state the role of covalent bonds in joining smaller molecules together to form polymers

Carbohydrates and Lipids – Biological Molecules (Cambridge AS & A‑Level Biology 9700)

Learning Objective

State the role of covalent bonds in joining smaller molecules together to form polymers.

Key Definitions (syllabus wording)

• Monomer – the smallest unit that can join with others to form a polymer (e.g., a monosaccharide, a fatty‑acid/glycerol unit).
• Polymer – a large molecule composed of many monomer units linked by covalent bonds.
• Disaccharide – a polymer of two monosaccharide units (e.g., maltose, sucrose).
• Polysaccharide – a polymer of three or more monosaccharide units (e.g., starch, glycogen, cellulose).

1. Role of Covalent Bonds in Polymerisation

• Condensation (dehydration) reaction: two monomers combine, a molecule of water (H₂O) is removed and a new covalent bond is formed.
• Hydrolysis: the reverse reaction; water is added to break the covalent bond and release the original monomers (important in digestion).
• The covalent bond created (glycosidic in carbohydrates, ester in lipids) is strong and stable, giving the polymer its structural integrity and determining many of its biological properties.

2. Testing for Biological Molecules (Syllabus 2.1)

Note: Non‑reducing sugars (e.g., sucrose) give a negative Benedict’s test unless first hydrolysed with acid to release free reducing ends.

3. Carbohydrates

3.1 Ring Forms of Glucose (Syllabus 2.2.1)

• Glucose predominantly exists as a six‑membered pyranose ring.
• The anomeric carbon (C‑1) can be in the α‑configuration (‑OH below the plane) or the β‑configuration (‑OH above the plane).
• The configuration determines the type of glycosidic bond formed and, consequently, the properties of the polymer (e.g., starch is α‑linked, cellulose is β‑linked).

3.2 Formation of Glycosidic Bonds

1. Two monosaccharides align; a hydroxyl group on one attacks the anomeric carbon of the other.
2. The attacking –OH oxygen forms a C–O bond with the anomeric carbon while the hydrogen from the attacking –OH and the hydroxyl hydrogen of the partner combine to give H₂O.
3. Release of water = condensation reaction; a covalent C–O–C **glycosidic bond** is created.
4. The orientation (α or β) of the new bond determines the polymer’s digestibility, solubility and mechanical strength.

3.3 Major Carbohydrate Polymers

3.4 Reducing vs. Non‑Reducing Sugars (Syllabus 2.1)

• Reducing sugar: free anomeric carbon capable of acting as a reducing agent (e.g., glucose, maltose). Gives a positive Benedict’s test.
• Non‑reducing sugar: anomeric carbon involved in a glycosidic bond (e.g., sucrose, cellulose). Does not reduce Cu²⁺ unless hydrolysed first.

3.5 Cross‑Reference to Later Topics

• The hydrolysis of starch and glycogen by amylase (Topic 3 – Digestion) breaks the α‑glycosidic bonds, releasing glucose for metabolism.
• Cellulose’s β‑linkages cannot be hydrolysed by human enzymes, explaining dietary fibre’s indigestibility and its role in gut health.

4. Lipids

4.1 Monomeric Units

• Fatty acids – long hydrocarbon chain (usually 14–22 C) terminating in a carboxyl group (–COOH). Can be saturated or unsaturated.
• Glycerol – a three‑carbon tri‑hydroxy alcohol (HO‑CH₂‑CHOH‑CH₂‑OH).

4.2 Formation of Ester Bonds (Triglycerides)

1. The carboxyl group of a fatty acid reacts with a hydroxyl group of glycerol.
2. The –OH hydrogen of glycerol and the –OH hydrogen of the carboxyl group combine to give H₂O.
3. The remaining oxygen of the carboxyl group forms an **ester linkage** (–CO‑O–) with the carbon of glycerol.
4. Repeating this three times yields a **triglyceride** (triacylglycerol). Each step is a condensation reaction.

4.3 Phospholipids (Syllabus 2.2)

• Structure: glycerol + two fatty‑acid chains + one phosphate group (often linked to choline, ethanolamine, serine, etc.).
• The phosphate group is attached by an ester bond; the head‑group is polar while the fatty‑acid tails are non‑polar, giving the molecule amphipathic properties.
• Amphipathicity is the basis of the **fluid‑mosaic model of biological membranes** (Topic 4 – Membranes).

4.4 Functional Significance of Lipid Polymers

• Energy storage: Triglycerides contain ~2 × more energy per gram than carbohydrates because of their reduced oxygen content.
• Structural role: Phospholipids form bilayers that act as selective barriers and platforms for membrane proteins.
• Insulation & protection: Subcutaneous fat reduces heat loss; adipose tissue cushions organs.
• Signalling: Fatty‑acid derivatives (e.g., prostaglandins) act as hormones; the phosphate head‑group can be modified to generate second messengers.

5. Comparison of Polymerisation in Carbohydrates and Lipids

6. Why Covalent Bonds Matter

• They provide the strong, stable connections required for the structural integrity of polymers.
• In carbohydrates, the **type (α or β) and position** of the glycosidic bond dictate digestibility, solubility, and mechanical strength (e.g., starch vs. cellulose).
• In lipids, the **ester linkage** anchors fatty‑acid chains to glycerol; the length, degree of saturation and the presence of a phosphate head‑group influence energy density, membrane fluidity and signalling capacity.
• Water is both a reactant (hydrolysis) and a by‑product (condensation), linking polymer chemistry to physiological processes such as digestion, metabolism and membrane dynamics.

7. Brief Overview of the Remaining Syllabus Topics (for completeness)

7.1 Proteins (Topic 2.3)

• Monomer: amino acid (α‑amino, α‑carboxyl, side‑chain R).
• Polymerisation: peptide bond (C–N) formed by condensation between the –COOH of one amino acid and the –NH₂ of another.
• Four levels of structure (primary → quaternary) and functional examples (haemoglobin, collagen, enzymes).

7.2 Water (Topic 2.4)

• Polarity and hydrogen‑bonding give water a high specific heat, surface tension and solvent power.
• These properties are essential for temperature regulation, transport of metabolites and the proper functioning of enzymes.

8. Summary

• Covalent bonds—glycosidic in carbohydrates and ester in lipids—join monomers through condensation reactions that release water.
• The orientation (α/β) and position of these bonds determine the physical properties, biological roles and digestibility of the resulting polymers.
• Key carbohydrate polymers: starch (α‑glucose, energy storage), glycogen (highly branched α‑glucose, rapid glucose release), cellulose (β‑glucose, structural fibre).
• Key lipid polymers: triglycerides (three ester‑linked fatty acids, dense energy store) and phospholipids (two fatty acids + phosphate ester, membrane formation).
• Understanding these covalent linkages underpins later topics such as digestion of polysaccharides (hydrolysis), membrane structure and fluidity (phospholipid bilayer), and energy metabolism.