Macromolecules and Their Properties

Overview of Cellular Macromolecules

  • Cells rely on three primary classes of biological macromolecules:
    • Polysaccharides (≈ carbohydrates)
    • Proteins
    • Nucleic acids
  • Shared features
    • Each macromolecule is a polymer—a long chain of repeating building blocks (monomers).
    • Specific monomer → polymer mapping:
    • Monosaccharides (simple sugars) → polysaccharides
    • Amino acids → proteins
    • Nucleotides → nucleic acids
    • All polymer-building processes require energy and involve removal of water.

Polymerisation & Covalent Bond Formation

  • Adjacent monomers are connected by covalent bonds—strong bonds that demand an energy input to form.
  • Energy source: generally the hydrolysis of a high-energy phosphate bond in ATP (or a similar nucleotide).
    ATP \rightarrow ADP + P_i + \text{energy}
  • Dehydration (condensation) reaction
    • Water (H2O) is expelled as the bond forms: \text{Monomer}1 + \text{Monomer}2 \xrightarrow[\text{energy}]{\text{dehydration}} \text{Monomer}1{-}\text{Monomer}2 + H2O
    • Mechanism applies equally to sugars, amino acids, and nucleotides.

Hydrolysis – The Reverse Process

  • Hydrolysis reaction cleaves polymers back to monomers by adding water, essentially the exact opposite of dehydration:
    \text{Polymer} + H2O \xrightarrow{\text{hydrolysis}} \text{Monomer}n
  • Practical cellular example: breaking stored glycogen into glucose → fuels ATP production.

Non-Covalent Interactions & 3-D Structure

  • Covalent backbones alone do not supply biological function; correct folding is essential.
  • In the aqueous cytoplasm, polymers spontaneously adopt specific 3-D conformations driven by non-covalent bonds:
    • Hydrogen bonds
    • Ionic (electrostatic) bonds
  • Properties of non-covalent bonds
    • Individually weak, form without external energy, and break easily.
    • In large numbers—dozens, hundreds, or thousands—they create substantial collective stability ("zipper" analogy).
  • DNA example
    • Two antiparallel nucleotide strands twist into a double helix.
    • Bases pair internally (non-covalent H-bonding) while the sugar–phosphate backbone remains solvent-exposed.

Cellular Functions of Macromolecules

  • Serve as sensors, transporters, regulators, binders, information warehouses, energy reservoirs, and structural components.
  • Effectiveness depends on precise 3-D conformation; misfolding generally inactivates function.

Carbohydrates (Polysaccharides)

  • Everyday term: carbohydrates—a major dietary category.
  • Core roles
    • Energy storage
    • Starch: plant storage polysaccharide (found in vegetables, grains).
    • Glycogen: animal storage polysaccharide (primarily in liver & muscle).
    • Both consist entirely of \alpha-glucose units.
    • Structural support
    • Cellulose: main structural component in plant cell walls.
    • Chitin: exoskeleton material in arthropods and cell walls of fungi.
  • Energy mobilization pathway
    • Glycogen \xrightarrow{\text{hydrolysis}} glucose \rightarrow ATP production.

Proteins

  • Diversity: thousands of distinct proteins, spanning catalytic, structural, transport, signaling, and regulatory roles.
  • Building blocks: 20 standard amino acids.
  • Formation
    • Amino acids linked by peptide bonds via dehydration → generate a polypeptide chain.
  • Folding & assembly
    • Polypeptide automatically folds into a unique conformation powered by hundreds of non-covalent (and occasional covalent, e.g., disulfide) interactions.
    • Some proteins function as single chains; others require oligomerisation (e.g., hemoglobin = 4 subunits).

Enzymatic Catalysis

  • Enzymes (nearly all proteins) accelerate reactions by lowering the activation energy barrier:
    \text{Reactants} \xrightarrow[\small \text{low } E_a]{\text{enzyme}} \text{Products}
  • Activation energy analogy: a hill reactants must climb before descending to products; enzymes reduce hill height.

Additional Protein Functions (from transcript examples)

  • Structural: e.g., collagen (major component of connective tissues, skin, and bone).
    (Transcript cuts off mid-sentence, but collagen cited as example.)

Key Concept Map & Connections

  • Dehydration vs. hydrolysis forms a fundamental biochemical cycle: build polymers → store information/energy/structure → break polymers → release information/energy/subunits.
  • Energy economy centers on ATP hydrolysis coupling to covalent bond formation.
  • Proper function hinges on hierarchical bonding:
    1. Covalent backbone confers permanence.
    2. Non-covalent network directs folding, specificity, and dynamic interactions.
  • Real-world relevance
    • Dietary choices (carbohydrate intake) directly influence glycogen stores and metabolic ATP generation.
    • Protein folding diseases (e.g., misfolded enzymes) illustrate critical dependence on non-covalent integrity.
    • Industrial enzymes harness activation-energy reduction to drive bio-manufacturing.

Ethical & Practical Implications (Discussed/Implied)

  • Nutritional balance: understanding carbohydrate storage (starch/glycogen) informs diet planning and diabetes management.
  • Biotechnology: manipulating polymerisation/hydrolysis reactions underpins genetic engineering, drug design, and synthetic biology.
  • Environmental impact: abundant structural polysaccharides (cellulose, chitin) offer renewable raw materials for sustainable products.