Overview
This lecture covers the core biological molecules in the AQA Biology A-Level syllabus, including their structures, properties, key chemical tests, and biological roles.
Monomers and Polymers
- Monomers are small units like glucose, amino acids, and nucleotides that join to form polymers.
- Polymers are large molecules made of repeating monomers via condensation reactions, releasing water.
- Hydrolysis adds water to break bonds between monomers.
Carbohydrates
- Carbohydrates are made of carbon, hydrogen, and oxygen.
- Monosaccharides (single sugars) include glucose, galactose, and fructose.
- Disaccharides are formed by joining two monosaccharides (e.g., maltose, sucrose, lactose) via glycosidic bonds.
- Polysaccharides include glycogen (animals), starch (plants), and cellulose (plants).
Structure and Function of Polysaccharides
- Glycogen and starch store energy; cellulose gives plant cell walls rigidity.
- Glycogen is highly branched for rapid energy release and is insoluble.
- Starch is a mix of amylose (unbranched, compact) and amylopectin (branched).
- Cellulose chains form microfibrils, strengthening plant cell walls.
Biochemical Tests for Carbohydrates
- Benedict’s test: detects reducing sugars by forming a red precipitate.
- Modified Benedict’s test: detects non-reducing sugars after hydrolysis.
- Iodine test: turns blue/black if starch is present.
Lipids
- Lipids (triglycerides, phospholipids) are made of carbon, hydrogen, and oxygen, soluble only in organic solvents.
- Triglycerides: one glycerol + three fatty acids joined by ester bonds.
- Saturated fats have no double bonds; unsaturated fats have double bonds, making them liquid at room temp.
- Phospholipids have a hydrophilic head and hydrophobic tails, forming cell membranes.
Lipid Tests
- Emulsion test: lipids turn solution cloudy-white when mixed with ethanol and water.
Proteins
- Proteins are polymers of amino acids linked by peptide bonds.
- Amino acids have an amino group, carboxyl group, and R group.
- Structure: primary (sequence), secondary (alpha helix/beta sheet), tertiary (3D shape), quaternary (multiple polypeptides).
- Biuret test detects proteins by turning solution purple if peptide bonds are present.
Enzymes
- Enzymes are globular proteins that catalyse reactions by lowering activation energy.
- Active sites bind substrates specifically, forming enzyme-substrate complexes (induced fit model).
- Factors affecting rate: temperature, pH, enzyme concentration, substrate concentration, inhibitors.
Nucleic Acids: DNA and RNA
- DNA/RNA are polymers of nucleotides (pentose sugar, phosphate, nitrogenous base).
- DNA nucleotides: deoxyribose + A, T, C, G; RNA: ribose + A, U, C, G.
- DNA forms a double helix with complementary base pairing; RNA is usually single-stranded.
DNA Replication
- Semi-conservative replication involves DNA helicase separating strands, DNA polymerase synthesising new strands, and complementary base pairing.
ATP (Adenosine Triphosphate)
- ATP is a nucleotide derivative: ribose, adenine, three phosphates.
- Hydrolysis of ATP (ATP hydrolase) releases energy and forms ADP + phosphate.
- ATP is an immediate energy source, easily regenerated from ADP.
Water and Inorganic Ions
- Water is polar, a metabolite, solvent, has high heat capacity and cohesive properties.
- Inorganic ions: hydrogen ions (pH), iron ions (haemoglobin), sodium ions (co-transport), phosphate ions (DNA, ATP).
Key Terms & Definitions
- Monomer — Small molecule that can join to form a polymer.
- Polymer — Large molecule made of repeating monomers.
- Condensation reaction — Joins molecules; releases water.
- Hydrolysis — Breaks bonds using water.
- Glycosidic bond — Bond joining two sugar molecules.
- Triglyceride — Lipid with glycerol and three fatty acids.
- Phospholipid — Lipid with glycerol, two fatty acids, one phosphate group.
- Amino acid — Monomer of proteins.
- Peptide bond — Joins amino acids.
- Enzyme — Protein catalyst.
- Nucleotide — Monomer of nucleic acids.
- Phosphodiester bond — Links nucleotides in DNA/RNA.
- ATP — Immediate energy-carrying molecule.
- Hydrogen bond — Weak bond; key in water, DNA, proteins.
Action Items / Next Steps
- Review the biochemical tests for carbohydrates, lipids, and proteins.
- Practice drawing structures of glucose (alpha and beta), triglycerides, phospholipids, and amino acids.
- Complete any assigned practice questions on enzyme activity and DNA replication.