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Biological Molecules

Carbs, lipids, proteins, DNA & food tests

Carbohydrates

Glucose (C₆H₁₂O₆): Simple sugar. Product of photosynthesis. Broken down in respiration for energy.

Starch: Polymer of glucose. Storage carbohydrate in plants (seeds, tubers). Insoluble.

Glycogen: Polymer of glucose. Storage carbohydrate in animals (liver, muscle). Branched structure, more soluble than starch.

Cellulose: Polymer of glucose. Structural — cell wall in plants. Not broken down by human digestive enzymes.

Lipids (Fats & Oils)

Made of: glycerol + 3 fatty acids

Saturated fats: No C=C double bonds. Solid at room temperature. Animal fats (butter, lard). Less healthy.

Unsaturated fats: C=C double bonds. Liquid at room temperature. Plant oils (olive, sunflower). More healthy.

Functions: Energy store (9 kJ/g — more than carbs!), insulation, protective cushioning, membrane structure

Proteins

Made of: amino acids linked by peptide bonds

There are 20 different amino acids. Order determines protein structure & function.

Functions: Enzymes (catalyse reactions), antibodies (immune response), haemoglobin (O₂ transport), muscle fibres, keratin (hair), collagen (connective tissue)

Essential for growth, repair, and making hormones

Food Tests — Iodine & Benedict's

Iodine test: Add iodine solution to sample.
Result: Blue-black = starch present; yellow/brown = no starch

Benedict's test: Add Benedict's solution. Heat to ~50°C (water bath).
Result: Orange/brick-red precipitate = reducing sugar (glucose, fructose); blue = no sugar

Glucose is a reducing sugar (has free C=O group). Sucrose is NOT reducing (no free C=O).

More Food Tests

Biuret test (protein): Add Biuret reagent (CuSO₄ + NaOH).
Result: Purple/mauve = protein present; blue = no protein

Ethanol emulsion test (fat/lipid): Mix sample with ethanol, then add water.
Result: White emulsion = fat present; colourless = no fat

DCPIP test (vitamin C): Add DCPIP (blue dye) to sample.
Result: Colourless = vitamin C present; stays blue = no vitamin C (vitamin C reduces DCPIP)

DNA — Deoxyribonucleic Acid

Double helix structure

Made of: Deoxyribose sugar + phosphate group + nitrogenous base (A, T, G, C)

Base pairing: A pairs with T (2 bonds), G pairs with C (3 bonds)

Function: Carries genetic code. Controls protein synthesis. Replicates via semi-conservative replication.

Human: ~3 billion base pairs, 46 chromosomes (23 pairs)

Exam Tips

✓ Know the food tests: iodine, Benedict's, biuret, ethanol, DCPIP

✓ Starch = plant storage; glycogen = animal storage

✓ Lipids have more energy (9 kJ/g) than carbs (17 kJ/g)

✓ Proteins are made of amino acids, not carbohydrates

✓ DNA base pairing: A-T, G-C

Key Takeaways

✅ Glucose → starch (plant) or glycogen (animal)

✅ Lipids: glycerol + 3 fatty acids

✅ Proteins: amino acids linked by peptide bonds

✅ Iodine = starch; Benedict's = sugar; Biuret = protein

✅ DNA: double helix, A-T and G-C pairing

Learn the tests — they love asking! 🧪✓