Organic Molecules: Carbon Chemistry and Biological Macromolecules

Organic Molecules

Significance of Carbon's Ability to Form Four Covalent Bonds

Carbon is a unique element in biological systems due to its ability to form four covalent bonds. This property allows carbon to serve as the backbone for a vast diversity of organic molecules essential for life.

• Tetravalence: Carbon has four valence electrons, enabling it to form four stable covalent bonds with other atoms, including hydrogen, oxygen, nitrogen, and other carbons.

• Structural Diversity: The ability to form single, double, and triple bonds, as well as branched chains and rings, allows for a wide variety of molecular structures.

• Biological Importance: This versatility is the foundation for the complexity of biological macromolecules such as carbohydrates, proteins, lipids, and nucleic acids.

• Example: Methane (CH4) is the simplest organic molecule, with carbon forming four single bonds with hydrogen atoms.

Functional Groups: Identification, Formulas, and Properties

Functional groups are specific groups of atoms within molecules that confer characteristic chemical properties and reactivity. Recognizing these groups is essential for understanding the behavior of organic molecules in biological systems.

Functional Group	Formula	Properties	Example
Hydroxyl	-OH	Polar, forms hydrogen bonds, increases solubility in water	Alcohols (e.g., ethanol)
Carbonyl	>C=O	Polar, found in sugars (as ketones or aldehydes)	Acetone (ketone), formaldehyde (aldehyde)
Carboxyl	-COOH	Acts as an acid (can donate H+), found in amino acids and fatty acids	Acetic acid
Amino	-NH2	Acts as a base (can accept H+), found in amino acids	Glycine
Sulfhydryl	-SH	Forms disulfide bonds, stabilizes protein structure	Cysteine
Phosphate	-OPO32-	Contributes negative charge, involved in energy transfer (e.g., ATP)	Glycerol phosphate
Methyl	-CH3	Nonpolar, affects gene expression when added to DNA	5-methyl cytosine

Dehydration Synthesis vs. Hydrolysis Reactions

Macromolecules are assembled and disassembled by two key types of reactions: dehydration synthesis and hydrolysis.

• Dehydration Synthesis (Condensation Reaction): A chemical reaction in which two monomers are covalently bonded to each other with the removal of a water molecule. This process builds polymers from monomers.

• Hydrolysis Reaction: A chemical reaction that breaks the covalent bond between two monomers by the addition of a water molecule. This process breaks down polymers into monomers.

• Example: Formation of a peptide bond between two amino acids (dehydration synthesis) and the breakdown of a disaccharide into monosaccharides (hydrolysis).

Anabolic vs. Catabolic Reactions

Metabolic pathways in cells can be classified as anabolic or catabolic based on whether they build up or break down molecules.

• Anabolic Reactions: Build complex molecules from simpler ones; require energy input. Example: Protein synthesis from amino acids.

• Catabolic Reactions: Break down complex molecules into simpler ones; release energy. Example: Cellular respiration breaking down glucose.

Endergonic vs. Exergonic Reactions

Chemical reactions can also be classified based on their energy changes.

• Endergonic Reactions: Absorb free energy from their surroundings; non-spontaneous. Example: Photosynthesis.

• Exergonic Reactions: Release free energy; spontaneous. Example: Breakdown of ATP to ADP and inorganic phosphate.

Key Equation:

• Gibbs Free Energy Change: $\Delta G = \Delta H - T\Delta S$

• Where $\Delta G$ is the change in free energy, $\Delta H$ is the change in enthalpy, $T$ is temperature in Kelvin, and $\Delta S$ is the change in entropy.