Overview
This lecture provides a summary of key biological molecules, their structures, functions, related biochemical tests, and their significance in living organisms for AQA Biology A-Level.
Monomers and Polymers
- Monomers are small units like glucose, amino acids, and nucleotides that join to form polymers.
- Polymers are large molecules made from many monomers joined by condensation reactions (removing water).
- Hydrolysis is the addition of water to break bonds between monomers.
Carbohydrates
- Carbohydrates consist of carbon, hydrogen, and oxygen, forming chains of sugar units.
- Monosaccharides are single sugar units (e.g., glucose, galactose, fructose).
- Disaccharides are formed by condensation of two monosaccharides (e.g., maltose, sucrose, lactose).
- Polysaccharides are many glucose units joined (e.g., glycogen, starch, cellulose).
- Glycogen is an energy storage molecule in animals, highly branched and insoluble.
- Starch stores energy in plants (amylose: unbranched; amylopectin: branched).
- Cellulose forms plant cell walls, consisting of β-glucose linked in strong microfibrils.
Biochemical Tests for Carbohydrates
- Benedict’s reagent tests for reducing sugars; a brick-red result indicates presence.
- Non-reducing sugars require acid hydrolysis and neutralization before Benedict’s test.
- Iodine/potassium iodide turns blue/black if starch is present.
Lipids
- Lipids are made of carbon, hydrogen, and oxygen, insoluble in water.
- Triglycerides: one glycerol and three fatty acids, joined by ester bonds.
- Saturated fatty acids have no C=C bonds; unsaturated have one or more, making them liquid at room temperature.
- Phospholipids: one fatty acid replaced by phosphate; form cell membranes with hydrophilic heads and hydrophobic tails.
- Emulsion test detects lipids by forming a cloudy-white solution.
Proteins
- Proteins are made of amino acid monomers joined by peptide bonds.
- Primary structure: sequence of amino acids.
- Secondary structure: alpha helix or beta pleated sheet stabilized by hydrogen bonds.
- Tertiary structure: 3D folding via disulfide bridges, ionic, and hydrogen bonds.
- Biuret test produces purple color if protein is present.
Enzymes
- Enzymes are globular proteins that catalyze reactions by lowering activation energy.
- The induced fit model describes how enzyme active sites mold to substrates.
- Rate of enzyme activity is affected by temperature, pH, enzyme and substrate concentration, and inhibitors.
Structure of DNA and RNA
- DNA: double helix of nucleotides (deoxyribose, phosphate, bases A, T, C, G).
- RNA: single polynucleotide chain (ribose, phosphate, bases A, U, C, G).
- Nucleotides connect via phosphodiester bonds.
- Base pairing: A-T (DNA), G-C; A-U (RNA).
DNA Replication & ATP
- DNA helicase separates strands; DNA polymerase joins new nucleotides.
- DNA replication is semi-conservative, ensuring genetic continuity.
- ATP releases energy when hydrolyzed to ADP and phosphate, catalyzed by ATP hydrolase.
Water & Inorganic Ions
- Water is polar, solvent, has high specific heat and large latent heat of vaporization.
- Cohesion allows water transport in plants, supports high surface tension.
- Key ions: H⁺ (pH), Fe²⁺ (hemoglobin), Na⁺ (co-transport), PO₄³⁻ (DNA, ATP).
Key Terms & Definitions
- Monomer — Small molecule that can join others to form a polymer.
- Polymer — Large molecule made from repeating monomers.
- Condensation reaction — Joins monomers by removing water.
- Hydrolysis — Breaks bonds by adding water.
- Glycosidic bond — Joins two sugar molecules.
- Ester bond — Joins fatty acids to glycerol in lipids.
- Peptide bond — Joins two amino acids in proteins.
- Enzyme — Biological catalyst, usually a protein.
- Active site — Region of enzyme that binds substrate.
- Nucleotide — Building block of DNA/RNA.
- Phosphodiester bond — Connects nucleotides in nucleic acids.
- ATP (Adenosine triphosphate) — Cell’s energy currency.
- Hydrogen bond — Weak bond important in protein and DNA structure.
Action Items / Next Steps
- Practice biochemical tests for carbohydrates, lipids, and proteins in lab.
- Review molecular structures of glucose, amino acids, and nucleotides.
- Complete assigned reading on enzyme kinetics and DNA replication.