Cholesterol is a type of

Correct — C, lipid. Cholesterol is a sterol — a lipid built on a rigid four-ring steroid skeleton with a hydroxyl group at one end and a hydrocarbon tail at the other. That structure makes it almost insoluble in water and soluble in organic solvents, which is the operational definition of a lipid. It is a normal component of the animal cell membrane, where it wedges between phospholipids and controls membrane fluidity, and it is the starting material from which the body makes steroid hormones, bile salts and vitamin D.

• (a)carbohydrate — Carbohydrates are hydrates of carbon with the general formula Cn(H2O)n and a chemistry dominated by hydroxyl and carbonyl groups — sugars, starch, cellulose and glycogen. Cholesterol has one hydroxyl group on a hydrocarbon ring system and fits none of that.
• (b)protein — Proteins are polymers of amino acids joined by peptide bonds and always contain nitrogen. Cholesterol contains no nitrogen and is not a polymer. The confusion usually comes from LDL and HDL, which are lipoproteins that carry cholesterol — the carrier is part protein, the cargo is not.
• (d)nucleic acid — Nucleic acids are polymers of nucleotides, each with a sugar, a phosphate and a nitrogenous base. Cholesterol has no phosphate, no base and no sugar.

Lipids are defined by solubility rather than by a shared structure: they are the biomolecules that dissolve poorly in water and well in organic solvents. The class therefore covers fats and oils (triglycerides), phospholipids, waxes and steroids. Cholesterol is the best-known steroid, and in animals it does two jobs — it sits in the plasma membrane regulating fluidity, and it is the precursor for cortisol, aldosterone, testosterone, oestrogen, bile acids and vitamin D. Plants make sterols of their own but not cholesterol, which is why a purely plant-based diet contains none.

The confusion this item exploits is the everyday association of cholesterol with 'good' and 'bad' forms, HDL and LDL. Those are lipoproteins — transport particles with a protein coat wrapped around a lipid core — and their protein content tempts a candidate towards option (b). Keep the cargo separate from the carrier: cholesterol itself is a steroid lipid, and the protein belongs to the particle that ferries it through the blood.

• Cholesterol is a sterol, and sterols are a subclass of lipids built on a four-ring steroid skeleton.
• It is a constituent of animal cell membranes, where it moderates fluidity.
• It is the precursor of steroid hormones, bile acids and vitamin D.
• HDL and LDL are lipoproteins that transport cholesterol; they are not cholesterol itself.
• Dietary cholesterol comes only from animal foods, since plants do not make it.

• Choosing 'protein' because of HDL and LDL — those are lipoproteins, the carriers.
• Assuming cholesterol is only harmful; it is a required membrane component and hormone precursor.
• Expecting a lipid to be a polymer like the other three classes; lipids are not polymers.

One-line classification, but it is set as a trap for candidates whose only exposure to the word is from health advertising.

• practice — not a real PYQ

  Cholesterol in the human body serves as the precursor for the synthesis of

  • (a)haemoglobin
  • (b)steroid hormones and bile acids
  • (c)insulin
  • (d)thyroxine

  Answer(b) steroid hormones and bile acids — vitamin D is made from a cholesterol derivative in the skin as well.

• practice — not a real PYQ

  Which one of the following is not a lipid?

  • (a)Wax
  • (b)Triglyceride
  • (c)Glycogen
  • (d)Phospholipid

  Answer(c) Glycogen — it is a branched polymer of glucose and therefore a carbohydrate.