Overview
This lecture covers AQA A-level Biology Topic 1, focusing on the structure, formation, and function of biological molecules: carbohydrates, lipids, proteins, nucleic acids, ATP, water, and inorganic ions, including their biochemical tests and importance.
Monomers, Polymers & Chemical Reactions
- Monomers are small units from which larger molecules are made.
- Polymers are molecules made from many monomers joined together.
- Key biological monomers: monosaccharides, amino acids, nucleotides.
- Key polymers: carbohydrates (starch, glycogen, cellulose), proteins, DNA, RNA.
- Condensation reactions join molecules, forming a chemical bond with elimination of water.
- Hydrolysis reactions break chemical bonds using water.
- In applied questions, name the molecules and type of bond formed or broken.
Carbohydrates
- Monosaccharides: glucose, fructose, galactose (monomers of carbohydrates).
- Disaccharides: maltose (glucose + glucose), lactose (glucose + galactose), sucrose (glucose + fructose).
- Polysaccharides: starch and glycogen (both made of alpha glucose; energy storage), cellulose (beta glucose; structural support).
- Glycosidic bonds form during condensation between monosaccharides.
- Alpha and beta glucose are isomers, differing at carbon 1’s position of H and OH.
Properties & Tests of Carbohydrates
- Starch: coiled structure (amylose) and branched (amylopectin), insoluble, rapid hydrolysis for energy in plants.
- Glycogen: highly branched, rapid hydrolysis for energy in animals.
- Cellulose: straight chains, hydrogen bonds form strong fibrils for cell wall strength.
- Test for starch: iodine turns from orange-brown to blue-black.
- Test for reducing sugars: Benedict’s reagent and heat, positive result goes from blue to green/yellow/orange/brick red.
- Test for non-reducing sugars: acid hydrolysis, neutralization, then Benedict’s test (positive = orange/red).
Lipids
- Triglycerides: made from one glycerol and three fatty acids via condensation; ester bonds form.
- Phospholipids: one glycerol, two fatty acids, one phosphate group.
- Fatty acids: R group can be saturated (single C-C bonds) or unsaturated (at least one C=C double bond).
- Triglycerides are energy stores, insoluble, release metabolic water.
- Phospholipids have hydrophilic heads and hydrophobic tails, form cell membranes.
- Lipid test: mix with ethanol then water; positive result is a white emulsion.
Proteins & Enzymes
- Proteins: polymers of amino acids (monomers), joined by peptide bonds via condensation.
- Levels of structure: primary (amino acid sequence), secondary (alpha helix/beta sheet, hydrogen bonds), tertiary (unique 3D fold, hydrogen/ionic/disulfide bonds), quaternary (multiple polypeptides).
- Biuret test: solution turns purple for proteins.
- Enzymes: proteins with specific tertiary structure; active site is substrate-specific.
- Enzyme action models: lock-and-key (fixed shape), induced fit (flexible, molds to substrate).
- Factors affecting enzymes: temperature, pH, substrate/enzyme concentration, inhibitors (competitive and non-competitive).
Nucleic Acids & ATP
- DNA and RNA are nucleic acids; polymers of nucleotides.
- DNA: deoxyribose sugar, bases A, T, C, G; double helix with complementary base pairing (A-T, C-G).
- RNA: ribose sugar, bases A, U, C, G; single-stranded, shorter.
- Phosphodiester bonds link nucleotides.
- DNA replication is semi-conservative: parental strand + new strand.
- ATP: nucleotide derivative, adenosine, ribose, three phosphates; immediate energy source.
- ATP hydrolysis (by ATP hydrolase) releases energy, forms ADP and Pi; resynthesized by ATP synthase.
Water & Inorganic Ions
- Water: polar; forms hydrogen bonds; metabolite, solvent, high heat capacity, high latent heat of vaporization, strong cohesion.
- Key inorganic ions: H+ (pH), Fe2+ (oxygen transport in hemoglobin), Na+ (co-transport), PO4^3- (DNA, RNA, ATP).
Key Terms & Definitions
- Monomer — Small molecule that joins with others to form a polymer.
- Polymer — Large molecule made by joining many monomers.
- Condensation Reaction — Joins two molecules with loss of water.
- Hydrolysis Reaction — Splits molecules using water.
- Glycosidic Bond — Bond between carbohydrate monomers.
- Peptide Bond — Bond between amino acids.
- Phosphodiester Bond — Bond between nucleotides.
- Saturated Fatty Acid — Fatty acid with only single C-C bonds.
- Unsaturated Fatty Acid — Fatty acid with at least one C=C bond.
- Enzyme-Substrate Complex — Temporary association when substrate binds to enzyme’s active site.
Action Items / Next Steps
- Review and memorize key definitions and biochemical tests.
- Practice drawing structures of glucose, amino acids, triglycerides, and nucleotides.
- Complete workbook exercises or flashcards where available.
- Prepare for application questions (naming bonds, describing reactions, enzymes, etc.).