define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide

Cambridge A‑Level Biology 9700 – Biological Molecules (Topic 2)

Learning Objectives

• Define the key terminology for carbohydrates, lipids and proteins.
• Describe the structural basis of monomers, polymers and macromolecules.
• Explain the main laboratory tests used to identify biological molecules and what each test indicates.
• Relate structure to function for carbohydrates, lipids and proteins, including the role of water.
• Apply knowledge to answer AO1–AO3 exam questions.

1. Key Terminology (Carbohydrates)

Relationships Between the Terms

1. Monosaccharides are the monomers of carbohydrate polymers.
2. Two monosaccharides join to give a disaccharide.
3. Many monosaccharides join to give a polysaccharide, which is a type of polymer.
4. Polysaccharides, together with proteins, nucleic acids and some lipids, are classified as macromolecules.

2. Laboratory Tests for Biological Molecules (Topic 2.1)

Practical Safety Checklist (AO3)

• 🔸 Use heat‑proof gloves when handling hot water baths.
• 🔸 Wear safety goggles for all tests involving acids, bases or concentrated reagents.
• 🔸 Work in a well‑ventilated area when using iodine or volatile solvents.
• 🔸 Dispose of copper‑containing waste in designated containers.
• 🔸 Neutralise acidic solutions before discarding.

3. Link to Water (Syllabus Requirement 2.2)

Water’s polarity and ability to form hydrogen bonds underlie the behaviour of all major biomolecules:

• Carbohydrates: The numerous –OH groups on monosaccharides make them highly soluble; hydrogen‑bonding explains why starch granules swell in hot water and why cellulose fibres are insoluble (extensive inter‑chain H‑bonds).
• Lipids: Their long non‑polar hydrocarbon chains are hydrophobic, leading to segregation from water and the formation of membranes, micelles and lipoprotein particles.
• Proteins: Polar side‑chains interact with water, stabilising secondary structures; hydrophobic residues are driven to the interior of globular proteins, a process essential for proper folding.

Thus, the physical properties of water (high specific heat, cohesion, solvent ability) are the basis for the structure‑function relationships of biomolecules.

4. Carbohydrate Structures

4.1 α‑ and β‑Glucose

Glucose exists in an open‑chain form and in cyclic hemiacetal forms. In aqueous solution the predominant cyclic forms are:

• α‑D‑glucose: the –OH on C‑1 is trans (below the plane) to the CH₂OH group on C‑5.
• β‑D‑glucose: the –OH on C‑1 is cis (above the plane) to the CH₂OH group on C‑5.

Both cyclise to a six‑membered pyranose ring.

4.2 Glycosidic Bonds & Condensation

• Formation of a glycosidic bond = dehydration synthesis (loss of H₂O).
• Bond notation examples:

  • α‑1,4‑glycosidic – amylose (starch) and glycogen.
  • β‑1,4‑glycosidic – cellulose.
  • α‑1,6‑glycosidic – branch points in glycogen and amylopectin.

4.3 Reducing vs. Non‑reducing Sugars

• Reducing sugar: possesses a free hemiacetal/hemiketal group that can open to an aldehyde/ketone and reduce Cu²⁺ in Benedict’s test (e.g., glucose, maltose).
• Non‑reducing sugar: both anomeric carbons are involved in glycosidic bonds, preventing the open‑chain form (e.g., sucrose).

4.4 Comparison: Storage vs. Structural Polysaccharides

5. Lipids

5.1 General Structure of a Triglyceride

• Glycerol backbone – three carbon atoms each bearing a –OH group.
• Three fatty‑acid chains esterified to the –OH groups.
• The resulting molecule is non‑polar and insoluble in water.

5.2 Phospholipids – Amphipathic Nature

• Glycerol backbone with two fatty‑acid tails (hydrophobic) and one phosphate‑containing head‑group (hydrophilic).
• Amphipathic molecules self‑assemble into bilayers because the tails avoid water while the heads interact with the aqueous environment.
• Key to membrane fluidity, formation of micelles, liposomes and biological membranes.

6. Proteins (Topic 2.3)

6.1 Amino‑Acid Structure

General formula:

\$\mathrm{H_2N\!-\!CH(R)\!-\!COOH}\$

• α‑Carbon – central carbon bearing four groups.
• R‑group (side chain) – determines the chemical nature of the amino acid (non‑polar, polar uncharged, acidic, basic).
• At physiological pH the amino group is protonated (–NH₃⁺) and the carboxyl group is deprotonated (–COO⁻), giving a zwitterion.

6.2 Peptide Bond Formation

• Condensation (dehydration) reaction between the –COOH of one amino acid and the –NH₂ of the next.
• Produces a covalent –CO‑NH– (amide) linkage and releases H₂O.
• The resulting chain is called a polypeptide.

6.3 Levels of Protein Structure

Why Structure Matters (Linking Levels to Function)

• Primary structure determines the chemical properties of side‑chains, influencing folding and active‑site composition.
• Secondary structure creates regular patterns (α‑helix, β‑sheet) that provide mechanical strength or elasticity.
• Tertiary structure positions functional groups in three‑dimensional space, creating active sites, binding pockets or channels.
• Quaternary structure enables cooperative interactions (e.g., haemoglobin’s allosteric O₂ binding) and formation of large fibrous assemblies (e.g., collagen).

Post‑Translational Modifications (Optional Depth for AO2)

• Phosphorylation – adds a phosphate group (often on Ser, Thr, Tyr) → alters activity or localisation.
• Glycosylation – attachment of carbohydrate chains → stabilises proteins, aids cell‑cell recognition.
• Acetylation, methylation, ubiquitination – regulate protein turnover and gene expression.

7. Water – Properties and Biological Relevance (Syllabus 2.4)

8. Key Points to Remember (Across All Biomolecules)

• Monomers join by dehydration synthesis (loss of H₂O); polymers are broken down by hydrolysis (addition of H₂O).
• Carbohydrate polymers differ in the type of glycosidic bond (α vs β) and degree of branching – these dictate whether the polymer is a storage (energy) or structural molecule.
• Lipids are characterised by long‑chain hydrocarbons; their hydrophobic nature makes them excellent energy stores and essential components of biological membranes.
• Protein function depends on the precise three‑dimensional shape, which is determined by the amino‑acid sequence and stabilised by a hierarchy of non‑covalent interactions and, where required, covalent disulfide bonds.
• Water’s unique properties provide the medium in which all these biomolecules act, influencing solubility, folding, and biochemical reactions.
• Laboratory tests exploit specific chemical properties (reduction of Cu²⁺, charge‑transfer complexes, precipitation of Cu⁺, peptide‑nitrogen complexation) to give rapid qualitative identification of biomolecules.

Action‑Oriented Review of the “Biological Molecules” Lecture Notes vs. Cambridge 9700 Syllabus (Topic 2)