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Biological Molecules Overview

Aug 3, 2025

Overview

This lecture covers the core biological molecules in A-level Biology, including their structures, functions, and key biochemical processes essential for understanding cell biology.

Monomers, Polymers, and Reactions

  • Monomers are small units that join to form larger molecules called polymers.
  • Examples of monomers: glucose, amino acids, nucleotides.
  • Polymers formed: starch, glycogen, proteins, DNA, RNA.
  • Condensation reactions join monomers, forming chemical bonds and releasing water.
  • Hydrolysis reactions break bonds between monomers using water.

Carbohydrates

  • Monosaccharides: glucose (alpha and beta), fructose, galactose; formula C6H12O6.
  • Disaccharides: sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose), joined by glycosidic bonds via condensation.
  • Polysaccharides: starch & cellulose (plants), glycogen (animals); made of glucose but different isomers (alpha for starch/glycogen, beta for cellulose).
  • Starch: storage in plants; amylose (1-4 bonds, helix, compact), amylopectin (1-4 & 1-6 bonds, branched).
  • Glycogen: more branched than starch, rapid glucose release in animals.
  • Cellulose: straight chains, 1-4 bonds, hydrogen bonds form strong fibres (fibrils) for cell wall strength.
  • All polysaccharides are large, insoluble, and do not affect cell water potential.

Lipids

  • Triglycerides: glycerol + 3 fatty acids (condensation), ester bonds formed, used for energy storage.
  • Saturated fatty acids: no double bonds between carbons.
  • Unsaturated fatty acids: at least one double bond between carbons.
  • Triglycerides: high energy storage, hydrophobic (do not affect osmosis), low mass.
  • Phospholipids: glycerol + 2 fatty acids + phosphate group; have hydrophilic heads and hydrophobic tails, form bilayers in water.

Proteins

  • Made from amino acid monomers (general structure: central carbon, H, amine (NH2), carboxyl (COOH), variable R group).
  • Dipeptide: 2 amino acids; polypeptide: many; joined by peptide bonds (condensation).
  • Protein structure:
    • Primary: amino acid sequence.
    • Secondary: alpha-helix, beta-pleated sheet (hydrogen bonds).
    • Tertiary: further folding (ionic, hydrogen, disulfide bonds), unique 3D shape.
    • Quaternary: more than one polypeptide chain.

Enzymes

  • Enzymes are specific tertiary-structured proteins that lower activation energy of reactions.
  • Active site is complementary to substrate.
  • Induced fit model: active site changes shape to fit substrate.
  • Factors affecting rate: temperature, pH, substrate/enzyme conc., inhibitors (competitive bind active site, non-competitive bind elsewhere).

Biochemical Tests

  • Starch: Iodine turns blue-black.
  • Reducing sugars: Benedict’s + heat, blue to green/yellow/orange/red.
  • Non-reducing sugars: Acid hydrolysis + Benedict’s, blue to orange/brick red.
  • Proteins: Biuret, blue to purple.
  • Lipids: Dissolve in ethanol, add water, white emulsion forms.

Nucleic Acids (DNA & RNA)

  • DNA: double helix, nucleotide monomers (deoxyribose, phosphate, A/T/C/G), strong sugar-phosphate backbone (phosphodiester bonds), base-paired strands (A=T, C≡G).
  • RNA: single-stranded, ribose sugar, bases A/U/C/G, shorter than DNA.
  • DNA replication is semi-conservative: helicase unwinds, DNA polymerase forms new strands.

ATP and Water

  • ATP: nucleotide derivative (ribose, adenine, 3 phosphates), immediate energy source, hydrolyzed to ADP + Pi.
  • Water: polar, high heat capacity, excellent solvent, metabolite, strong cohesion, high latent heat of vaporization.

Inorganic Ions

  • Hydrogen ions: affect pH, enzyme activity.
  • Iron ions: part of hemoglobin, oxygen transport.
  • Sodium ions: co-transport of glucose/amino acids, nerve impulses.
  • Phosphate ions: found in DNA, RNA, ATP; enable bond formation and energy transfer.

Key Terms & Definitions

  • Monomer — Small molecule that can join others to form a polymer.
  • Polymer — Large molecule made from many monomers bonded together.
  • Condensation Reaction — Joins molecules by forming bonds and releasing water.
  • Hydrolysis Reaction — Breaks bonds using water.
  • Glycosidic Bond — Bond between carbohydrate molecules.
  • 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.
  • Saturated Fatty Acid — No double bonds between carbons.
  • Unsaturated Fatty Acid — At least one double bond between carbons.
  • Active Site — Region on enzyme where substrate binds.
  • Competitive Inhibitor — Binds to active site, blocks substrate.
  • Non-Competitive Inhibitor — Binds elsewhere, changes enzyme shape.

Action Items / Next Steps

  • Review and draw structures of alpha/beta glucose, amino acids, nucleotides.
  • Practice biochemical test methods and expected results.
  • Read textbook sections on each biological molecule for deeper understanding.

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