Chapter 3: Carbon and the Molecular Diversity of Life

Introduction

This chapter explores the central role of carbon in the chemistry of life, focusing on how carbon's unique properties enable the formation of a vast array of biological molecules. The chapter also introduces the four major classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids.

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

Electron Configuration and Bonding

• Electron configuration determines an atom’s chemical characteristics and bonding behavior.

• Carbon has four valence electrons, allowing it to form up to four covalent bonds with other atoms.

• This versatility enables carbon to form large, complex, and diverse molecules essential for life.

Shapes of Organic Molecules

• When carbon forms four single covalent bonds, the resulting shape is tetrahedral.

• When two carbons are joined by a double bond, the atoms bonded to them lie in the same plane as the carbons.

Table: Shapes of Three Simple Organic Molecules

Valence and Molecular Diversity

• Valence is the number of covalent bonds an atom can form.

• Carbon’s valence of four allows it to bond with many elements, including hydrogen, oxygen, and nitrogen.

• Carbon atoms can bond to each other, forming chains, branched molecules, or rings, contributing to molecular diversity.

Concept 3.2: Macromolecules Are Polymers, Built from Monomers

Polymers and Monomers

• Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• Monomer: The repeating units that serve as the building blocks of a polymer.

• Three of the four classes of life’s organic molecules—carbohydrates, proteins, and nucleic acids—are polymers.

Synthesis and Breakdown of Polymers

• Dehydration reaction: Monomers are joined by covalent bonds through the loss of a water molecule.

• Hydrolysis: Polymers are disassembled to monomers by the addition of water, breaking covalent bonds.

Concept 3.3: Carbohydrates Serve as Fuel and Building Material

Monosaccharides

• Monosaccharides are the simplest carbohydrates (simple sugars), with molecular formulas usually multiples of CH2O.

• Glucose (C6H12O6) is the most common monosaccharide.

• Monosaccharides are classified by the number of carbons and the position of the carbonyl group (aldose or ketose).

Disaccharides

• Disaccharide: Formed when a dehydration reaction joins two monosaccharides via a glycosidic linkage.

• Sucrose (table sugar) is the most prevalent disaccharide.

Polysaccharides

• Polysaccharides are polymers of sugars and serve storage and structural roles.

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

• Glycogen: Storage polysaccharide in animals, mainly in liver and muscle cells.

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

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

Concept 3.4: Lipids Are a Diverse Group of Hydrophobic Molecules

General Properties

• Lipids are hydrophobic molecules, mostly hydrocarbons with nonpolar covalent bonds.

• They do not form true polymers and are generally not large enough to be considered macromolecules.

Fats (Triglycerides)

• Constructed from glycerol (a three-carbon alcohol) and fatty acids (carboxyl group attached to a hydrocarbon chain).

• Three fatty acids are joined to glycerol by ester linkages, forming a triacylglycerol (triglyceride).

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

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

• Fats are a compact energy storage form, providing more than twice the energy per gram as polysaccharides.

Phospholipids

• Major component of cell membranes.

• Composed of two fatty acids (hydrophobic tails) and a phosphate group (hydrophilic head) attached to glycerol.

• In water, phospholipids self-assemble into a bilayer, forming the basic structure of cell membranes.

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.

Concept 3.5: Proteins Include a Diversity of Structures, Resulting in a Wide Range of Functions

Functions of Proteins

• Proteins account for more than 50% of the dry mass of most cells.

• Functions include catalysis (enzymes), defense, storage, transport, cellular communication, movement, and structural support.

Amino Acids and Polypeptides

• Amino acids are organic molecules with amino and carboxyl groups, differing in their side chains (R groups).

• There are 20 different amino acids, classified by the properties of their side chains.

• Polypeptides are polymers of amino acids linked by peptide bonds.

• A protein consists of one or more polypeptides folded into a specific three-dimensional structure.

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 in the backbone.

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

• Quaternary structure: Association of multiple polypeptide chains.

Protein Denaturation

• Physical and chemical conditions (pH, salt, temperature) can cause denaturation, the loss of a protein’s native structure and function.

• Denaturation is sometimes reversible if the primary structure remains intact.

Example: Sickle-Cell Disease

• A single amino acid substitution in hemoglobin causes sickle-cell disease, altering red blood cell shape and function.

Concept 3.6: Nucleic Acids Store, Transmit, and Help Express Hereditary Information

Roles of Nucleic Acids

• Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two types of nucleic acids.

• DNA stores genetic information; RNA is involved in protein synthesis and gene expression.

Structure of Nucleic Acids

• Nucleic acids are polymers called polynucleotides, made of monomers called nucleotides.

• Each nucleotide consists of a nitrogenous base, a pentose sugar (deoxyribose in DNA, ribose in RNA), and one or more phosphate groups.

• Nitrogenous bases are classified as pyrimidines (cytosine, thymine, uracil) or purines (adenine, guanine).

• DNA contains adenine (A), thymine (T), cytosine (C), and guanine (G); RNA contains uracil (U) instead of thymine.

Polynucleotide Structure

• Nucleotides are joined by phosphodiester linkages, forming a sugar-phosphate backbone with nitrogenous bases as appendages.

• Polynucleotides have directionality, with a 5' end and a 3' end.

DNA and RNA Molecules

• DNA is typically a double helix with two antiparallel strands held together by hydrogen bonds between complementary bases (A-T, G-C).

• RNA is usually single-stranded, but complementary base pairing can occur within or between RNA molecules (A-U, G-C).

Table: Comparison of DNA and RNA

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard biology curriculum.