Exercises — Vitamins — fat-soluble (A, D, E, K) vs water-soluble (B-complex, C)
The picture above is the map we will keep returning to. Left branch = non-polar tail → dissolves into fat → gets stored → the consequences of storage. Right branch = polar groups → dissolves in water → flushed out → the consequences of no storage. Every exercise is really just a walk down one of these two branches.
Level 1 — Recognition
(Can you name and classify? No reasoning chains yet — just retrieval.)
Recall Solution 1.1
Fat-soluble vitamins: A, D, E, K. Mnemonic: "All Dogs Eat Kibble" → A, D, E, K. Everything else (all B-complex + C) is water-soluble.
Recall Solution 1.2
- A → Retinol
- D → Calciferol
- E → Tocopherol
- K → Phylloquinone
- C → Ascorbic acid
Recall Solution 1.3
- C → Scurvy (bleeding gums)
- A → Night blindness / xerophthalmia
- B₁ (thiamine) → Beri-beri
- B₃ (niacin) → Pellagra
- D → Rickets (children) / osteomalacia (adults)
Level 2 — Application
(Apply the "like dissolves like" rule to predict a property.)
Recall Solution 2.1
Structure → solubility: a long hydrocarbon tail is non-polar; by "like dissolves like" it dissolves into the body's non-polar fat (membranes, adipose tissue), not into watery blood. Solubility → fate: fat-soluble ⇒ stored in liver and adipose tissue. Conclusion: stored, not excreted in urine. (This describes a fat-soluble vitamin like A/D/E/K.)
Recall Solution 2.2
(a) Those groups are polar and form strong hydrogen bonds with water → water-soluble. (b) Water-soluble ⇒ dissolves in blood ⇒ not stored; excess leaves in urine. (c) No store means reserves run out quickly ⇒ deficiency appears fast (weeks).
Recall Solution 2.3
Vitamin C (water-soluble): excess dissolves in blood and is flushed out in urine → toxicity is rare. The body simply cannot hoard it. Vitamin D (fat-soluble): excess dissolves into fat and is stored → repeated mega-doses accumulate → hypervitaminosis (hypercalcaemia) → dangerous. Same "20× dose," opposite verdict — because the solubility, not the size of the dose, decides fate.
Level 3 — Analysis
(Given a symptom or scenario, work backward to the vitamin AND justify each inference.)
Recall Solution 3.1
(a) Bleeding gums + poor wound healing → collagen is failing → Vitamin C (ascorbic acid; needed for collagen synthesis). Disease = scurvy. (b) C is water-soluble → not stored, so with no fresh intake the body's tiny reserve empties within weeks, and symptoms appear on the voyage. (c) Fresh fruit / vegetables (citrus) — the classic fix that gave British sailors the nickname "limeys."
Recall Solution 3.2
Vitamin K — required to synthesise prothrombin, a clotting factor. Mechanism: part of our Vitamin K supply is made by gut bacteria. Broad-spectrum antibiotics kill those bacteria → less K produced → less prothrombin → slower clotting → prolonged bleeding time. Note the diet is "normal," so the clue is the antibiotic, pointing to the bacterial source of K rather than to food.
Recall Solution 3.3
(a) Vitamin D (calciferol); disease = rickets. (b) D promotes absorption of Ca²⁺ and phosphate → needed to mineralise (harden) bone. Without it, bones stay soft and bend under weight. (c) It is unusual because the skin can synthesise Vitamin D from sunlight — so "cannot be synthesised by the body" is only fully true when sunlight is lacking. This is why sun-poor + no dietary source together cause deficiency.
Level 4 — Synthesis
(Combine several rules or compare cases across both families.)
Recall Solution 4.1
(a) X (no harm on excess) is water-soluble; Y (toxic accumulation) is fat-soluble. (b) Excess X → urine (fast route); Y → bile/faeces (slow route). This follows because water-soluble things travel in watery blood → kidney → urine, while fat-soluble things ride bile. (c) On withdrawal, the water-soluble X deficits appear faster — it was never stored. Y, having a store, buffers a withdrawal for much longer. Big idea: the same property (storage) explains both "safe to overdose" and "slow to become deficient" for Y, and both "hard to overdose" and "fast to become deficient" for X. One property, four consequences.
Recall Solution 4.2
Fat-soluble (non-polar skeleton): Stored ⇒ (1) not needed daily, (2) excess accumulates → toxic (hypervitaminosis), (3) excreted slowly via bile/faeces, (4) deficiency onset is slow.
Water-soluble (polar groups, H-bonding): Not stored ⇒ (1) needed regularly, (2) rarely toxic, (3) excreted fast via urine, (4) deficiency onset is fast. Notice both fans of four consequences unfold from one solubility fact — that's the synthesis.
Recall Solution 4.3
Vitamins have negligible caloric value — they are not fuel, so they cannot supply energy directly. They act as coenzymes (see Enzymes and Coenzymes) that help enzymes release energy already stored in food (carbohydrates/fats); they regulate the machinery but are not the fuel it burns.
Level 5 — Mastery
(Multi-step, includes a degenerate/edge case and a quantitative estimate.)
Recall Solution 5.1
The general rule is a tendency, and B₁₂ is the noted exception: the body keeps a small B₁₂ store (chiefly in the liver, bound to proteins), unlike most water-soluble vitamins. So the chain becomes: water-soluble but exceptionally stored ⇒ deficiency is slow (years), unlike Vitamin C which has essentially no store ⇒ fast (weeks). Lesson: the rule predicts the default; always check for the flagged exception (B₁₂) before applying it blindly.
Recall Solution 5.2
Step 1 — absorption: fat-soluble vitamins (A, D, E, K) are absorbed dissolved in dietary fat. If fat isn't absorbed, these vitamins ride out with it → fat-soluble vitamins are at risk. Step 2 — consequence: low Vitamin K → less prothrombin → prolonged clotting time / bleeding tendency. So a fat-absorption problem selectively starves the fat-soluble family — the water-soluble B/C group is largely spared because they don't need fat to be absorbed.
Recall Solution 5.3
Vitamin A: store lasts 90 days, only 45 elapsed → days of reserve remain → NOT yet depleted (half the store left). Vitamin C: store lasts 30 days, but 45 days elapsed → depleted at day 30, so by day 45 it has been empty for 15 days → depleted. Shortfall as percentage of the 30-day store: the deficit beyond the store is days. So the person is 50% past exhausting their Vitamin C store, while Vitamin A is only halfway used up. This quantifies why C deficiency (scurvy) strikes long before A deficiency on the same fast.
Wrap-up recall
Recall One-line summaries of every level
- L1: A D E K are fat-soluble; C = ascorbic acid; know names + diseases.
- L2: structure → polarity → solubility → storage → toxicity/frequency.
- L3: symptom → vitamin → mechanism → why now.
- L4: one solubility fact fans out into four consequences.
- L5: edge cases — B₁₂ is stored; A/D/E/K need fat to be absorbed.
Recall The single most reusable question
Whenever stuck, ask: "Does it dissolve in fat or water?" ::: Fat → stored → toxic-if-excess, slow deficiency, bile excretion. Water → flushed → rarely toxic, fast deficiency, urine excretion. (Exception: B₁₂ stored.)
Related vault topics: Biomolecules · Lipids · Carbohydrates · Hormones · Enzymes and Coenzymes · Solubility — like dissolves like.