids, to where they are needed for growth Science, Class X, Life Processes, p.95.

Sources: Science-Class VII . NCERT(Revised ed 2025), Life Processes in Plants, p.137, 143, 147; Environment, Shankar IAS Academy .(ed 10th), Agriculture, p.363; Science , class X (NCERT 2025 ed.), Life Processes, p.95

2. Enzymes as Biological Catalysts (basic)

Imagine your body is a high-speed chemical factory. Without workers, the machines (chemicals) would sit idle for years. Enzymes are these workers—specialized proteins that act as biological catalysts. A catalyst is a substance that speeds up a chemical reaction without being consumed or permanently changed in the process. In nature, enzymes are so fundamental that they even determine physical traits. For instance, if an enzyme responsible for producing growth hormones works efficiently, a plant grows tall; if a genetic alteration makes that enzyme less efficient, the plant remains short Science, class X (NCERT 2025 ed.), Heredity, p.131.

In our daily lives, enzymes are the master "disassemblers." In the human digestive tract, the pancreas and small intestine release enzymes like trypsin (which breaks down proteins into amino acids) and lipase (which breaks down fats into fatty acids and glycerol) Science, class X (NCERT 2025 ed.), Life Processes, p.86. These reactions would take years to happen at room temperature without enzymes, but they happen in seconds within us. When these enzymes are hindered—for example, by environmental pollutants—organisms can suffer from malnutrition because they lose the ability to break down vital nutrients like lipids Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.78.

A fascinating example of "applied chemistry" in your kitchen is why onions make you cry. When you cut an onion, you rupture its cells, bringing together enzymes and sulfur compounds that were previously kept in separate compartments. An enzyme called alliinase reacts with amino acid sulfoxides to create sulfenic acids. Then, a second enzyme, lachrymatory-factor synthase, converts these into a volatile gas (syn-propanethial-S-oxide). When this gas reaches the moisture in your eyes, it reacts to form a mild sulfuric acid (H₂SO₄). Your eyes then produce tears to dilute and wash away this irritant. This entire rapid-fire sequence is powered by the precision and speed of biological catalysts.

Sources: Science, class X (NCERT 2025 ed.), Heredity, p.131; Science, class X (NCERT 2025 ed.), Life Processes, p.86; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.78

3. Plant Defense Mechanisms: Secondary Metabolites (intermediate)

In the plant kingdom, survival is a game of chemical warfare. Since plants cannot run away from predators, they produce secondary metabolites—complex compounds that aren't strictly necessary for growth or reproduction but are vital for defense. While primary metabolites like glucose and amino acids provide energy Science, Class X (NCERT 2025 ed.), Control and Coordination, p.112, secondary metabolites act as the plant's "chemical armor" against herbivores and pathogens Environment, Shankar IAS Academy, Agriculture, p.354.

The onion (Allium cepa) provides a classic example of this applied chemistry in our kitchens. The onion plant absorbs sulfur from the soil to create specialized amino acid sulfoxides. Within the plant cell, these compounds are kept strictly separate from an enzyme called alliinase. This separation is a safety catch—the "weapon" is only armed when the plant is physically attacked.

When you cut or peel an onion, you rupture these cellular compartments. The resulting chemical chain reaction is swift:

• Step 1: The enzyme alliinase reacts with the sulfoxides to produce sulfenic acids.
• Step 2: A second enzyme, lachrymatory-factor synthase, converts these acids into a volatile gas called syn-propanethial-S-oxide.
• Step 3: This gas wafts upward and contacts the moisture on your eyes.
• Step 4: The gas reacts with the water in your tears to form a very mild sulfuric acid (H₂SO₄).

Your brain detects this irritation and signals the lachrymal glands to produce more tears to dilute and wash away the acid. This is a highly effective deterrent; in nature, a herbivore that experiences burning eyes after one bite is unlikely to finish the meal Environment and Ecology, Majid Hussain, Basic Concepts of Environment and Ecology, p.30.

Sources: Science, Class X (NCERT 2025 ed.), Control and Coordination, p.112; Environment, Shankar IAS Academy, Agriculture, p.354; Environment and Ecology, Majid Hussain, Basic Concepts of Environment and Ecology, p.30

4. Organic Acids and Bases in Everyday Chemistry (basic)

In our daily lives, we encounter acids and bases not just in a laboratory, but in our kitchens and gardens. Organic acids are naturally occurring compounds found in plants and animals. Unlike strong mineral acids like Hydrochloric acid (HCl), organic acids like Ethanoic acid (commonly known as Acetic acid) are considered weak acids because they do not ionize completely in water Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.73. These acids give many foods their characteristic sour taste—for instance, Lactic acid in curd and Citric acid in lemons Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28.

A fascinating real-world application of this chemistry occurs when we cut an onion. This isn't just a mechanical process; it's a series of rapid chemical reactions. Onions absorb sulfur from the soil to create specific amino acid sulfoxides. When you cut the onion, you rupture its cells, allowing an enzyme called alliinase to break down these sulfoxides into sulfenic acids. A second enzyme then converts these into a volatile gas called syn-propanethial-S-oxide. When this gas hits the moisture on the surface of your eyes, it reacts to form a very mild solution of sulfuric acid (H₂SO₄). To protect your cornea from this irritant, your lachrymal glands produce tears to dilute and wash the acid away.

Understanding the strength of these substances is often done using the pH scale. A neutral substance, like pure water, has a pH of 7.0; the lower the pH, the more acidic the substance Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Environmental Degradation and Management, p.8. While the acids in our food are weak, even a mild acid formed unexpectedly—like the one in our eyes when cutting onions—is enough to trigger a strong biological defense mechanism.

To help you remember where these common organic acids come from, refer to this table:

Natural Source	Organic Acid Present
Vinegar	Acetic Acid (Ethanoic Acid)
Orange / Lemon	Citric Acid
Tamarind	Tartaric Acid
Tomato	Oxalic Acid
Ant / Nettle Sting	Methanoic Acid

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.73; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Environmental Degradation and Management, p.8

5. Volatile Compounds and Gas Diffusion (intermediate)

At the heart of many sensory experiences—from the fragrance of a perfume to the sting of a freshly cut onion—lies the concept of volatility. Volatility refers to the tendency of a substance to vaporize or turn into a gas at room temperature. In our environment, Volatile Organic Compounds (VOCs) are prevalent in indoor sources like perfumes, air fresheners, and furniture polish. These substances easily enter the air we breathe and can cause immediate health effects such as irritation of the eyes, nose, and throat Shankar IAS Academy, Environmental Pollution, p.66. Once these compounds become airborne, they move through a process called gas diffusion, traveling from areas of high concentration to low concentration until they reach our sensory receptors.

A fascinating everyday application of this chemistry occurs in the kitchen. Onions are biological masters at absorbing sulfur from the soil to create defense mechanisms. When you cut or peel an onion, you rupture its cells, bringing together enzymes and amino acid sulfoxides that were previously separated. Specifically, the enzyme alliinase reacts with these sulfoxides to produce sulfenic acids, which are then converted by a second enzyme (lachrymatory-factor synthase) into a highly volatile gas called syn-propanethial-S-oxide. Because this compound is a gas, it diffuses through the air and reaches your eyes.

When this volatile sulfur-based gas encounters the moisture on the surface of your eye (the cornea), a chemical reaction occurs: it forms a mild sulfuric acid (H₂SO₄). This acid is an irritant that triggers the lachrymal glands to produce tears as a protective reflex to dilute and wash away the acid. This is why sulfur is the fundamental culprit behind your tears. While we often think of sulfur compounds like sulfur dioxide (SO₂) as industrial pollutants that cause environmental damage like chlorosis in plants Shankar IAS Academy, Environmental Pollution, p.69, the onion uses its own specialized sulfur chemistry as a natural deterrent against being eaten.

Substance Type	Examples	Common Effects
Volatile Organic Compounds (VOCs)	Perfumes, Glues, Formaldehyde	Nausea, eye/throat irritation Shankar IAS Academy, Environmental Pollution, p.66
Volatile Sulfur Compounds	Hydrogen Sulphide, syn-propanethial-S-oxide	Tears (lachrymatory), pungent odors, environmental acidity

Sources: Shankar IAS Academy, Environmental Pollution, p.66; NCERT Science class X, Carbon and its Compounds, p.68; Shankar IAS Academy, Environmental Pollution, p.69; PMF IAS Physical Geography, Earth's Atmosphere, p.270

6. The Biochemistry of Onion Lachrymation (exam-level)

Concept: The Biochemistry of Onion Lachrymation

7. Solving the Original PYQ (exam-level)

Now that you have mastered the basics of plant biochemistry and enzymatic reactions, this question serves as a perfect application of how cellular structures interact when damaged. In your previous lessons, you learned that plants absorb minerals from the soil to synthesize defense compounds. In the case of onions, they specifically concentrate sulfur to create precursor molecules. When you cut the onion, you are physically rupturing the vacuoles and cell walls, allowing enzymes like alliinase to mix with sulfur-containing amino acid sulfoxides. This is a classic example of a latent chemical reaction that only triggers upon external stimulus—in this case, your kitchen knife.

To navigate to the correct answer, follow the chemical breadcrumbs: the reaction produces sulfenic acids, which are then reorganized by a second enzyme into a volatile gas called syn-propanethial-S-oxide. Because this gas is volatile, it drifts upward and reacts with the water in your eyes to form a diluted version of sulfuric acid. Your brain detects this irritation and signals your tear glands to flush the acid away. Therefore, while several processes are involved, the entire reaction is fundamentally dependent on the sulfur in the cell. This mechanism is detailed extensively in Molecules of the Month by the University of Bristol.

As an aspiring civil servant, you must watch out for UPSC traps like options (B), (C), and (D). While onions do contain carbon and amino acids, these are universal building blocks of all organic life and do not explain the specific irritant property of onions. The UPSC often includes a partially correct answer like "amino acid" to see if you can identify the specific element (sulfur) that gives the molecule its unique functionality. Fat is virtually non-existent in onions, making it a clear outlier. Always look for the specific active element that drives the unique phenomenon described in the question.