Carbon and the Molecular Diversity of Life: Study Notes

Chapter 3: Carbon and the Molecular Diversity of Life

Introduction

This chapter explores the central role of carbon in the structure and function of biological molecules. It covers the chemical properties of carbon, the diversity of organic molecules, and the four major classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids.

Carbon: The Basis of Molecular Diversity

Electron Configuration and Bonding

Carbon's unique electron configuration allows it to form diverse molecules by bonding with up to four other atoms.

• Valence electrons: Carbon has four valence electrons, enabling it to form four covalent bonds.

• Tetrahedral geometry: In molecules with multiple carbons, each carbon bonded to four other atoms forms a tetrahedral shape.

• Double bonds: When two carbon atoms are joined by a double bond, the atoms attached to them lie in the same plane.

Valence and Covalent Compatibility

The number of covalent bonds an atom can form is called its valence. Carbon's electron configuration allows it to bond with many elements, including hydrogen, oxygen, and nitrogen.

• Versatility: Carbon can form large, complex molecules, including chains and rings.

• Examples: Carbon dioxide (CO2), hydrocarbons, and organic acids.

Shapes of Simple Organic Molecules

Organic molecules can be represented in various models to illustrate their shapes and bonding.

Molecule	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Methane	CH4	H | H–C–H | H	Shows tetrahedral geometry	Compact, spherical representation
Ethane	C2H6	H–C–C–H (with hydrogens attached)	Shows two tetrahedral carbons	Space-filling model of two connected spheres
Ethene (ethylene)	C2H4	H2C=CH2	Planar structure due to double bond	Flat, space-filling representation

Classes of Biological Molecules

Overview

Carbon forms the structural basis for four important classes of biological molecules:

• Carbohydrates: Polymers of sugars, such as starch and glucose.

• Proteins: Polymers of amino acids, with diverse functions.

• Nucleic acids: Polymers of nucleotides, such as DNA and RNA.

• Lipids: Hydrophobic molecules, including fats, phospholipids, and steroids (not true polymers).

Key Terms

• Carbon

• Amino acids

• Carbohydrates

• Hydroxyl

• Amino

• Methyl

• Phosphate

• Carboxylic

• Carbonyl

• Sulfhydryl

• ATP

• DNA

• Fatty acid

• Hydrogen bond

• Hydrophilic

• Hydrophobic

• Lipids

• Lipid bilayer

• Macromolecule

• Monosaccharide

• Monomer

• Nucleotide

• Polymer

• Protein

• RNA

• Sugar

• Double helix

• Antiparallel

• Beta sheet

• Alpha helix

• Primary structure

• Secondary structure

• Tertiary structure

• Quaternary structure

• Fatty acids

• Triglycerol

Summary Table: Four Classes of Biological Molecules

Class	Monomer	Polymer	Example	Function
Carbohydrates	Monosaccharide	Polysaccharide	Starch, glucose	Energy storage, structure
Proteins	Amino acid	Polypeptide	Enzymes, hemoglobin	Catalysis, transport, structure
Nucleic acids	Nucleotide	Polynucleotide	DNA, RNA	Genetic information, protein synthesis
Lipids	Fatty acid, glycerol	Not true polymers	Triglycerides, phospholipids	Energy storage, membranes

Conclusion

Carbon's ability to form four covalent bonds makes it the foundation for the molecular diversity essential to life. The four major classes of biological molecules—carbohydrates, lipids, proteins, and nucleic acids—are built on carbon skeletons and are central to the structure and function of living organisms.