22 · Biological Molecules

· Biological Molecules — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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2 · Biological Molecules — Carbohydrates and Their Tests — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe monomers and polymers, the isomers of glucose, disaccharides, polysaccharides and the biochemical tests for carbohydrates.Monomer — a small subunit that…

Macromolecule
Why carbon
Monosaccharides
Link to polymers
Definition
Storage properties

Section 1 of 10Monomers and polymers

Monomer

Monomer a small subunit that bonds with many others to form a polymer, e.g. a monosaccharide, amino acid or nucleotide.

Polymer

Polymer a large molecule made of many repeating monomers joined by condensation reactions.

Macromolecule

Macromolecule a very large molecule of about 1000 or more atoms;
- all polymers are macromolecules but not all macromolecules are polymers.
Triglycerides are large but not made of repeating identical units.

Why carbon

Why carbon each carbon forms four strong covalent bonds, and carbon can form chains, branches and rings.

Section 2 of 10Glucose isomers

Monosaccharides

Monosaccharides classed by carbon number: trioses (3C), pentoses (5C, ribose) and hexoses.
- (6C, glucose, fructose, galactose)

Glucose

C₆H₁₂O₆, existing as alpha and beta isomers that differ only in the position of the –OH on carbon 1.
In alpha-glucose the –OH is below the ring, and in beta-glucose it is above.

Link to polymers

Link to polymers alpha-glucose forms starch and glycogen, and beta-glucose forms cellulose.

Section 3 of 10Disaccharides

Formation
- two monosaccharides join by a glycosidic bond in a condensation reaction.

Disaccharide	Made from	Bond
Maltose	Glucose + glucose	alpha-1,4
Sucrose	Glucose + fructose	alpha-1,2
Lactose	Glucose + galactose	beta-1,4

Uses
- maltose is the sugar in germinating seeds, lactose is milk sugar and sucrose is the transport sugar of plants.

Section 4 of 10Polysaccharides

Definition

Definition polymers of many monosaccharides joined by glycosidic bonds.

Starch

Starch plant store made of amylose and amylopectin.

Glycogen

Glycogen animal and fungal store, more highly branched than amylopectin.

Cellulose

Cellulose structural polysaccharide of plant cell walls.

Storage properties

Storage properties starch and glycogen are insoluble and compact, so they have no osmotic effect and store a lot of glucose.

Section 5 of 10Test for reducing sugars

Test for reducing sugars

Add Benedict's solution to the sample.

Heat in a water bath at about 80 °C or boiling for about 5 minutes.

Observe the colour change: blue, green, yellow, orange, then brick-red precipitate of copper(I) oxide.

Result

Result a precipitate shows a reducing sugar; the further the colour change, the higher the concentration.

Reducing sugars

Reducing sugars all monosaccharides, and maltose and lactose.

Section 6 of 10Test for non-reducing sugars and starch

First do a Benedict's test; if it is negative, take a fresh sample.

Boil with dilute hydrochloric acid to hydrolyse the glycosidic bond.

Neutralise with sodium hydrogencarbonate, since Benedict's needs alkaline conditions.

Repeat the Benedict's test; a brick-red precipitate shows a non-reducing sugar such as sucrose.

Starch test
- add iodine in potassium iodide solution; orange-brown turns blue-black.

Section 7 of 10Cellulose in plant cell walls

Cellulose in plant cell walls

Structure

Structure beta-glucose residues are alternately inverted by 180°, forming straight unbranched chains.

Strength

Strength many hydrogen bonds between parallel chains form microfibrils with high tensile strength.

Function

Function walls resist bursting when cells are turgid and remain freely permeable.

Fibre

Fibre most animals cannot make cellulase, so cellulose passes through as dietary fibre.

Section 8 of 10More from the specification

Triglyceride synthesis via ester bonds

Condensation of one glycerol molecule with three fatty acid chains forms three ester linkages, releasing three water molecules.

Phospholipid bilayer amphipathic properties

Phospholipids possess hydrophilic phosphate heads and hydrophobic hydrocarbon tails, spontaneously assembling into stable double-layered cell membranes.

Saturated vs unsaturated fatty acid chains

Saturated fatty acids contain only single C-C bonds with straight chains that pack closely,
whereas unsaturated fatty acids feature cis double bonds creating kinks.

Phospholipid

Glycerol, two fatty acids and a phosphate group, which is amphipathic.

Hydrophobic

Water-hating; non-polar and unable to dissolve in water.

Lipids

Concentrated energy, insulation, cell membranes.

Fatty acid

A hydrocarbon chain with a carboxyl group.

Section 9 of 10Key terms

Term	Meaning
Monomer	A small subunit that joins with others to form a polymer.
Polymer	A large molecule made of many repeating monomers.
Macromolecule	A very large molecule containing about 1000 or more atoms.
Isomer	A molecule with the same molecular formula but a different structure.
Glycosidic bond	A covalent bond between two monosaccharides formed by condensation.
Reducing sugar	A sugar that can donate electrons, giving a positive Benedict's test.
Polysaccharide	A polymer of many monosaccharides.

Section 10 of 10Exam technique

Saying cellulose is made of alpha-glucose — it is made of beta-glucose with alternate residues inverted.

Forgetting to neutralise before the Benedict's test after acid hydrolysis — the test needs alkaline conditions.

Saying starch and glycogen are equally branched — glycogen is more highly branched.

Writing that hydrolysis removes water — it adds water.

Last updated: 15 October 2026