OCR (A) Biology A-Level: Biological Molecules

Water

• Polar Molecule: Uneven charge distribution; oxygen is slightly negative, hydrogen slightly positive.
• Metabolite: Involved in condensation and hydrolysis.
• Solvent: Facilitates metabolic reactions.
• High Specific Heat Capacity: Requires much energy to warm, minimizing temperature fluctuations, acts as a buffer.
• Large Latent Heat of Vaporization: Evaporation cools with minimal water loss.
• Cohesion: Supports water transport and high surface tension.
• Density: Ice less dense than water, floats, insulates bodies of water, aids organism survival.
• Incompressibility: Provides support.

Monomers and Polymers

• Monomers: Small units, e.g., monosaccharides (glucose), amino acids, nucleotides.
• Polymers: Made from monomers via condensation (releases water) or hydrolysis (adds water).

Carbohydrates

• Composition: Carbon, hydrogen, oxygen; long sugar chains (saccharides).
• Types: Monosaccharides, disaccharides, polysaccharides.
• Monosaccharides: Glucose (main respiration substrate); alpha and beta isomers.
• Disaccharides:
  • Maltose: 2 glucose
  • Sucrose: Glucose + fructose
  • Lactose: Glucose + galactose
• Polysaccharides:
  • Glycogen & Starch: From alpha glucose.
  • Cellulose: From beta glucose.
• Glycogen: Main energy storage in animals; quick release due to side branches.
• Starch: Energy storage in plants; composed of amylose (compact) and amylopectin (branched).
• Cellulose: Plant cell wall component; strong microfibrils from hydrogen bonding.

Lipids

• Solubility: In organic solvents like alcohol.
• Types:
  • Saturated: No C-C double bonds, solid at room temperature.
  • Unsaturated: One/more C-C double bonds, liquid at room temperature.
• Triglycerides: Glycerol + 3 fatty acids, energy reserves, ester bonds.
• Phospholipids: Glycerol + 2 fatty acids + phosphate group; form micelles in water.

Inorganic Ions

• Roles:
  • Hydrogen ions: pH determinant.
  • Iron ions: Component of haemoglobin.
  • Sodium ions: Co-transport glucose/amino acids.
  • Phosphate ions: Component of DNA and ATP.

Proteins

• Amino Acids: Building blocks; 20 types, peptide bonds.
• Structure Levels:
  • Primary: Amino acid sequence.
  • Secondary: Alpha helix or beta pleated sheet via hydrogen bonding.
  • Tertiary: 3D globular (e.g., enzymes) or fibrous (e.g., keratin) structure.
  • Quaternary: Multiple polypeptides (e.g., haemoglobin with haem groups).
• Examples:
  • Collagen: Fibrous, strong, connective tissues.
  • Haemoglobin: Globular, oxygen transport in blood.

Tests

• Benedict's Test: For reducing sugars; red precipitate if present.
• Biuret Test: For proteins; lilac color indicates presence.
• Emulsion Test: For lipids; cloudy solution if present.
• Iodine Test: For starch; yellow to blue/black color change.