Carbohydrates and Lipids

A-Level Biology ยท Biological Molecules

Biological molecules and polymers

Living things are built from a few key groups of molecules. Many are polymers โ€” long chains of repeating monomers joined by condensation reactions (which remove a molecule of water and form a bond) and broken by hydrolysis reactions (which add water to break the bond).

Carbohydrates

Made of carbon, hydrogen and oxygen. The monomers are monosaccharides.

  • Monosaccharides: single sugars โ€” glucose (ฮฑ and ฮฒ forms), fructose, galactose.
  • Disaccharides: two monosaccharides joined by a glycosidic bond (condensation):
  • glucose + glucose โ†’ maltose
  • glucose + fructose โ†’ sucrose
  • glucose + galactose โ†’ lactose
  • Polysaccharides: many monosaccharides:
  • Starch (amylose + amylopectin) โ€” energy store in plants; coiled, compact, insoluble (so no osmotic effect).
  • Glycogen โ€” energy store in animals; highly branched for rapid glucose release.
  • Cellulose โ€” made of ฮฒ-glucose; straight chains with H-bonds forming strong fibres for plant cell walls.

Testing for sugars/starch

  • Reducing sugars: heat with Benedict's solution โ†’ brick-red precipitate.
  • Non-reducing sugars: boil with acid first (to hydrolyse), neutralise, then Benedict's.
  • Starch: add iodine solution โ†’ blue-black.

Lipids

Lipids (fats and oils) are not polymers. The main types:

  • Triglycerides: one glycerol + three fatty acids, joined by ester bonds (condensation). Used for energy storage, insulation and protection. High energy density.
  • Phospholipids: glycerol + two fatty acids + a phosphate group. The phosphate "head" is hydrophilic; the fatty-acid "tails" are hydrophobic. This is why they form the bilayer of cell membranes.
  • Saturated fatty acids have no C=C double bonds; unsaturated have one or more (kinked chains, liquid at room temperature).

Test for lipids: the emulsion test โ€” mix with ethanol then water โ†’ a white/milky emulsion if lipid is present.

Worked example

Explain why cellulose is suited to forming cell walls but starch is not.

  • Cellulose is made of ฮฒ-glucose, forming straight chains held by many hydrogen bonds into strong microfibrils โ€” ideal for structural support. Starch coils into a compact store and lacks this strength. โœ“

Common mistakes

  • Forgetting bonds: glycosidic (carbohydrates), ester (lipids), peptide (proteins).
  • Saying lipids are polymers โ€” they aren't (triglycerides are assembled from glycerol + fatty acids, not repeating monomers).
  • Mixing up the food tests (Benedict's = sugars, iodine = starch, emulsion = lipids).

Exam tips

  • Learn condensation (join, remove water) vs hydrolysis (break, add water).
  • Link structure to function (glycogen branched โ†’ fast release; cellulose H-bonded โ†’ strength).
  • Know all the biochemical food tests and results.

Key facts to remember

  • Monomers โ†’ polymers via condensation (removes water); broken by hydrolysis (adds water).
  • Carbohydrates: monosaccharides โ†’ di/polysaccharides via glycosidic bonds (starch/glycogen = stores, cellulose = structural ฮฒ-glucose).
  • Lipids: triglycerides (glycerol + 3 fatty acids, ester bonds) and phospholipids (hydrophilic head/hydrophobic tails โ†’ membranes).
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