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📌Definition Table

Term	Definition
Lipid	Hydrophobic organic molecule including fats, oils, waxes, steroids, and phospholipids.
Triglyceride	Lipid formed from glycerol and three fatty acids.
Fatty Acid	Hydrocarbon chain with a carboxyl group; may be saturated or unsaturated.
Saturated Fatty Acid	Fatty acid with no double bonds between carbon atoms.
Unsaturated Fatty Acid	Fatty acid with one or more double bonds between carbon atoms.
Phospholipid	Lipid with two fatty acids and a phosphate group attached to glycerol; amphipathic.

📌Introduction

Lipids are a diverse group of hydrophobic molecules that play key roles in energy storage, membrane structure, insulation, and signalling. Their nonpolar nature makes them insoluble in water but soluble in nonpolar solvents. Lipids are not true polymers, as they are not composed of repeating monomers, but they are built from smaller units like glycerol and fatty acids.

❤️ CAS Link: Create a dietary awareness campaign showing the health effects of different fats, including omega-3 benefits and risks of trans fats.

📌 Structure and Types of Lipids

• Triglycerides: Glycerol + three fatty acids linked via ester bonds.

• Fatty acids can be saturated (straight chains, solid at room temp) or unsaturated (kinked chains, liquid at room temp).
• Monounsaturated: One double bond; Polyunsaturated: multiple double bonds.
• Phospholipids: Amphipathic molecules forming the bilayer of membranes.

• Steroids: Four fused carbon rings; include cholesterol, testosterone, and oestradiol.
• Waxes provide waterproofing in plants and animals.

🧠 Examiner Tip: Always specify that phospholipids are amphipathic, as this links structure directly to membrane function.

📌 Properties and Biological Roles

• High energy yield per gram — ideal for long-term energy storage.
• Insolubility in water prevents interference with osmotic balance.
• Provide insulation in animals (e.g., blubber in whales).
• Aid buoyancy in aquatic animals due to low density.
• Serve as precursors for hormones and signalling molecules.
• Contribute to waterproofing in plants (cuticle) and animals (feathers).

🌍 Real-World Connection: Polar bears rely on stored lipid reserves for insulation and energy during fasting periods in Arctic winters.

📌 Phospholipid Bilayer and Membranes

• Phospholipids arrange with hydrophobic tails inwards and hydrophilic heads outwards.
• Creates a selectively permeable barrier controlling entry and exit of substances.
• Allows diffusion of nonpolar molecules (O₂, CO₂, steroid hormones).
• Embedded proteins assist in transport of polar molecules and ions.
• Fluidity affected by fatty acid composition and cholesterol content.
• Essential for compartmentalisation of cellular processes.

📝 Paper 2: Data Response Tip: For diagrams showing triglyceride formation, label ester bonds, glycerol, fatty acids, and water molecules — missing labels lose easy marks.

📌Definition Table

Term	Definition
Monosaccharide	Single sugar unit with general formula CₙH₂ₙOₙ (e.g., glucose, ribose).
Disaccharide	Sugar composed of two monosaccharides joined by a glycosidic bond.
Polysaccharide	Large carbohydrate polymer of monosaccharide units linked by glycosidic bonds.
Glycosidic Bond	Covalent bond between two monosaccharides formed in a condensation reaction.
Isomer	Molecules with the same chemical formula but different structural arrangements.
Glycoprotein	Protein with carbohydrate chains attached, often involved in cell recognition.

📌Introduction

Carbohydrates are organic molecules made of carbon, hydrogen, and oxygen in a 1:2:1 ratio. They range from small, soluble monosaccharides to large, insoluble polysaccharides. Carbohydrates serve as energy sources, structural components, and recognition molecules in cells. Variations in structure, such as α- and β-glucose, lead to diverse biological functions.

❤️ CAS Link: Conduct a nutrition awareness project teaching communities how to interpret carbohydrate content on food labels and its link to energy needs.

📌 Monosaccharides

• Basic building blocks of carbohydrates.
• Common examples: glucose, galactose, fructose, ribose.
• α-glucose and β-glucose differ in orientation of hydroxyl group on C1 — critical for polysaccharide structure.
• Highly soluble in water, making them easy for transport in the bloodstream or sap.
• Serve as immediate energy sources in cellular respiration.
• Ribose is a key component of RNA and ATP.

🧠 Examiner Tip: In structural questions, always draw α- and β-glucose accurately — hydroxyl group position on C1 must be correct for marks.

📌 Disaccharides

• Formed by condensation of two monosaccharides.
• Examples:
  • Maltose = glucose + glucose
  • Sucrose = glucose + fructose
  • Lactose = glucose + galactose
• Glycosidic bonds (α-1,4 or β-1,4) determine digestibility and function.
• Soluble but less so than monosaccharides.
• Transport form of sugar in some organisms (e.g., sucrose in plants).

🌍 Real-World Connection: Lactose intolerance is caused by lack of lactase enzyme, leading to gastrointestinal symptoms when lactose remains undigested.

📌 Polysaccharides

• Long chains of monosaccharides linked by glycosidic bonds.
• Starch: Plant energy store — amylose (unbranched helix) and amylopectin (branched).
• Glycogen: Animal energy store, highly branched for rapid glucose release.
• Cellulose: Structural polysaccharide in plant cell walls; β-glucose with alternating orientation for hydrogen bonding.
• Insoluble, making them good for storage or structure.
• Different branching patterns affect digestibility and function.

📝 Paper 1 Tip: When given structural diagrams, identify monosaccharide units, glycosidic bonds, and orientation — small details often determine the mark.

📌Definition Table

Term	Definition
Carbon Skeleton	The chain or ring of carbon atoms forming the structural backbone of an organic molecule.
Functional Group	A specific group of atoms that imparts characteristic chemical properties to a molecule.
Monomer	Small, repeating unit that can be joined to form polymers.
Polymer	Large molecule made of repeating monomer units.
Condensation Reaction	Chemical reaction where monomers join, releasing water.
Hydrolysis Reaction	Chemical process that breaks polymers into monomers using water.

📌Introduction

Carbon’s ability to form four covalent bonds and create diverse structures underpins all biological macromolecules. The arrangement of carbon atoms — in chains, rings, and branched frameworks — allows the formation of an immense variety of molecules, from small sugars to large proteins. The process of linking smaller subunits into large macromolecules involves condensation reactions, while breakdown involves hydrolysis.

❤️ CAS Link: Organise a hands-on molecular modelling workshop for younger students, using kits to build carbon-based macromolecules and demonstrate condensation vs. hydrolysis.

📌 Properties of Carbon

• Carbon forms four covalent bonds (tetravalency) allowing stable and diverse molecules.
• Can form single, double, and triple bonds with itself or other atoms.
• Structures include straight chains, branched chains, and rings.
• Carbon-carbon bonds are strong and stable, allowing large complex molecules.
• Functional groups (e.g., hydroxyl, carboxyl, amino) attached to the carbon skeleton influence molecular properties.
• This versatility is the basis for the diversity of life’s molecules.

🧠 Examiner Tip: When explaining carbon’s importance, always mention both bonding capacity and structural diversity for maximum marks.

📌 Monomers, Polymers, and Macromolecules

• Monomers are small units like monosaccharides, amino acids, nucleotides, and fatty acids.
• Polymers are long chains of monomers linked by covalent bonds (e.g., starch, proteins, DNA).
• Macromolecules are large biological molecules that may be polymers (carbohydrates, proteins, nucleic acids) or non-polymers (lipids).
• Polymerisation occurs via condensation reactions (water released).
• Breakdown occurs via hydrolysis (water added).
• The type and sequence of monomers determine the function of the macromolecule.

🌍 Real-World Connection: The condensation reaction between glucose molecules to form starch is exploited in food manufacturing for texture and storage stability.

📌 Condensation and Hydrolysis Reactions

Condensation: Joins monomers → polymer + water; catalysed by specific enzymes.

Hydrolysis: Breaks polymers → monomers by adding water; catalysed by hydrolase enzymes.

Reactions are reversible under appropriate conditions.

Examples:

• Glucose + glucose → maltose + water (condensation).
• Maltose + water → glucose + glucose (hydrolysis).

Both reactions are crucial in metabolism (anabolism vs. catabolism).

Energy changes: condensation often requires energy input; hydrolysis releases energy.

📝 Paper 2: Data Response Tip: If given molecular diagrams, always identify reactants, products, and water molecules in condensation/hydrolysis — these marks are easy to secure but often lost through oversight.