Question bank — Lipids — fatty acids, triglycerides, phospholipids; saponification

False. Lipids are defined by a property (insolubility in water), not a shared structure — fatty acids, triglycerides, phospholipids and steroids look very different yet all count.

True. No C=C means straight chains that stack tightly → stronger Van der Waals Forces → more energy needed to melt. The kink from a cis double bond spoils this packing.

False, and a classic trap. Each cis double bond adds a kink, so more double bonds = worse packing = lower melting point. "Double bond" sounds strong but here it weakens the intermolecular contact.

False. All three glycerol –OH groups hold hydrophobic fatty-acid tails, so the whole molecule is hydrophobic. Only phospholipids (one tail swapped for a polar phosphate) are amphipathic.

Half-true. It reverses the bond (ester → carboxylate), but esterification releases water while saponification consumes it AND the base traps the product, so it's not a simple reversal — it's driven one way.

True. Both are amphipathic — hydrophobic tail hides from water, hydrophilic head faces it. Soap forms micelles around grease; phospholipids form bilayers; same "hide the tails" logic.

False. Glycerol is a product of saponification (bonds broken) but a reactant in building a triglyceride (bonds made). Don't confuse the two directions.

False. A fatty acid needs a long unbranched hydrocarbon chain plus the –COOH. Acetic acid (${\text{CH}}_3\text{COOH}$) is a carboxylic acid but far too short to be a fatty acid.

Error: in basic medium the released acid is instantly neutralised, so you get 3 sodium stearate (the soap, ${\text{RCOO}}^{-}{\text{Na}}^{+}$), not free stearic acid. Free acid appears only in acidic hydrolysis.

Error: 3 waters. Each ester bond is one condensation, and three ester bonds form, so three ${\text{H}}_2\text{O}$ leave — one per bond.

Error: only one –OH holds a phosphate; the other two still hold fatty-acid tails. That one swap is exactly what creates the two-tails-one-head amphipathic shape.

Error: it's the opposite — fatty-acid carbons are highly reduced (many C–H bonds, little oxygen). Oxidising those C–H bonds via Energy Metabolism — Beta Oxidation releases the big energy (~9 vs ~4 kcal/g).

Error: acid hydrolysis is an equilibrium and reverses easily. Base is used for soap because it converts the acid to a non-reactive salt, trapping the reaction forward — that's why it's saponification.

Error: the kink comes from a cis C=C double bond in the tail, which bends the chain like a hockey stick. The –COOH head is the same in saturated and unsaturated acids.

Error: the heads are hydrophilic, so they face the water on both surfaces; the hydrophobic tails tuck inward away from water. You've inverted the Cell Membrane Structure.

Because the body builds them two carbons at a time from acetyl (a 2-carbon unit), so the total keeps landing on an even number.

Fat is anhydrous — it stores without dragging water along, while carbohydrate stores are hydrated. So fat is lighter and denser per usable joule.

The base grabs each released fatty acid and turns it into a carboxylate salt that cannot re-form the ester, so the equilibrium is trapped in the forward direction (see Hydrolysis Reactions).

A cis bond bends the chain, ruining tight packing; a trans bond keeps the chain nearly straight, so packing (and melting point) stays close to the saturated case.

Physics alone: water pushes the hydrophobic tails together and lets the hydrophilic heads face water, and the lowest-energy arrangement of that compromise is a bilayer sheet — no template needed.

It encodes the two properties that matter at a glance — chain length (18 → melting trend) and unsaturation (1 double bond → kink, reactivity) — in two numbers.

The kinks vanish, chains straighten, packing improves → melting point rises, turning liquid oil into a solid fat. This is exactly how margarine is made.

No. Saturation refers only to C=C double bonds; a branch affects packing but a chain with no C=C is saturated regardless of branching.

Same reaction, but you get potassium salts of the fatty acids — softer, more soluble "soft soaps" — plus glycerol. The choice of base changes the counter-ion, not the mechanism.

No. Its hydrocarbon tail is too short to be meaningfully hydrophobic, so there's no strong drive to bury tails and form a micelle — it just dissolves. You need a long tail, i.e. an actual fatty-acid salt (see Soaps and Detergents).

Two-tailed (phospholipid) favours flat bilayers/membranes; one-tailed (soap) favours curved micelles. Head-to-tail geometry decides the shape that self-assembles.

It could not form a triglyceride (needs 3 ester attachment points); at most a di-substituted glyceride, changing both energy-storage capacity and the phospholipid architecture.