Carbon and the Molecular Diversity of Life & Structure and Function of Large Biological Molecules

Carbon and the Molecular Diversity of Life

Overview: Carbon—The Backbone of Life

Carbon is the foundational element in organic molecules, forming the backbone of the major macromolecules essential for life. Its unique bonding properties allow for the formation of a vast diversity of molecular structures.

• Organic chemistry is the study of carbon-containing compounds, which range from simple molecules to complex macromolecules.

• Major elements in living organisms include C, H, O, N, S, and P, with carbon being central due to its tetravalence (ability to form four covalent bonds).

• The versatility of carbon enables the diversity of life’s molecular architecture.

Electron Configuration and Bonding

The electron configuration of carbon determines its chemical behavior and bonding capacity.

• Carbon has four valence electrons, allowing it to form four covalent bonds with a variety of atoms.

• This tetravalence leads to tetrahedral geometry in molecules like methane, and planar geometry when double bonds are present.

Valence and Bonding Partners

The number of unpaired electrons in the valence shell determines the number of bonds an atom can form (its valence).

• Carbon’s most frequent bonding partners are hydrogen, oxygen, and nitrogen.

Examples of Carbon Compounds

• Carbon can bond with atoms other than hydrogen, such as in carbon dioxide (CO2) and urea (CO(NH2)2).

Carbon Skeletons and Molecular Diversity

Carbon chains form the skeletons of most organic molecules, varying in length, branching, double bond position, and ring formation.

• These variations contribute to the diversity and complexity of organic molecules.

Hydrocarbons

Hydrocarbons are organic molecules consisting only of carbon and hydrogen. They are hydrophobic and can store large amounts of energy, as seen in fats.

Isomers

Isomers are compounds with the same molecular formula but different structures and properties.

• Structural isomers: Different covalent arrangements of atoms.

• Cis-trans (geometric) isomers: Same covalent bonds but different spatial arrangements.

• Enantiomers: Mirror images of each other, important in pharmaceuticals as only one enantiomer may be biologically active.

Functional Groups

Functional groups are specific groups of atoms attached to carbon skeletons that determine the chemical properties of organic molecules. The seven most important functional groups in biology are:

• Hydroxyl (-OH)

• Carbonyl (C=O)

• Carboxyl (-COOH)

• Amino (-NH2)

• Sulfhydryl (-SH)

• Phosphate (-PO43-)

• Methyl (-CH3)

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is an important organic phosphate that stores and transfers energy within cells.

• ATP consists of adenosine attached to three phosphate groups.

• Hydrolysis of ATP releases energy used for cellular work.

The Structure and Function of Large Biological Molecules

Classes of Biological Macromolecules

All living things are composed of four major classes of large biological molecules:

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

Macromolecules are polymers built from monomers, except for lipids, which are not true polymers.

Polymer Synthesis and Breakdown

• Dehydration reaction: Joins monomers by removing a water molecule, forming a new bond.

• Hydrolysis: Breaks polymers into monomers by adding a water molecule.

Carbohydrates

Carbohydrates include sugars and their polymers. They serve as fuel and building material.

• Monosaccharides: Simple sugars (e.g., glucose, C6H12O6), classified by the location of the carbonyl group and the number of carbons.

• Disaccharides: Formed by joining two monosaccharides via a glycosidic linkage.

• Polysaccharides: Polymers of sugars with storage (e.g., starch, glycogen) or structural (e.g., cellulose, chitin) roles.

Lipids

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids. They are not true polymers.

• Fats: Composed of glycerol and fatty acids, function in energy storage, insulation, and protection.

• Saturated fats: No double bonds, solid at room temperature (e.g., animal fats).

• Unsaturated fats: One or more double bonds, liquid at room temperature (e.g., plant and fish oils).

• Phospholipids: Two fatty acids and a phosphate group attached to glycerol; form cell membranes.

• Steroids: Four fused carbon rings; cholesterol is a key example.

Proteins

Proteins are polymers of amino acids and perform a vast array of functions, including catalysis, structure, transport, and defense.

• Amino acids: Organic molecules with amino and carboxyl groups; differ by their R groups.

• Peptide bonds: Link amino acids to form polypeptides.

• Protein structure: Four levels—primary (sequence), secondary (α-helix, β-sheet), tertiary (3D folding), quaternary (multiple polypeptides).

• Denaturation: Loss of protein structure and function due to environmental changes.

Nucleic Acids

Nucleic acids (DNA and RNA) store, transmit, and help express hereditary information.

• Nucleotides: Monomers consisting of a nitrogenous base, a pentose sugar, and phosphate group.

• DNA: Double helix, antiparallel strands, base pairing (A-T, G-C).

• RNA: Single-stranded, base pairing (A-U, G-C).

• Gene expression: DNA directs synthesis of mRNA, which is translated into protein.

Summary Table: Functional Groups

Chemical Group	Compound Name	Examples
Hydroxyl (-OH)	Alcohol	Ethanol
Carbonyl (C=O)	Ketone, Aldehyde	Acetone, Propanal
Carboxyl (-COOH)	Carboxylic acid	Acetic acid
Amino (-NH2)	Amine	Glycine
Sulfhydryl (-SH)	Thiols	Cysteine
Phosphate (-PO43-)	Organic phosphate	Glycerol phosphate
Methyl (-CH3)	Methylated compound	5-Methyl cytidine

Key Equations

• ATP Hydrolysis: $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{Energy}$

Things to Know

• How carbon’s electron configuration explains its ability to form large, complex, diverse organic molecules

• How carbon skeletons vary and contribute to molecular diversity

• The three types of isomers: structural, geometric (cis-trans), and enantiomers

• The major functional groups, their structures, and properties

• How ATP functions as the primary energy transfer molecule in cells

• The four major classes of biological molecules and their functions

• The formation and breakdown of polymers

• The four levels of protein structure

• Differences between pyrimidines and purines, nucleotides and nucleosides, ribose and deoxyribose, and the 5′ and 3′ ends of nucleic acids