Carbon and the Molecular Diversity of Life

Introduction to Organic Molecules and Biomolecules

Organic molecules, which contain carbon, are the foundation of life. The diversity and complexity of these molecules arise from the unique bonding properties of carbon atoms. Four main classes of biomolecules—carbohydrates, lipids, proteins, and nucleic acids—are essential for cellular structure and function.

3.1 Carbon Atoms Can Form Diverse Molecules by Bonding to Four Other Atoms

Properties of Carbon

Carbon atoms have four valence electrons, allowing them to form up to four covalent bonds with other atoms. This property enables the construction of large, complex, and varied molecules.

• Valence: The number of covalent bonds an atom can form, determined by unpaired electrons in the outer shell.

• Tetrahedral Geometry: When bonded to four other atoms, carbon forms a tetrahedral shape. Double bonds create planar (flat) molecules.

Formation of Bonds with Carbon

Carbon can bond with many elements, most commonly hydrogen, oxygen, and nitrogen, as well as with other carbon atoms. This allows for the formation of long chains, branched molecules, and rings, which serve as the skeletons of organic molecules.

• Hydrocarbons: Molecules containing only carbon and hydrogen; important components of many organic molecules.

• Carbon Dioxide (CO2): An example of a simple carbon compound.

Variation in Carbon Skeletons

Carbon chains can vary in length, branching, double bond position, and ring formation, contributing to molecular diversity.

• Length: Chains can be short or long.

• Branching: Chains may be unbranched or branched.

• Double Bonds: The position and number of double bonds can vary.

• Rings: Some carbon skeletons form rings.

Isomers: Structural Diversity

Isomers are compounds with the same molecular formula but different structures and properties. There are three main types:

• Structural Isomers: Differ in covalent arrangement of atoms.

• Cis-trans Isomers (Geometric Isomers): Differ in spatial arrangement around double bonds.

• Enantiomers: Mirror images of each other; only one form is usually biologically active.

Functional Groups

Functional groups are specific chemical groups attached to carbon skeletons that participate in chemical reactions and confer specific properties to molecules. Seven functional groups are most important in biological chemistry: hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl.

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is an organic molecule that stores and releases energy for cellular processes. Hydrolysis of ATP releases energy by removing a phosphate group, forming ADP (adenosine diphosphate).

3.2 Macromolecules Are Polymers, Built from Monomers

Polymers and Monomers

Most macromolecules are polymers, long molecules made of repeating units called monomers. The four major classes of macromolecules are carbohydrates, proteins, nucleic acids, and (with some exceptions) lipids.

• Dehydration Reaction: Joins two monomers by removing a water molecule, forming a covalent bond.

• Hydrolysis Reaction: Breaks a covalent bond by adding water, splitting a polymer into monomers.

• Enzymes: Catalyze both dehydration and hydrolysis reactions.

Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates (simple sugars) and serve as the building blocks for more complex carbohydrates. They have the general formula (CH2O)n and are classified by the number of carbons and the position of the carbonyl group.

• Examples: Glucose (C6H12O6), fructose, ribose.

• Ring Formation: In aqueous solutions, most five- and six-carbon sugars form rings.

Disaccharides

Disaccharides are formed by joining two monosaccharides via a glycosidic linkage (covalent bond formed by dehydration reaction). Examples include sucrose (glucose + fructose), lactose, and maltose.

Polysaccharides

Polysaccharides are large carbohydrate polymers with storage or structural roles.

• Starch: Storage polysaccharide in plants, composed of glucose monomers.

• Glycogen: Storage polysaccharide in animals, highly branched, stored in liver and muscle cells.

• Cellulose: Structural polysaccharide in plant cell walls; differs from starch in glycosidic linkage orientation.

• Chitin: Structural polysaccharide in arthropod exoskeletons and fungal cell walls; contains nitrogen.

Lipids

General Properties

Lipids are hydrophobic molecules that do not form true polymers. They are mainly composed of hydrocarbons and serve as energy storage, structural components of membranes, and signaling molecules.

Fats (Triglycerides)

Fats are constructed from glycerol and three fatty acids, joined by ester linkages. They are efficient energy storage molecules.

• Saturated Fats: No double bonds; solid at room temperature; mostly animal fats.

• Unsaturated Fats: One or more double bonds; liquid at room temperature; mostly plant and fish fats.

• Trans Fats: Unsaturated fats with trans double bonds; often produced industrially.

Phospholipids

Phospholipids consist of two fatty acids, a phosphate group, and glycerol. They are amphipathic, with hydrophilic heads and hydrophobic tails, and form the bilayer structure of cell membranes.

Steroids

Steroids are lipids with a carbon skeleton consisting of four fused rings. Cholesterol is a key steroid, maintaining membrane fluidity and serving as a precursor for other steroids such as hormones.

Proteins

Structure and Function

Proteins are polymers of amino acids and perform a vast array of functions, including catalysis (enzymes), defense, transport, signaling, movement, and structural support. The function of a protein is determined by its three-dimensional structure.

• Amino Acids: Monomers with a central alpha carbon, amino group, carboxyl group, hydrogen, and variable R group (side chain).

• Peptide Bonds: Covalent bonds linking amino acids in a polypeptide chain.

Levels of Protein Structure

• Primary Structure: Unique sequence of amino acids.

• Secondary Structure: Coils and folds (α-helix, β-pleated sheet) stabilized by hydrogen bonds.

• Tertiary Structure: Overall 3D shape due to interactions among R groups (hydrophobic interactions, disulfide bridges, ionic bonds, hydrogen bonds).

• Quaternary Structure: Association of two or more polypeptides (e.g., hemoglobin).

Protein Folding and Denaturation

Protein function depends on correct folding. Physical and chemical conditions (temperature, pH, salt concentration) can disrupt folding, causing denaturation and loss of function. Some proteins can refold (renaturation), but many cannot.

Nucleic Acids

DNA and RNA: Structure and Function

Nucleic acids store, transmit, and help express hereditary information. DNA encodes genetic instructions; RNA is involved in protein synthesis. Both are polymers of nucleotides.

• Nucleotide: Composed of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• Nucleoside: Nitrogenous base + sugar (no phosphate).

• DNA: Double-stranded, deoxyribose sugar, bases A, T, G, C.

• RNA: Single-stranded, ribose sugar, bases A, U, G, C.

Polynucleotide Structure

Nucleotides are joined by phosphodiester linkages, forming a sugar-phosphate backbone. DNA strands are antiparallel and held together by complementary base pairing (A-T, G-C).

Genomics and Proteomics

Genomics is the study of whole sets of genes and their interactions, while proteomics is the study of large sets of proteins. Advances in these fields, aided by bioinformatics, have revolutionized our understanding of biology and evolution.

Summary Table: Major Classes of Biomolecules