Notes on Carbon, Organic Molecules, Functional Groups, and Macromolecules

Carbon and the Central Role of Carbon in Life

• Carbon is the backbone of organic chemistry; it has four valence electrons and can form four covalent bonds at once, enabling long chains and rings. This tetravalence makes carbon the basis for diverse organic molecules and for life as we know it.
• When you look at carbon on the periodic table, it has valence 4, so it can form four single covalent bonds to other atoms, allowing branches, chains, and rings.
• Because of its ability to form multiple bonds and structural diversity, carbon-based chemistry leads to hydrocarbons, chains, and heteroatom-containing molecules that define biology.
• Organic molecules are primarily carbon skeletons with hydrogens and then heteroatoms (O, N, P, S) as substituents; the exact length, branching, and functional groups determine identity and function.
• A simple way to think about it: carbon skeletons are “scaffolds” that other atoms and groups plug into, creating the vast array of biomolecules.

Hydrocarbons and Lipids: What They Are and What They Are Not

• Hydrocarbons are molecules composed only of carbon and hydrogen (no oxygen): they are carbon-hydrogen chains or rings.
  • They are considered organic, but you typically won’t find pure hydrocarbons performing as the sole constituents of body tissues.
• Hydrocarbons are nonpolar overall and largely hydrophobic; the lack of heteroatoms means little to no hydrogen-bonding capability.
• Lipids contain hydrocarbon-like tails but are not pure hydrocarbons because they include oxygen-containing groups (e.g., glycerol backbone with hydroxyl groups and fatty acids with carboxyl groups).
• Lipids are not polymers in the strict sense; triglycerides, for example, have a glycerol backbone linked to three fatty acids via ester bonds.
• In biology, hydrocarbons are important in energy storage and membrane structure, but living organisms generally require oxygen-containing functional groups in most biomolecules.
• A common lipid example: a triglyceride consisting of glycerol plus three fatty acids; the three fatty acids provide long hydrocarbon chains, while glycerol provides oxygen-containing sites that form ester bonds with the fatty acids.

The Eight Functional Groups Found in Organic Molecules

• Functional groups are reactive clusters of atoms within organic molecules that determine chemical properties and behavior. The eight main groups discussed are: 1) Hydroxyl group: $- ext{OH}$
  • Polar and hydrophilic; forms hydrogen bonds; common in alcohols (e.g., ethanol, glycerol).
    2) Carboxyl group: $- ext{COOH}$
  • Polar and acidic; donates a proton to become $- ext{COO}^-$; a key feature of organic acids like acetic acid.
    3) Ketone group: $- ext{C(=O)-}$
  • A carbonyl within the carbon chain; polar; found in many sugars and organic compounds.
    4) Aldehyde group: $- ext{CHO}$
  • Polar; carbonyl at the end of a carbon chain; reactive and important in metabolism and chemistry of sugars.
    5) Amino group: $- ext{NH}_2$
  • Contains nitrogen; proton acceptor (base); polar; common in amino acids.
    6) Phosphate group: $- ext{PO}_4^{2-}$ (often with OH groups)
  • Highly polar; multiple oxygens; crucial in energy transfer (ATP), nucleic acids, and phospholipids; can act as a base.
    7) Sulfhydryl group: $- ext{SH}$
  • Polar-ish; similar to hydroxyl in some chemistry;important for protein folding via disulfide bonds and redox reactions.
    8) Methyl group: $- ext{CH}_3$
  • Nonpolar and hydrophobic; contains only carbon and hydrogen; acts as a nonpolar modifier.
• Polar vs nonpolar trend:
  • All functional groups listed above except methyl are polar (hydrophilic) due to electronegative atoms like O, N, P, S.
  • Methyl is the notable nonpolar group, lacking heteroatoms beyond C and H.
• Examples and context:
  • Hydroxyl groups appear in alcohols (ethanol) and glycerol, contributing to water solubility and hydrogen bonding.
  • Carboxyl groups are acidic and prevalent in fatty acids and amino acids (as part of the amino acid backbone).
  • Ketones and aldehydes are common carbonyl-containing groups in sugars and metabolic intermediates.
  • Amino groups are central to amino acids; their basic, proton-accepting character influences protein structure and function.
  • Phosphate groups are key in energy chemistry (ATP) and nucleic acids; they enable charge interactions and solubility.
  • Sulfhydryl groups participate in disulfide bond formation in proteins, impacting folding and stability.
  • Methyl groups are common in modifying enzyme binding, protein folding, and signaling interactions by modulating hydrophobicity.
• Practical takeaway: in many biological molecules, the presence and arrangement of these groups determine polarity, solubility, reactivity, and functional roles.

Polarity and Hydrogen Bonding: Key Concepts

• Electronegative atoms (O, N, P, S) attract electrons more strongly, creating partial charges in bonds with hydrogen.
• Hydrogen bonds form when a hydrogen attached to O or N interacts with another electronegative atom’s lone pair (O or N). This underpins water solubility and the structure of biomolecules.
• Oxygen’s higher electronegativity pulls shared electrons toward itself, creating partial negative charges on O and partial positive on the bonded hydrogen, enabling hydrogen bonding.
• These interactions explain why hydroxyl, carboxyl, carbonyl, amino, phosphate, and sulfhydryl groups contribute to hydrophilicity and molecular interactions with water.

Lipids, Glycerol, and Fatty Acids

• Lipids are not polymers; they do not form long repeating monomer chains like carbohydrates, proteins, or nucleic acids.
• A common lipid structure is triglyceride: glycerol backbone (three carbons with hydroxyl groups) esterified to three fatty acids (long hydrocarbon tails).
• Fatty acid tails are largely hydrocarbon (nonpolar), making that portion hydrophobic, while glycerol and the ester linkages introduce oxygen-containing groups (polar regions).
• This combination yields amphipathic molecules, which is essential for membrane structure and lipid signaling.

Macromolecules and Polymerization

• Four major classes of organic macromolecules in biology: carbohydrates, lipids, proteins, and nucleic acids.
• Polymers are formed by linking monomers; three key polymer types are:
  • Carbohydrates (monosaccharides → polysaccharides like starch, cellulose)
  • Proteins (amino acids → polypeptides)
  • Nucleic acids (nucleotides → DNA/RNA)
• Lipids are not polymers and do not form long repeating chains.
• Dehydration synthesis (condensation) creates polymers by removing water:
  • General form: ext{Monomer}1 + ext{Monomer}2
    ightarrow ext{Polymer} + ext{H}_2 ext{O}
  • For a polymer composed of n monomers, the number of water molecules removed is $n-1$.
  • Example: five monomers → four water molecules removed: $5-1 = 4$.
• Hydrolysis is the reverse process: adding water to break a polymer into monomers.

Carbohydrates: Sugars and Nomenclature

• Carbohydrates = sugars; monosaccharides are the building blocks; polysaccharides are polymeric carbohydrates.
• Most sugars end with the suffix "-ose" (e.g., glucose, fructose, galactose).
• General empirical formula for simple sugars is typically $ext{C}<em>x ext{H}</em>{2x} ext{O}_x$, reflecting carbon, hydrogen, and oxygen in a 1:2:1 ratio for many sugars, though there are exceptions and variations (e.g., amino sugars with nitrogen).
• Carbohydrate polymers include starch (composed of amylose and amylopectin) and others; note that starch is not typically called a single "ose" molecule, but is built from glucose units (amylose/amylopectin).
• The three elements found in carbohydrates are carbon, hydrogen, and oxygen: $ext{C, H, O}$, typically in a 1:2:1 ratio in simple sugars.
• Water is the most abundant inorganic molecule in the body; carbohydrates provide energy and structural roles in biomolecules.

Bonds, Bond Energy, and Chemical Relationships

• Not all chemical bonds have the same energy; bond energies vary by bond type (e.g., C–H vs C–O). You do not need to memorize exact bond energies, but you should understand that bond energies influence reactivity and stability.
• In organic chemistry, high-energy bonds (like many C–O and C=O bonds) contribute to the energy landscape of metabolic reactions and polymerization.
• The presence of polar bonds (in groups like OH, COOH, NH2, PO4) influences solubility and intermolecular interactions, including hydrogen bonding with water.

Summary of Key Takeaways

• Carbon’s tetravalence enables the vast diversity of organic molecules and biomolecules.
• Pure hydrocarbons are carbon–hydrogen-only; living systems rely on oxygen-containing functional groups that modify polarity and reactivity.
• There are eight major functional groups (OH, COOH, C=O in ketones, CHO in aldehydes, NH2, PO4, SH, CH3); all but methyl are polar.
• Macromolecules include carbohydrates, lipids, proteins, and nucleic acids; carbohydrates, proteins, and nucleic acids are polymers; lipids are not.
• Polymers form via dehydration synthesis (condensation); hydrolysis breaks polymers by adding water; for n monomers, there are n−1 water molecules released during assembly.
• Hydrogen bonding and electronegativity drive polarity, solubility, and molecular interactions in biology.
• Small structural changes in functional groups (e.g., in steroids like estrogen vs. testosterone) can lead to large differences in biological function.
• Water is the primary inorganic molecule in the body, but the vast majority of biology arises from organic molecules built on carbon skeletons with diverse functional groups.