Carbohydrates and Lipids

A-Level Biology · Biological Molecules

Biological molecules and polymers

Living things are built from a few key groups of molecules. Many are polymers — 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 — glucose (α and β forms), fructose, galactose.
Disaccharides: two monosaccharides joined by a glycosidic bond (condensation):
glucose + glucose → maltose
glucose + fructose → sucrose
glucose + galactose → lactose
Polysaccharides: many monosaccharides:
Starch (amylose + amylopectin) — energy store in plants; coiled, compact, insoluble (so no osmotic effect).
Glycogen — energy store in animals; highly branched for rapid glucose release.
Cellulose — made of β-glucose; straight chains with H-bonds forming strong fibres for plant cell walls.

Testing for sugars/starch

Reducing sugars: heat with Benedict's solution → brick-red precipitate.
Non-reducing sugars: boil with acid first (to hydrolyse), neutralise, then Benedict's.
Starch: add iodine solution → 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 — mix with ethanol then water → 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 β-glucose, forming straight chains held by many hydrogen bonds into strong microfibrils — ideal for structural support. Starch coils into a compact store and lacks this strength. ✓

Common mistakes

Forgetting bonds: glycosidic (carbohydrates), ester (lipids), peptide (proteins).
Saying lipids are polymers — 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 → fast release; cellulose H-bonded → strength).
Know all the biochemical food tests and results.

Key facts to remember

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