Molecules to Matabolism

Carbon has 4 valence electrons → forms 4 stable covalent bonds. This lets it build long chains, branches and rings, supporting an enormous diversity of molecules — none of which other elements can match at biological temperatures.

Four classes of biological macromolecule:

Class	Monomer	Examples	Main role
Carbohydrate	Monosaccharide (glucose)	Starch, glycogen, cellulose	Short-term energy, structure
Lipid	Glycerol + fatty acids	Triglycerides, phospholipids, steroids	Long-term energy, membranes, signalling
Protein	Amino acid	Enzymes, antibodies, haemoglobin	Catalysis, transport, structure
Nucleic acid	Nucleotide	DNA, RNA	Information storage and transfer

Condensation reactions join monomers into polymers, releasing water: $n \text{ monomer} \to \text{polymer} + (n-1) \text{ H}_2\text{O}.$

Hydrolysis reactions break polymers using water: $\text{polymer} + \text{ H}_2\text{O} \to \text{monomers}.$

Same reaction in reverse — both are catalysed by enzymes.

Metabolism is the sum of all chemical reactions occurring in a cell. It has two complementary halves:

Anabolism — building large molecules from small. Endergonic (requires energy input, often ATP). Examples:

• Protein synthesis (amino acids → polypeptide).
• Photosynthesis (CO₂ + H₂O → glucose).
• DNA replication.

Catabolism — breaking down large molecules into small. Exergonic (releases energy, used to make ATP). Examples:

• Cell respiration (glucose → CO₂ + H₂O + ATP).
• Digestion of proteins → amino acids.

Coupling. Cells couple exergonic and endergonic reactions: catabolic reactions release energy that drives anabolic ones via ATP.

ATP is the universal energy currency — its hydrolysis (ATP → ADP + Pᵢ) releases ~30 kJ/mol used to power anabolic reactions throughout the cell.