state that triglycerides are non-polar hydrophobic molecules and describe the molecular structure of triglycerides with reference to fatty acids (saturated and unsaturated), glycerol and the formation of ester bonds

Cambridge AS & A Level Biology (9700) – Biological Molecules

Learning Objective (AO1)

State that triglycerides are non‑polar, hydrophobic molecules and describe their molecular structure with reference to fatty acids (saturated and unsaturated), glycerol and the formation of ester bonds.

1. Overview of the Biological‑Molecule Block

• The syllabus examines four major classes of biomolecules: carbohydrates, lipids, proteins and water.
• Each class is linked to later topics such as cell membranes, enzymes, metabolism and homeostasis.
• Key assessment points: naming, structural formulae, structure‑function relationships, physical properties and biological roles.

2. Classification of Lipids

Cambridge groups lipids into three hierarchical categories.

3. Carbohydrates – Quick Reference & Tests

3.1 Common Carbohydrate Tests

4. Proteins – Structure, Interactions & Examples

• Monomer: Amino acid – general formula NH₂–CHR–COOH.
• Peptide bond: Condensation of the carboxyl group of one amino acid with the amino group of the next, releasing H₂O.

4.1 Levels of Protein Structure

1. Primary: Linear sequence of amino acids.
2. Secondary: Local folding into α‑helices or β‑sheets (hydrogen‑bond stabilised).
3. Tertiary: Three‑dimensional shape formed by interactions between side‑chains (hydrophobic interactions, disulphide bridges, ionic bonds, hydrogen bonds).
4. Quaternary: Association of two or more polypeptide subunits.

4.2 Important Inter‑molecular Interactions

• Hydrogen bonds
• Ionic (salt) bridges
• Hydrophobic interactions
• Disulphide (–S–S–) bonds (cysteine)

4.3 Representative Examples (syllabus expectations)

5. Water – Physicochemical Properties & Biological Importance

• Polarity & hydrogen‑bonding: Each molecule can form up to four hydrogen bonds, giving water a high cohesion, surface tension and solvent power.
• Specific heat (4.18 J g⁻¹ °C⁻¹): Large amount of energy required to change temperature – crucial for temperature regulation in organisms.
• Latent heat of vaporisation (2260 J g⁻¹): Provides cooling through sweating and transpiration.
• Universal solvent: Dissolves ionic and polar substances, allowing biochemical reactions to occur in aqueous media.
• Role in hydrolysis & condensation: Water is a reactant in hydrolysis (e.g., breaking glycosidic or peptide bonds) and a product in condensation (e.g., formation of ester or peptide bonds).

6. Lipids – Focus on Triglycerides

6.1 Why Triglycerides Are Non‑Polar & Hydrophobic

• Each molecule contains three long hydrocarbon chains (fatty‑acid tails) that consist almost entirely of C–H bonds – non‑polar.
• The glycerol backbone contributes only three oxygen atoms that are tied up in ester linkages; the overall molecule is dominated by the non‑polar region.
• Water, being polar, cannot form favourable interactions with this extensive non‑polar surface, so triglycerides are insoluble in aqueous media.

6.2 Molecular Components

1. Glycerol: A three‑carbon tri‑hydroxy alcohol, formula C₃H₈O₃.
2. Fatty acids: Long‑chain carboxylic acids, general formula CH₃(CH₂)_nCOOH (n ≈ 14–22).

6.3 Fatty‑Acid Types

6.4 Formation of Ester Bonds (Esterification)

Condensation (dehydration) of a carboxyl group of a fatty acid with a hydroxyl group of glycerol:

Repeating the reaction three times links three fatty‑acid chains to the three –OH groups of glycerol, giving a tri‑acylglycerol and releasing three molecules of water:

6.5 Overall Structural Formula (shorthand)

Each R represents the hydrocarbon tail of a fatty acid (saturated or unsaturated). The three tails may be identical (e.g., tripalmitin) or different, giving rise to the great diversity of natural fats and oils.

6.6 Energy Content & Physical State

Melting‑point trend: Saturated fatty‑acid chains pack tightly → higher melting point (solid fats). Each cis‑double bond introduces a kink, reducing packing efficiency → lower melting point (liquid oils).

6.7 Functional Significance of Triglyceride Structure

• Energy storage: Oxidation of the long hydrocarbon chains yields ~9 kcal g⁻¹, more than double the energy from carbohydrates or proteins.
• Insulation & buoyancy: Hydrophobic nature prevents water loss and provides a lightweight energy reserve.
• Membrane precursor: Fatty acids are liberated for the synthesis of phospholipids, the main components of the fluid‑mosaic cell membrane.
• Physical state: Saturated fats are solid at room temperature; unsaturated fats are liquid because double‑bond kinks hinder tight packing.

7. Other Lipid Classes (required by the syllabus)

7.1 Phospholipids

• Structure: Glycerol backbone + two fatty‑acid tails (hydrophobic) + phosphate group attached to a polar head group (e.g., choline, serine).
• Amphipathic nature – one side hydrophobic, the other hydrophilic – drives formation of bilayers.
• Key role: Core component of the fluid‑mosaic cell membrane; provides a barrier to polar molecules while allowing membrane proteins to function.

7.2 Sterols (e.g., Cholesterol)

• Structure: Four fused carbon rings with a hydroxyl group and a hydrocarbon tail.
• Amphipathic – the small –OH group interacts with the polar head region of the membrane, the rigid ring system embeds in the hydrophobic core.
• Functions: Modulates membrane fluidity, precursor for steroid hormones and bile acids.

8. Links to Other Syllabus Topics

• Cell membranes & transport (Section 4): Understanding the amphipathic nature of phospholipids (derived from glycerol and fatty acids) explains why the membrane core is impermeable to polar solutes.
• Enzymes (Section 3): Lipases catalyse the hydrolysis of triglyceride ester bonds during digestion – the reverse of esterification.
• Energy & respiration (Section 12 A‑Level): β‑oxidation of fatty‑acid chains yields acetyl‑CoA, feeding the Krebs cycle and oxidative phosphorylation.
• Homeostasis (Section 14 A‑Level): Hormonal regulation (insulin, glucagon, epinephrine) controls mobilisation of stored triglycerides in adipose tissue to maintain blood glucose levels.

9. Practical Skills Related to Biological Molecules

1. Solubility test (lipids): Mix a few drops of oil with water; observe phase separation to demonstrate hydrophobicity.
2. Saponification (triglyceride hydrolysis): Treat a fat with NaOH; the resulting soap (fatty‑acid salts) and glycerol illustrate ester‑bond cleavage.
3. Melting‑point determination: Compare a saturated fat (e.g., butter) with an unsaturated oil (e.g., olive oil) to relate degree of saturation to physical state.
4. Carbohydrate testing: Perform Benedict’s, iodine, and Molisch tests; hydrolyse a polysaccharide with dilute acid and retest to show conversion of non‑reducing sugars into reducing sugars.
5. Protein denaturation: Add heat or acid to egg‑white; observe loss of tertiary structure (coagulation) and discuss the role of hydrogen bonds and disulphide bridges.

10. Summary Points (AO1)

• Triglycerides are non‑polar, hydrophobic molecules because they consist mainly of long hydrocarbon fatty‑acid tails.
• They are formed by esterification of one glycerol molecule with three fatty‑acid molecules, producing three ester bonds and three molecules of water.
• Fatty‑acid tails may be saturated (no C=C) or unsaturated (one or more C=C); saturation influences packing density, melting point and physical state.
• Energy yield of triglycerides (~9 kcal g⁻¹) is roughly double that of carbohydrates or proteins (~4 kcal g⁻¹).
• Triglycerides serve as long‑term energy stores, insulation, buoyancy aids and as a source of fatty acids for membrane phospholipids.
• Other lipid classes (phospholipids, sterols) are essential for membrane structure and signalling; they are built from the same basic glycerol‑fatty‑acid building blocks.
• Understanding the chemistry of carbohydrates, proteins, lipids and water provides the foundation for later topics on membranes, enzymes, metabolism and homeostasis.

11. Suggested Diagram (Insert in teaching material)

A labelled schematic should show:

• Glycerol backbone (three carbon atoms, each bearing an –OH group before esterification).
• Three ester linkages (–COO–) joining the glycerol to three fatty‑acid tails.
• One saturated tail (straight) and one unsaturated tail (kinked) to illustrate structural diversity.
• Optional: a separate inset of a phospholipid molecule highlighting the hydrophilic head group and the two hydrophobic tails.