Overview
This lecture covers AQA A-Level Biology Topic 1, focusing on biological molecules: their structure, function, and key biochemical processes.
Monomers, Polymers & Reactions
- Monomers are small units from which larger molecules are made.
- Polymers are molecules made from many monomers joined together.
- Main biological monomers: monosaccharides, amino acids, nucleotides.
- Main biological polymers: carbohydrates (starch, cellulose, glycogen), proteins, DNA, and RNA.
- Condensation reactions join molecules, forming bonds and releasing water.
- Hydrolysis reactions break bonds using water.
Carbohydrates
- Monosaccharides (single sugar units): glucose, fructose, galactose.
- Disaccharides: formed from two monosaccharides by condensation (maltose = glucose+glucose; lactose = glucose+galactose; sucrose = glucose+fructose).
- Polysaccharides: starch and glycogen (alpha glucose), cellulose (beta glucose).
- Glycosidic bonds join monosaccharides.
- Starch/glycogen: energy storage; cellulose: structural support in plants.
- Starch and glycogen are insoluble, do not affect osmosis.
- Starch: coiled and branched for compact storage and rapid hydrolysis.
- Glycogen: highly branched for rapid glucose release in animals.
- Cellulose: straight chains with hydrogen bonds for rigidity; every other beta glucose inverted.
Carbohydrate and Sugar Testing
- Starch: add iodine; blue-black indicates presence.
- Reducing sugars: Benedict's reagent + heat; green/yellow/orange/red indicates presence.
- Non-reducing sugars: after negative reducing sugars test, hydrolyse with acid, neutralize, then Benedict's; orange/brick red indicates presence.
Lipids
- Triglycerides: 1 glycerol + 3 fatty acids (ester bonds via condensation).
- Phospholipids: 1 glycerol + 2 fatty acids + 1 phosphate group.
- Fatty acids: saturated (all single bonds), unsaturated (at least one double bond).
- Triglycerides: energy storage, insoluble, release water when oxidized.
- Phospholipids: hydrophilic head and hydrophobic tails, form cell membranes.
- Lipid test: add ethanol, then water; white emulsion forms if present.
Proteins
- Amino acids are protein monomers; 20 different types based on R group.
- Amino acids join by peptide bonds (condensation).
- Protein structure: Primary (amino acid sequence), Secondary (alpha helix/beta sheet, H-bonds), Tertiary (3D folding, ionic/disulfide/H-bonds), Quaternary (multiple polypeptides).
- Structure determines function; sequence changes alter activity.
- Protein test: Biuret reagent turns purple if proteins are present.
Enzymes
- Enzymes are proteins with specific active sites due to tertiary structure.
- They catalyze reactions by lowering activation energy.
- Lock and key model: enzyme active site is a fixed shape.
- Induced fit model: active site molds around substrate.
- Factors: temperature, pH (denaturation at extremes), substrate/enzyme concentration, inhibitors.
- Competitive inhibitors: bind active site; can be outcompeted by substrate.
- Non-competitive inhibitors: bind elsewhere, change active site shape; substrate can't outcompete.
Nucleic Acids
- DNA: deoxyribose, bases A, T, C, G; double-stranded helix.
- RNA: ribose, bases A, U, C, G; single-stranded, shorter.
- Nucleotides joined by phosphodiester bonds (condensation).
- DNA strands joined by H-bonds between complementary bases (A-T, C-G).
- DNA replication is semiconservative: each new DNA has one parental, one new strand.
- DNA helicase and DNA polymerase are key enzymes for replication.
ATP & Water
- ATP: adenosine triphosphate (adenine, ribose, 3 phosphates); immediate energy source.
- ATP hydrolysis (by ATP hydrolase) releases energy and inorganic phosphate.
- Water: polar, forms hydrogen bonds.
- Key properties: metabolite, solvent, high heat capacity, large latent heat of vaporization, strong cohesion.
Inorganic Ions
- Hydrogen ions (H⁺): affect pH.
- Iron ions (Fe²⁺): oxygen transport in hemoglobin.
- Sodium ions (Na⁺): cotransport for absorption.
- Phosphate ions (PO₄³⁻): components of DNA, RNA, ATP.
Key Terms & Definitions
- Monomer — small unit that can join to form polymers.
- Polymer — large molecule made of monomers.
- Condensation reaction — joins molecules, releases water.
- Hydrolysis reaction — splits molecules, uses water.
- Glycosidic bond — bond between sugars in carbohydrates.
- Ester bond — bond between glycerol and fatty acids in lipids.
- Peptide bond — bond between amino acids in proteins.
- Phosphodiester bond — bond between nucleotides in nucleic acids.
- Hydrophilic — interacts with water.
- Hydrophobic — repels water.
Action Items / Next Steps
- Fill in workbook or table summaries alongside these notes.
- Practice drawing and labeling structures for glucose, amino acids, nucleotides, etc.
- Memorize key equations and definitions for each biomolecule.
- Review enzyme inhibition graphs and biochemical test steps.