Q.Which of the following acids is a vitamin?
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Water Soluble Vitamins: The First Meeting
Imagine you drop a spoonful of sugar into a glass of water. It dissolves completely, disappears into the liquid, and you can't see it anymore — but it's still there, molecule by molecule, spread evenly throughout the water. That's exactly what "water soluble" means: the substance mixes with water at the molecular level.
Now, a vitamin is a tiny organic compound your body needs in small amounts to function properly — it helps enzymes do their jobs, keeps your skin healthy, helps you see, and so on. Your body cannot make most vitamins on its own, so you must get them from food.
Put the two together: a water soluble vitamin is a vitamin that dissolves in water. This single property — solubility — determines almost everything about how these vitamins behave in your body.
The Precise Statement
Water soluble vitamins are vitamins that dissolve in water, are not stored in significant amounts in the body, and are excreted in urine when consumed in excess. They must be consumed regularly (daily or almost daily) to prevent deficiency.
The group includes the B-complex vitamins (B1, B2, B3, B5, B6, B7, B9, B12) and Vitamin C. That's 9 vitamins total.
Why Solubility Matters So Much
Think about fat. Fat and water don't mix — oil floats on top of water. Fat soluble vitamins (A, D, E, K) behave like oil: they dissolve in fat, get stored in your liver and fatty tissues, and can stay in your body for weeks or months. You don't need them every single day.
Water soluble vitamins are the opposite. Because they dissolve in water, they travel freely in your blood (which is mostly water). But your kidneys are constantly filtering your blood and removing excess water-soluble substances into urine. So if you take more Vitamin C than your body needs right now, the extra gets flushed out within hours.
This does not mean you can take unlimited amounts safely. Very high doses of some water soluble vitamins (especially B6 and niacin/B3) can cause toxicity — just not as easily as fat soluble vitamins.
The Practical Consequences
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You need them frequently — ideally every day. Missing a day won't cause immediate harm, but weeks of low intake will lead to deficiency diseases (scurvy for Vitamin C, beriberi for B1, pellagra for B3, etc.).
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Cooking destroys them — because they dissolve in water, boiling vegetables leaches vitamins into the cooking water. If you throw that water away, you lose the vitamins. Steaming or microwaving with minimal water preserves them better.
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They are fragile — heat, light, and air can break them down. That's why fresh fruits and vegetables lose vitamin content over time.
The Two Families
| Family | Members | Key Role |
|--------|---------|----------| …
Why this formula?
Water-Soluble Vitamins: Understanding Why They Behave the Way They Do
Let’s start with the core idea: Water-soluble vitamins are a group of organic compounds that dissolve in water and are not stored in large amounts in the body. The key “formulae” here are not mathematical equations but chemical structures and physiological principles that explain their solubility, absorption, and excretion.
1. Why Are They Water-Soluble? The Chemical Reason
The fundamental “formula” is the presence of polar functional groups (like –OH, –COOH, –NH₂, –C=O) in their molecular structure.
- Polar groups form hydrogen bonds with water molecules.
- Example: Vitamin C (ascorbic acid) Its structure has multiple –OH groups and a lactone ring.
C6H8O6(with 4 –OH groups)
Each –OH can hydrogen-bond with water, making it highly soluble.
Why this matters:
Unlike fat-soluble vitamins (A, D, E, K) which are nonpolar and stored in fatty tissues, water-soluble vitamins dissolve in blood and interstitial fluid. This means they are rapidly absorbed but also quickly excreted by the kidneys.
Key takeaway: Solubility is not magic — it’s a direct consequence of molecular polarity.
2. The “No Storage” Principle: Why Excretion Happens
The body has no specific binding proteins to store most water-soluble vitamins (except B₁₂).
- Once absorbed, they circulate freely in blood.
- Kidneys filter blood continuously.
- If plasma concentration exceeds the renal threshold, excess is excreted in urine.
Derivation of the excretion logic:
Let Cp = plasma concentration of vitamin, T = renal threshold.
- If Cp≤T → all filtered vitamin is reabsorbed.
- If Cp>T → excess (Cp−T) is lost in urine.
Why this holds:
The transport proteins in kidney tubules have a maximum capacity (saturation). Once saturated, the extra vitamin cannot be reabsorbed.
Exam tip: This is why water-soluble vitamins need daily intake — no storage means deficiency can develop in weeks.
3. The B-Complex “Formula”: Coenzyme Activation
Most B vitamins are precursors to coenzymes. The “formula” here is:
Vitamin (inactive)enzymeCoenzyme (active)
Example: Thiamine (B₁) → Thiamine pyrophosphate (TPP)
- Thiamine has a thiazole ring and a pyrimidine ring.
- In the body, it gets phosphorylated (adds two phosphate groups).
- The active form TPP is a coenzyme for decarboxylation reactions (e.g., in Krebs cycle).
Why this matters for exams:
You don’t need to memorize every step — just understand that the vitamin itself is not the active molecule. The body modifies it to create the functional coenzyme.
4. Vitamin C: The Redox “Formula”
Vitamin C (ascorbic acid) acts as a reducing agent. Its key reaction:
Ascorbic acid⇌Dehydroascorbic acid+2H++2e−
Why this holds:
The molecule has an enediol group (–C(OH)=C(OH)–) which can donate electrons.
- In the reduced form (ascorbic acid), it has two –OH groups on adjacent carbons.
- Upon oxidation, these become a diketone (C=O groups).
- This reversible redox pair allows vitamin C to scavenge free radicals and regenerate other antioxidants (like vitamin E).
Key exam point: The ability to donate electrons is why vitamin C is essential for collagen synthesis (proline hydroxylation requires reducing power).
5. Absorption: The “Active Transport” Formula …
The key idea is that ascorbic acid is the chemical name for Vitamin C, a water-soluble vitamin. The other options are not vitamins: aspartic acid is an amino acid, adipic acid is a dicarboxylic acid used in nylon production, and saccharic acid is an oxidation product of glucose.
- Identify which of the given acids is an essential micronutrient required in the diet. …
The question asks which acid is a vitamin. Ascorbic acid is vitamin C, a water-soluble vitamin essential for collagen synthesis and antioxidant function. The correct answer is (ii) Ascorbic acid.
This is a straightforward classification question from biochemistry. The key is to recall which of these compounds is an essential micronutrient that the human body cannot synthesise and must obtain from the diet.
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Aspartic acid is a non-essential amino acid. It is a building block of proteins and can be synthesised by the body. It is not a vitamin.
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Ascorbic acid is the chemical name for vitamin C. It is a water-soluble vitamin, meaning it dissolves in water and is not stored in large amounts. It acts as a cofactor for several enzymes (e.g., in collagen synthesis) and as a powerful antioxidant. Humans lack the enzyme L-gulonolactone oxidase needed to synthesise it, so it must come from food.
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Adipic acid is a dicarboxylic acid used industrially in nylon production. It has no vitamin activity and is not required in the human diet.
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Saccharic acid (also called glucaric acid) is a sugar acid derived from glucose. It is not a vitamin; it is a metabolic byproduct. …
Concept: Vitamins as Essential Organic Compounds
Vitamins are organic compounds required in small amounts for normal metabolic functions. They are not synthesized in sufficient quantities by the body and must be obtained from diet. The key is to recognize which acid among the options is a vitamin.
Method: Recall of Common Vitamin Names
Steps:
- Identify the vitamin form of each acid
- Aspartic acid → an amino acid, not a vitamin
- Ascorbic acid → Vitamin C (water-soluble vitamin)
- Adipic acid → a dicarboxylic acid used in nylon production, not a vitamin
- Saccharic acid → an oxidation product of glucose, not a vitamin …
Here are the common mistakes students make on this question, along with how to avoid each.
1. Confusing "Vitamin" with "Amino Acid"
- The Mistake: Students see the word "acid" and the suffix "-ic" and immediately think of amino acids (the building blocks of proteins). Aspartic acid (A) is a well-known amino acid, so many pick it without reading the question carefully.
- Why it happens: In biology and chemistry chapters, "amino acids" are drilled heavily. The brain defaults to the most familiar option.
- How to Avoid: Read the question stem twice. The question asks for a vitamin, not an amino acid. Before looking at the options, ask yourself: "Which of these is a vitamin I have studied?" If you don't recognize the name as a vitamin, eliminate it.
2. Assuming "Ascorbic" is a Complex Name for a Lab Chemical
- The Mistake: Students think "Ascorbic acid" sounds like a synthetic lab compound (like adipic or saccharic acid) and dismiss it.
- Why it happens: The common name for Vitamin C is rarely used in everyday conversation. Students memorize "Vitamin C" but forget its chemical name.
- How to Avoid: Memorize the chemical names of common vitamins.
- Vitamin C = Ascorbic acid (think: anti-scorbutic, meaning prevents scurvy).
- Vitamin B1 = Thiamine
- Vitamin B2 = Riboflavin
- Vitamin B3 = Niacin
- Vitamin B12 = Cyanocobalamin
- Vitamin A = Retinol
- Vitamin D = Calciferol
- Key trick: The word "ascorbic" is directly linked to scurvy (the disease caused by its deficiency). This is a classic exam linkage.
3. Falling for the "Acid" Trap (Adipic & Saccharic)
- The Mistake: Students pick Adipic acid (C) or Saccharic acid (D) because they sound like "vitamin-like" names or because they confuse "Saccharic" with "Saccharin" (an artificial sweetener).
- Why it happens: The names sound scientific and important. Students guess based on sound rather than knowledge.
- How to Avoid: Know the industrial uses of these acids.
- Adipic acid: Used to make nylon (a polymer). Not a vitamin.
- Saccharic acid: An oxidation product of glucose (sugar). Not a vitamin.
- Aspartic acid: An amino acid (found in proteins). Not a vitamin.
- Ascorbic acid: Vitamin C. This is the only one that is an essential micronutrient. …
- COMEDK 2025Set 2025-E1 markMCQQ.Which of the following is a water soluble vitamin? (A) Vitamin C (B) Vitamin E (C) Vitamin A (D) Vitamin D
›Reveal solutionSolution
Vitamins are classified as fat-soluble (A, D, E, K) or water-soluble (B-complex and C). Vitamin C is water-soluble; the others listed are fat-soluble. The correct option is (A).
Concept & Intuition
Vitamins are organic compounds our bodies need in small amounts. Their solubility determines how they are absorbed, transported, and stored. Fat-soluble vitamins (A, D, E, K) dissolve in fats and are stored in the liver and fatty tissues — so you don’t need them every day. Water-soluble vitamins (the B-complex and vitamin C) dissolve in water, are not stored in large amounts, and are excreted in urine — so you need them more regularly. The question tests this basic classification.
Step-by-step reasoning
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Recall the two main groups
- Fat-soluble: Vitamins A, D, E, K.
- Water-soluble: B-complex (B₁, B₂, B₃, B₅, B₆, B₇, B₉, B₁₂) and Vitamin C.
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Examine each option
- (A) Vitamin C — Ascorbic acid. Known to dissolve in water; famously found in citrus fruits. Belongs to the water-soluble group.
- (B) Vitamin E — Tocopherol. Fat-soluble; stored in body fat.
- (C) Vitamin A — Retinol. Fat-soluble; stored in the liver.
- (D) Vitamin D — Calciferol. Fat-soluble; produced in skin upon sun exposure. …
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- KCET 2020Set A-11 markMCQQ.Which one of the following vitamins is not stored in adipose tissue ? (A) E (B) A (C) B6 (D) D
›Reveal solutionSolution
Fat-soluble vitamins (A, D, E, K) are stored in adipose tissue/liver; the water-soluble ones (B-group, C) are not — and B6 is water-soluble.
Step 1 — The classifying principle: solubility.
Vitamins are classified by the solvent they dissolve in, and that single property decides whether the body can stockpile them:
- Fat-soluble vitamins: A, D, E, K. They are non-polar (long hydrocarbon/isoprenoid skeletons), so they partition into lipids. They are absorbed with dietary fat, transported in lipoproteins, and stored in the liver and adipose (fat) tissue. Because they accumulate, they need not be eaten daily — and an excess can be toxic (hypervitaminosis).
- Water-soluble vitamins: the B-complex (B1,B2,B6,B12, niacin, folate…) and vitamin C. They carry polar –OH, –NH2, –COOH groups, dissolve in the aqueous body fluids, and are not stored in fat. Whatever the body does not immediately use is filtered by the kidney and excreted in urine, so they must be supplied regularly in the diet.
Step 2 — Screen the four options.
- (A) Vitamin E (tocopherol) — fat-soluble → is stored in adipose tissue. …
- KCET 2019Set A-11 markMCQQ.The vitamin that helps in clotting of blood is (A) A (B) B2 (C) C (D) K
›Reveal solutionSolution
The vitamin essential for blood clotting is vitamin K, which activates clotting factors in the liver. The correct option is (D).
The question tests a straightforward fact from human physiology: which vitamin is directly involved in the coagulation (clotting) cascade? The key is to connect each vitamin to its primary function — not just memorise names, but understand why a deficiency leads to bleeding disorders.
Vitamin K is a fat-soluble vitamin that acts as a cofactor for an enzyme that modifies certain proteins (clotting factors II, VII, IX, X) in the liver. Without this modification, these proteins cannot bind calcium and cannot participate in the clotting cascade. That’s why newborns get a vitamin K shot — their gut hasn’t yet colonised the bacteria that synthesise it.
Let’s check each option:
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Vitamin A — crucial for vision (retinal), immune function, and epithelial cell health. No role in clotting. A deficiency causes night blindness, not bleeding.
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Vitamin B₂ (riboflavin) — part of coenzymes FAD and FMN, involved in energy metabolism (electron transport chain). No connection to coagulation.
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Vitamin C (ascorbic acid) — needed for collagen synthesis (wound healing), antioxidant function, and iron absorption. While scurvy causes poor wound healing and gum bleeding, that’s due to weak blood vessels, not a failure of the clotting mechanism itself. Vitamin C does not directly participate in the clotting cascade. …
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