AQA Biology A-Level

Introduction to Biological Molecules

• All life on Earth shares a common chemistry, providing indirect evidence for evolution.
• Cells contain a few groups of carbon-based compounds that interact similarly.
• Key molecules:
  • Carbohydrates: Used as respiratory substrates; form structural components of plasma membranes and cell walls.
  • Lipids: Used in plasma membranes, hormones, and as respiratory substrates.
  • Proteins: Form cell structures, enzymes, chemical messengers, and blood components.
  • Nucleic acids: Carry genetic code; common genetic code across organisms is evidence for evolution.
• Water is the most common component of cells, critical in the search for extraterrestrial life.

3.1.1 Monomers and Polymers

• Monomers: Smaller units that form larger molecules.
• Polymers: Made from many monomers joined together.
• Examples:
  • Monosaccharides, amino acids, nucleotides.
• Reactions:
  • Condensation reaction: Joins two molecules, forms a bond, eliminates water.
  • Hydrolysis reaction: Breaks a bond, uses water.

3.1.2 Carbohydrates

• Monosaccharides: Building blocks of carbohydrates (e.g., glucose, galactose, fructose).
• Disaccharides: Formed by condensation of two monosaccharides.
  • Maltose: Two glucose molecules.
  • Sucrose: Glucose + fructose.
  • Lactose: Glucose + galactose.
• Polysaccharides: Formed from many glucose units.
  • Glycogen and Starch: From α-glucose.
  • Cellulose: From β-glucose.
• Structure-function relationship in animal and plant cells.
• Biochemical tests:
  • Benedict's solution for sugars.
  • Iodine for starch.

3.1.3 Lipids

• Triglycerides and phospholipids.
  • Triglycerides: Formed by glycerol + three fatty acids.
  • Phospholipids: One fatty acid substituted by phosphate.
• Fatty acids can be saturated or unsaturated.
• Emulsion test for lipids.

3.1.4 Proteins

3.1.4.1 General Properties

• Amino acids: Monomers of proteins.
  • General structure includes NH2 (amine), COOH (carboxyl), R (side chain).
• Dipeptides: Two amino acids.
• Polypeptides: Many amino acids.
• Proteins' structure (primary to quaternary) relates to function.
• Biuret test for proteins.

3.1.4.2 Enzymes

• Lower activation energy.
• Induced-fit model: Active site complements substrate.
• Factors affecting enzyme reactions:
  • Enzyme/substrate concentration, inhibitors, pH, temperature.
• Enzymes catalyse intracellular and extracellular reactions.

3.1.5 Nucleic Acids

3.1.5.1 Structure of DNA and RNA

• DNA: Genetic information holder.
• RNA: Transfers genetic info from DNA to ribosomes.
  • DNA/RNA are nucleotide polymers.
• DNA structure: Double helix, hydrogen bonds between bases.
• RNA: Short polynucleotide chain.

3.1.5.2 DNA Replication

• Semi-conservative replication ensures genetic continuity.
• Steps: Unwinding, hydrogen bond breakage, nucleotide attraction, role of polymerase.

3.1.6 ATP

• Adenosine triphosphate (ATP): Energy currency.
• Hydrolysis to ADP + Pi releases energy; catalysed by ATP hydrolase.
• ATP resynthesised by condensation of ADP + Pi.

3.1.7 Water

• Major cell component.
• Properties:
  • Metabolite in reactions.
  • Solvent for reactions.
  • High heat capacity.
  • Large latent heat of vaporisation.
  • Strong cohesion.

3.1.8 Inorganic Ions

• Exist in cytoplasm/body fluids in various concentrations.
• Specific roles based on properties (e.g., hydrogen ions, iron ions, sodium ions, phosphate ions).