2.3 Carbon

Carbon is the backbone of life, forming diverse organic molecules used by living organisms. Its unique bonding properties allow it to create complex structures like proteins, carbohydrates, and lipids. These molecules are the building blocks of cells and drive biological processes.

Functional groups and hydrocarbon types give organic molecules their specific properties and roles in organisms. Understanding these structures is key to grasping how biomolecules interact and function within living systems, from DNA replication to enzyme activity.

Carbon and Organic Molecules

Carbon's molecular versatility

• Carbon has four valence electrons enabling formation of four covalent bonds
  • Forms stable covalent bonds with many elements (C, H, O, N, S, P)
• Carbon forms single, double, or triple bonds
  • Single bonds share one electron pair
  • Double bonds share two electron pairs
  • Triple bonds share three electron pairs
• Carbon forms long chains, branched structures, and ring structures
  • Strong stable carbon-carbon bonds
  • Enables formation of diverse organic molecules with various shapes and sizes (proteins, carbohydrates, lipids)
• Carbon atoms can undergo hybridization, allowing for different bonding geometries

Impact of functional groups

• Functional groups are specific atom arrangements that impart distinct chemical properties
• Common functional groups in biological molecules
  • Hydroxyl (-OH) increases water solubility and reactivity (sugars, alcohols)
  • Carbonyl (C=O) includes aldehydes and ketones involved in biochemical reactions (amino acids, carbohydrates)
  • Carboxyl (-COOH) is acidic and found in amino acids and fatty acids
  • Amino (-NH2) is basic and found in amino acids and nucleotides
  • Phosphate (-PO4) is acidic and found in nucleotides and phospholipids
  • Sulfhydryl (-SH) stabilizes protein structure through disulfide bonds (cysteine)
• Functional groups determine molecular chemical behavior
  • Influence solubility, reactivity, and molecular interactions (enzyme-substrate binding)
• Functional groups shape the specific biological roles of molecules in living organisms (DNA, enzymes, hormones)
• Some molecules exhibit resonance, where electrons are delocalized across multiple atoms

Hydrocarbon types in organisms

• Hydrocarbons are organic molecules composed of only carbon and hydrogen
• Three main hydrocarbon types
  1. Alkanes have single bonds between carbon atoms
     • General formula $C_nH_{2n+2}$
     • Examples (methane $CH_4$, ethane $C_2H_6$, propane $C_3H_8$)
     • Saturated, nonpolar, and hydrophobic
  2. Alkenes have one or more double bonds between carbon atoms
     • General formula $C_nH_{2n}$
     • Examples (ethene $C_2H_4$, propene $C_3H_6$)
     • Unsaturated, nonpolar, and hydrophobic
  3. Alkynes have one or more triple bonds between carbon atoms
     • General formula $C_nH_{2n-2}$
     • Examples (ethyne $C_2H_2$, propyne $C_3H_4$)
     • Unsaturated, nonpolar, and hydrophobic
• Hydrocarbons serve as the backbone for many biological molecules
  • Lipids are long-chain hydrocarbons with additional functional groups (fatty acids, waxes)
  • Steroids have four fused hydrocarbon rings with attached functional groups (cholesterol, hormones)

Structural features of organic molecules

• Isomers: Molecules with the same molecular formula but different structural arrangements
• Chirality: The property of a molecule that is not superimposable on its mirror image
• Aromaticity: A property of cyclic compounds with delocalized electrons, conferring stability