📖 Notes
LESSON 03 OF 17
The Building Blocks of Life
The Building Blocks of Life
Carbohydrates, proteins, lipids, and nucleic acids — their structures, functions, and the biochemical tests MDCAT loves to ask about.
Four Macromolecules
The Building Blocks, At a Glance
🍞
Carbohydrates
Mono-, di- and polysaccharides built from sugar units — the body's primary quick energy source.
🧬
Proteins
Built from just 20 amino acids, folded into 1°/2°/3° structures that determine every enzyme's function.
🧈
Lipids
Glycerol + 3 fatty acids. Saturated bonds pack solid; double bonds keep them liquid at room temp.
🧵
Nucleic Acids
DNA and RNA — the double- and single-stranded molecules that store and carry genetic instructions.
Carbohydrates
Sugars, From Simple to Complex
- Monosaccharides: glucose, fructose, galactose
- Disaccharides: sucrose, lactose, maltose
- Polysaccharides: starch, glycogen, cellulose
- Tests: Benedict's (reducing sugars), Iodine (starch)
Proteins
- Monomers: 20 amino acids
- 1° structure = amino acid sequence
- 2° structure = α-helix or β-sheet (held by hydrogen bonds)
- 3° structure = full 3D shape
- Test: Biuret (turns purple)
Lipids
- Triglyceride = glycerol + 3 fatty acids
- Saturated = no double bonds (solid at room temp)
- Unsaturated = has double bonds (liquid at room temp)
- Test: Emulsion test (turns white/cloudy)
DNA vs RNA
Quick Comparison
REFERENCEDNA: double-stranded, deoxyribose sugar, bases A-T-C-G, found in nucleus RNA: single-stranded, ribose sugar, bases A-U-C-G, found in nucleus + cytoplasm
💡
Exam tip
MDCAT frequently pairs a biochemical test with its result and asks you to identify the molecule. Memorise the four tests above as pairs, not just individually.