Carbohydrates and Lipids – Key Concepts

Page 1 – Significance of the Coiled Structure of Polymer P (Starch)

• The helix (coiled) configuration packs many glucose sub-units into a small volume, making the macromolecule compact.
• Compactness → efficient storage inside plastids (plants) or granules (animals).
• A dense, space-saving shape minimises osmotic effect in cytoplasm.

Page 2 – Converting Starch to a Respiratory Substrate

1. Hydrolysis by the enzyme α-amylase breaks the α-1,4 glycosidic bonds in starch.
2. Product = free α-glucose monomers.
3. α-glucose enters glycolysis and aerobic/anaerobic respiration to release ATP energy.

(Any synonym such as “broken down / converted / produced / transformed” is acceptable.)

Page 3 – Formation of Maltose (Condensation)

• Maltose = disaccharide formed from two α-glucose molecules.
• Reaction = condensation (dehydration synthesis).
• One molecule of water (H₂O) is removed.
• Bond formed: α-1,4-glycosidic linkage between the –OH on C-1 of the first glucose and the –OH on C-4 of the second glucose.
• Enzyme that catalyses both directions = maltase.

\text{α-Glucose}{(1)} + \text{α-Glucose}{(2)} \xrightarrow[\small maltase]{\small condensation} \text{Maltose} + \text{H}_2\text O

Page 4 – Breakdown of Maltose (Hydrolysis)

• Hydrolysis = reverse reaction; water is added.
• Maltase again catalyses cleavage of the α-1,4 bond → two α-glucose molecules.
• Reaction is reversible (condensation ⇌ hydrolysis).

Page 5 – Topic Transition

• Beginning of Lipids section.

Page 6 – Basic Definition of Lipids

• Organic compounds containing C, H, O.
• Proportion of O lower than in carbohydrates.
• Insoluble in water, but soluble in non-polar organic solvents (alcohol, ether, chloroform).

Page 7 – General Properties of Lipids

• Not true polymers; grouped by hydrophobicity.
• General empirical formula often simplified as \text Cn\text H{2n}\text O_2 (varies).
• Energy density: many C–H bonds → > 2× energy per gram compared with carbohydrates.
• Hydrophobic behaviour ⇒ little/no affinity for water.
• Soluble in ether, acetone, chloroform, hot alcohol.

Page 8 – Three Major Classes of Lipids

1. Triglycerides – fats (solid) & oils (liquid).
2. Phospholipids – e.g. lecithin.
3. Steroids – e.g. cholesterol.

Page 9 – Structure of Glycerol

• Glycerol = 3-carbon alcohol (propane-1,2,3-triol).
• Each carbon bears a hydroxyl group (–OH), enabling ester linkage formation.

Page 10 – Structure of Fatty Acids

• Long unbranched hydrocarbon chain (commonly 16–18 C).
• Terminal carboxylic acid group (–COOH) → acidic property.

Page 11 – Formation of Triglycerides

• Triglyceride = glycerol + 3 fatty acids.
• Bond: ester linkage (between –OH of glycerol & –COOH of fatty acid).
• 3 water molecules removed in condensation.

Page 12 – Condensation vs Hydrolysis of Triglycerides

Condensation:
\text{Glycerol} + 3\;\text{Fatty acids} \xrightarrow{\small condensation} \text{Triglyceride} + 3\;\text{H}_2\text O

Hydrolysis (reverse):
\text{Triglyceride} + 3\;\text{H}_2\text O \xrightarrow{\small lipase} \text{Glycerol} + 3\;\text{Fatty acids}

Page 13 – Example Lipid Components

• Linoleic acid (poly-unsaturated).
• Stearic acid (saturated).
• Illustrations show esterified fatty acids on glycerol backbone.

Page 14 – Saturated vs Unsaturated Fatty Acids

• Stearic acid (C₁₈:0) – no C=C, straight chain → packs tightly → solid at room T.
• Oleic acid (C₁₈:1 Δ9) – one cis C=C, introduces a “kink” → prevents tight packing → liquid.
• Figure 1.17 emphasises state at room temperature (solid vs liquid) & source (animal vs plant oil).

Page 15 – Comparison Table (Triglycerides)

Similarities:
(a) Both triglycerides; (b) Formed by condensation; (c) Hydrolysis yields glycerol + FA; (d) Energy storage.

Differences:
• Unsaturated fats – ≥1 double bond, liquid, chemically reactive, lower LDL, plant oils (olive, sunflower, corn).
• Saturated fats – no double bonds, solid, less reactive, elevate LDL, animal fats (lard, butter, full-cream milk).

Page 16 – Phospholipids Overview

• Contain phosphate group ⇒ amphipathic.
• One glycerol + two FA chains + phosphate (often with choline, ethanolamine, etc.).
• Hydrophilic “head” (phosphate) & hydrophobic “tails” (fatty acid chains).
• Example: lecithin in cell membranes.

Page 17 – Membrane Role of Phospholipids

1. Form lipid bilayers – structural basis of all cell membranes.
2. Provide membrane fluidity.
3. Permit diffusion of lipid-soluble & small non-polar molecules.
4. Lecithin first identified phospholipid (phosphatidylcholine).

Page 18 – Steroids

• Characteristic backbone: four fused rings (3 × 6-C + 1 × 5-C = 17 C).
• Variations arise from side-chain length & functional groups.
• Examples: cholesterol, testosterone.

Page 19 – Cholesterol Structure & Labelling

• 27 carbon atoms in four fused rings + hydrocarbon tail.
• Hydrophilic end: single hydroxyl (–OH) group at C-3 interacts with water.
• Hydrophobic end: rings + isooctyl tail, lipid-soluble.

(Drawing should highlight polar –OH vs non-polar ring system.)

Page 20 – Properties of Cholesterol

• Amphipathic: tiny polar head, large non-polar body.
• Dissolves in non-polar solvents; poorly soluble in water except for –OH interactions.

Page 21 – Metabolic Roles of Cholesterol

1. Precursor for myelin sheath formation → rapid nerve impulse conduction.
2. Substrate for steroid hormone synthesis (progesterone, testosterone, etc.).
3. Precursor of bile salts – emulsify dietary fats.
4. Precursor of vitamin D – Ca²⁺ homeostasis, healthy bones/teeth.

Page 22 – General Importance of Lipids

1. Energy source: high C–H bond density; yield 38\;\text J\,\text g^{-1} (≈ 2× carbohydrates).
2. Energy storage: reduces body mass for mobility; packed in seeds & adipocytes.
3. Thermal insulation: adipose under dermis; blubber in whales.
4. Mechanical protection: fat cushions organs (e.g., kidneys).
5. Neural insulation: constituent of myelin.
6. Metabolic water: oxidation of fats supplies water (desert fauna).
7. Membrane components: phospholipids + cholesterol maintain fluidity.
8. Hormonal & signalling precursors: steroid hormones coordinate physiology.
9. Cuticular waxes: reduce transpiration & pathogen entry in plants.
10. Vitamin solvents: dissolve & transport fat-soluble vitamins (A, D, E, K).