Chapter 3: Carbon and the Molecular Diversity of Life – Study Notes

Carbon and the Molecular Diversity of Life

Introduction

Carbon is the foundational element of life, forming the backbone of the major biological macromolecules. Its unique chemical properties allow for the formation of a vast array of complex molecules essential for cellular structure and function.

The Four Major Classes of Biological Molecules

Overview of Macromolecules

• Carbohydrates: Serve as fuel and building material. Example: Starch (polymer of glucose).

• Proteins: Perform a wide range of functions including catalysis, structure, and signaling. Example: Enzymes, structural proteins.

• Nucleic Acids: Store and transmit hereditary information. Example: DNA and RNA.

• Lipids: Diverse group, not true polymers, important for energy storage and membrane structure. Example: Fats, phospholipids, steroids.

Three of these classes (carbohydrates, proteins, nucleic acids) are polymers made from monomers. Lipids are not polymers but are large macromolecules.

Formation of Bonds with Carbon

Covalent Bonding and Molecular Shape

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

• This property enables carbon to form large, complex molecules with various shapes, including chains and rings.

• When carbon forms four single bonds, the molecule adopts a tetrahedral shape.

• Double bonds between carbon atoms restrict rotation, resulting in planar regions within molecules.

Examples of Simple Carbon Compounds

Comparison of Methane, Ethane, and Ethene

The following table compares the molecular formula, structural formula, and 3D shape of three simple hydrocarbons:

Molecule and Molecular Shape	Molecular Formula	Structural Formula	Ball-and-Stick Model
Methane	CH4	H | H–C–H | H	Tetrahedral (all H atoms equidistant from C)
Ethane	C2H6	H H | | H–C–C–H | | H H	Two tetrahedral carbons joined by a single bond
Ethene (ethylene)	C2H4	H2C=CH2	Planar, due to double bond

Valences of the Major Elements of Organic Molecules

Valence and Covalent Bonding

The valence of an atom is the number of covalent bonds it can form, determined by the number of electrons needed to fill its valence shell.

Element	Hydrogen	Oxygen	Nitrogen	Carbon
Electrons Needed to Fill Valence Shell	1	2	3	4
Valence (Number of Bonds Formed)	1	2	3	4

• Key Point: The electron configuration of carbon allows it to bond covalently with many different elements, contributing to the diversity of organic molecules.

Example

• Methane (CH4): Carbon forms four single covalent bonds with hydrogen, resulting in a stable, tetrahedral molecule.

Additional info: The ability of carbon to form stable bonds with other elements, especially hydrogen, oxygen, and nitrogen, is fundamental to the structure and function of biomolecules.