Carbon and the Molecular Diversity of Life

Introduction to Biomolecules

Living organisms are composed of a vast array of organic molecules, all of which are based on the versatile element carbon. The four main classes of biomolecules—carbohydrates, lipids, proteins, and nucleic acids—are essential for life’s structure and function.

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

Properties of Carbon

Carbon is unique in its ability to form large, complex, and diverse molecules. This versatility arises from its four valence electrons, allowing it to form four covalent bonds with a variety of atoms, including other carbon atoms. This property enables the construction of an immense variety of organic molecules, including macromolecules essential for life.

• Organic compounds are molecules containing carbon and are found in all living organisms.

• Macromolecules are large molecules composed of thousands of covalently connected atoms.

• The four main classes of biomolecules are: carbohydrates, lipids, proteins, and nucleic acids.

Formation of Bonds with Carbon

The number of covalent bonds an atom can form is called its valence, determined by the number of unpaired electrons in its outer shell. Carbon’s valence of four allows it to form single, double, or even triple bonds, resulting in a variety of molecular shapes, including tetrahedral and planar structures.

• Single bonds create a tetrahedral geometry.

• Double bonds result in planar (flat) molecules.

Carbon Bonding Diversity

Carbon can bond with many elements, most commonly hydrogen, oxygen, and nitrogen. It can also bond with other carbon atoms, forming long chains, branched molecules, or rings. Hydrocarbons, which contain only carbon and hydrogen, are common components of many organic molecules, such as fats.

Carbon Skeletons

Carbon chains form the skeletons of most organic molecules. These skeletons can vary in:

• Length

• Branching

• Double bond position

• Presence of rings

Isomers

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

1. Structural isomers: Differ in the covalent arrangement of atoms.

2. Cis-trans (geometric) isomers: Differ in spatial arrangement around a double bond.

3. Enantiomers: Mirror images of each other, often with only one form biologically active.

Functional Groups

Functional groups are specific groups of atoms attached to carbon skeletons that participate in chemical reactions and confer specific properties to molecules. Seven functional groups are most important in the chemistry of life: 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. ATP consists of adenosine attached to three phosphate groups. 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 synthesis and breakdown of polymers involve dehydration (condensation) and hydrolysis reactions, respectively. Enzymes catalyze these reactions.

• Dehydration reaction: Joins two monomers by removing a water molecule.

• Hydrolysis reaction: Breaks a bond between monomers by adding water.

Carbohydrates

Monosaccharides

Carbohydrates are sugars and polymers of sugars. The simplest carbohydrates are monosaccharides (simple sugars), which generally have molecular formulas that are multiples of CH2O. Glucose (C6H12O6) is the most common monosaccharide. Monosaccharides are classified by the number of carbons and the placement of the carbonyl group (aldose or ketose).

• Monosaccharides serve as major nutrients and as raw materials for building other molecules.

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

Disaccharides

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

Polysaccharides

Polysaccharides are carbohydrate polymers with storage or structural roles. Their function depends on the type of monomers and the glycosidic linkages.

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

• Glycogen: Storage polysaccharide in animals, highly branched polymer of glucose.

• Cellulose: Structural polysaccharide in plant cell walls, straight and unbranched polymer of glucose.

• Chitin: Structural polysaccharide in arthropod exoskeletons and fungal cell walls, similar to cellulose but with nitrogen-containing groups.

Lipids

Properties and Types of Lipids

Lipids are hydrophobic molecules that do not form true polymers. They are mainly composed of hydrocarbons and are important for energy storage, membrane structure, and signaling. Major types include fats, phospholipids, and steroids.

Fats (Triglycerides)

Fats are constructed from glycerol and three fatty acids, joined by ester linkages. They serve as energy storage molecules. Fatty acids can be saturated (no double bonds, solid at room temperature) or unsaturated (one or more double bonds, liquid at room temperature).

Phospholipids

Phospholipids consist of two fatty acids, a phosphate group, and glycerol. They are major components of cell membranes, forming bilayers with hydrophilic heads facing outward and hydrophobic tails inward.

Steroids

Steroids are lipids with a carbon skeleton consisting of four fused rings. Cholesterol is an important steroid in animal cell membranes and a precursor for other steroids such as hormones.

Proteins

Structure and Function

Proteins are polymers of amino acids and account for more than 50% of the dry mass of most cells. They perform a wide range of functions, including catalysis (enzymes), defense, storage, transport, communication, movement, and structural support. The function of a protein is determined by its unique three-dimensional structure.

Amino Acids and Polypeptides

Amino acids are organic molecules with a central alpha carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable R group (side chain). The sequence of amino acids determines the protein’s structure and function. Amino acids are linked by peptide bonds to form polypeptides.

Levels of Protein Structure

• Primary structure: Unique sequence of amino acids.

• Secondary structure: Coils and folds (α helix and β pleated sheet) due to hydrogen bonding.

• Tertiary structure: Overall 3D shape due to interactions among R groups (hydrophobic interactions, disulfide bridges, etc.).

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

Protein Structure and Disease

A single amino acid substitution can drastically affect protein function, as seen in sickle-cell disease, where a change in hemoglobin’s primary structure leads to abnormal red blood cell shape and function.

Denaturation

Protein structure can be affected by physical and chemical conditions such as temperature, pH, and salt concentration. Denaturation is the loss of a protein’s native structure, rendering it biologically inactive. This process is sometimes reversible.

Nucleic Acids

Structure and Function

Nucleic acids store, transmit, and help express hereditary information. The two types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Genes are made of DNA, which directs its own replication and the synthesis of RNA and proteins (gene expression).

Nucleotide Structure

Nucleic acids are polymers called polynucleotides, made of monomers called nucleotides. Each nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups. Nitrogenous bases are classified as pyrimidines (C, T, U) or purines (A, G). DNA contains deoxyribose; RNA contains ribose.

DNA and RNA Structure

Adjacent nucleotides are joined by phosphodiester linkages, forming a sugar-phosphate backbone. DNA is typically double-stranded, forming a double helix with complementary base pairing (A-T, G-C). RNA is usually single-stranded, but can form complex structures through internal base pairing.

Genomics and Proteomics

Modern biology uses genomics (study of whole sets of genes) and proteomics (study of whole sets of proteins) to analyze biological information. Bioinformatics applies computational tools to manage and analyze large datasets, such as those generated by the Human Genome Project.

DNA and Proteins as Tape Measures of Evolution

Comparing DNA and protein sequences among organisms reveals evolutionary relationships. Closely related species have more similar DNA sequences, reflecting their shared ancestry.