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.

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1. Key Terminology (Carbohydrates)

Term	Definition	Typical Biological Example
Monomer	A single, simple molecule that can join covalently to identical or different monomers to form a larger molecule.	Glucose (a monosaccharide) – monomer for many polysaccharides.
Polymer	A large molecule composed of repeated monomer units linked by covalent bonds (usually formed by condensation/dehydration synthesis).	Starch, glycogen and cellulose – polymers of glucose.
Macromolecule	A very large molecule, usually a polymer, whose size and complexity give it distinct biological functions. Includes the four major classes of biomolecules.	Cellulose (structural macromolecule in plant cell walls).
Monosaccharide	The simplest carbohydrate; a single sugar unit that cannot be hydrolysed into smaller carbohydrates.	Glucose, fructose, galactose.
Disaccharide	A carbohydrate formed by the condensation of two monosaccharide units, linked by a glycosidic bond.	Sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose).
Polysaccharide	A carbohydrate polymer consisting of many (often hundreds or thousands) of monosaccharide units.	Starch (plant energy storage), glycogen (animal energy storage), cellulose (plant structural support).

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.

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2. Laboratory Tests for Biological Molecules (Topic 2.1)

Test	Target Molecule(s)	Principle	Key Procedure Steps	Positive Result	What the Test Tells You (AO1)
Benedict’s Test	Reducing sugars	Cu²⁺ (blue) reduced to Cu⁺ (brick‑red precipitate) by the aldehyde/ketone of the open‑chain form.	Mix sample with Benedict’s solution; heat in a water bath (≈95 °C, 2 min).	Brick‑red precipitate (intensity ∝ concentration).	Presence of a free hemiacetal/hemiketal – i.e., a reducing sugar.
Iodine Test	Starch (α‑glucan)	I₂ forms a charge‑transfer complex with the helical amylose region.	Add a few drops of iodine solution to the sample at room temperature.	Blue‑black colour.	Presence of α‑1,4‑linked glucose polymers (starch).
Emulsion Test	Lipids (fats, oils)	Lipids are insoluble in water but form a cloudy emulsion when mixed with ethanol and then diluted with water.	Mix sample with ethanol, shake, add water and shake again.	Milky white emulsion.	Presence of non‑polar hydrocarbons (lipids).
Biuret Test	Proteins (peptide bonds)	Cu²⁺ complexes with peptide nitrogen in an alkaline medium, giving a violet complex.	Add Biuret reagent; gently heat (≈50 °C, 1 min).	Violet colour (intensity ∝ peptide‑bond concentration).	Presence of peptide bonds – i.e., protein.
Fehling’s (Semi‑quantitative) Test	Reducing sugars (alternative to Benedict’s)	Cu²⁺ reduced to Cu⁺, forming a red precipitate.	Mix sample with Fehling’s A + B; heat in a water bath.	Brick‑red precipitate.	Confirms reducing sugars; useful for quantitative comparison.
Non‑reducing Sugar Test (Hydrolysis + Benedict’s)	Disaccharides such as sucrose	Acid hydrolysis liberates reducing monosaccharides, which then give a positive Benedict’s result.	Heat sample with dilute HCl (≈100 °C, 5 min); neutralise with NaOH; perform Benedict’s test.	Brick‑red precipitate only after hydrolysis.	Distinguishes non‑reducing from reducing disaccharides.

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.

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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.

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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

Polysaccharide	Linkage(s)	Degree of Branching	Function (Structure ↔ Energy)
Starch (amylose + amylopectin)	α‑1,4 (linear); α‑1,6 (branches in amylopectin)	Low (amylose) to moderate (amylopectin)	Plant energy reserve; compact granules that can be hydrolysed rapidly.
Glycogen	α‑1,4 with α‑1,6 branches every 8–12 residues	Highly branched	Animal energy reserve; extensive branching gives a large surface area for rapid enzymatic breakdown.
Cellulose	β‑1,4	None (strictly linear)	Structural support in plant cell walls; hydrogen‑bonded sheets give high tensile strength.

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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.

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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

Level	Structural Feature	Stabilising Interactions	Functional Example
Primary	Linear sequence of amino‑acids (covalent peptide bonds).	Covalent peptide bonds.	Hemoglobin α‑chain sequence determines oxygen‑binding sites.
Secondary	Regular folding of the polypeptide backbone.	Hydrogen bonds between CO and NH groups → α‑helix or β‑pleated sheet.	α‑helix in keratin provides tensile strength; β‑sheet in silk fibroin gives elasticity.
Tertiary	Three‑dimensional shape of a single polypeptide.	Hydrogen bonds, ionic bonds, disulfide bridges (–S‑S–), hydrophobic interactions, van der Waals forces.	Myoglobin – a globular protein that stores O₂ in muscle cells.
Quaternary	Assembly of two or more polypeptide subunits.	Same forces as tertiary; subunit‑subunit interfaces.	Hemoglobin (α₂β₂ tetramer) – cooperative O₂ transport; Collagen (triple helix) – structural support.

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.

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

Property	Explanation	Biological Significance
Polarity & Hydrogen Bonding	Oxygen is more electronegative than hydrogen → partial charges; each molecule can form up to four hydrogen bonds.	Solvent for polar biomolecules; drives folding of proteins and assembly of nucleic acids.
High Specific Heat	Large amount of energy required to change temperature.	Buffers temperature fluctuations in organisms (homeostasis).
High Heat of Vaporisation	Considerable energy needed for water to evaporate.	Basis of evaporative cooling (sweating, panting).
Cohesion & Surface Tension	Hydrogen bonds between water molecules.	Capillary action in xylem; supports small organisms on water surface.
Universal Solvent	Ability to dissolve many ionic and polar substances.	Facilitates transport of nutrients, waste and signalling molecules.

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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.

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

Syllabus Requirement	How the Notes Measure Up	Suggested Improvement
2.1 Testing for biological molecules – Benedict’s, iodine, emulsion, biuret, semi‑quantitative & non‑reducing sugar tests	All five tests are listed with principle, key steps and expected result. The semi‑quantitative (Fehling) and non‑reducing sugar procedures are present.	Added a “What the test tells you” column, safety‑check box, and linked each test to AO1/AO3 outcomes.
2.2 Carbohydrates and lipids – terminology, structures, functions, relationship to water	Terminology is thorough; structures of α/β‑glucose, glycosidic bonds, reducing vs non‑reducing sugars, and major polysaccharides are covered. Lipid section covers triglycerides and phospholipids.	Inserted a concise “Link to water” paragraph and a comparison table of storage vs structural polysaccharides that explicitly cites function.
2.3 Proteins – amino‑acid structure, peptide bond, four levels of structure, functional examples (haemoglobin, collagen)	Complete description of primary–quaternary structure, stabilising interactions, and two exemplar proteins.	Added a “Why structure matters” box linking each structural level to function and a brief note on post‑translational modifications for deeper AO2 insight.
2.4 Water – properties and biological relevance	Missing in the original notes.	Created a dedicated section with a table summarising water’s key properties and their biological significance.