Chapter 4 – Carbon and the Molecular Diversity of Life: Study Notes

Carbon and the Molecular Diversity of Life

Introduction

Carbon is a fundamental element in biological molecules, providing the backbone for the vast diversity of organic compounds found in living organisms. Its unique bonding properties enable the formation of complex molecules essential for life.

Major Elements in Biological Molecules

Uniformity Across Life

• Major elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Sulfur (S), and Phosphorus (P) are present in similar proportions in all living organisms.

• These elements form the basis of most biological molecules.

Properties of Carbon

Tetravalence and Bonding

• Valence electrons: Carbon has four valence electrons, allowing it to form four covalent bonds (tetravalence).

• Bond types: Carbon can form single, double, or triple covalent bonds with other atoms.

• Molecular diversity: Carbon atoms can link together in chains, branch, or form rings, resulting in a wide variety of molecular structures.

The Formation of Bonds with Carbon

Bonding Capabilities

• With four valence electrons, carbon can form stable covalent bonds with many elements, including H, O, N, S, and P.

• This enables the formation of large, complex molecules essential for life.

• In molecules with multiple carbons, each carbon bonded to four other atoms adopts a tetrahedral shape.

• When two carbons are joined by a double bond, the atoms attached to those carbons lie in the same plane as the carbons (planar geometry).

Carbon Skeletons and Molecular Diversity

Carbon Skeletons

• Carbon can bond to other carbons, forming the skeletons of organic molecules.

• These skeletons can vary in length, branching, and ring formation, contributing to molecular diversity.

• Properties of a carbon-containing molecule depend on its carbon skeleton and chemical (functional) groups attached to it.

Examples of Carbon Skeletons

Molecule and Shape	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Methane (tetrahedral)	CH4	H | H–C–H | H	Ball-and-stick representation (tetrahedral)	Space-filling representation
Ethane	C2H6	H H | | H–C–C–H | | H H	Ball-and-stick representation (two tetrahedra joined)	Space-filling representation
Ethene (ethylene, planar)	C2H4	H2C=CH2	Ball-and-stick representation (planar)	Space-filling representation

Additional info: The table above summarizes how carbon's bonding leads to different molecular shapes, which in turn affect molecular properties.

Valence and Bonding Capacity

Valence Electrons and Covalent Bonds

• The number of unpaired electrons in the valence shell of an atom determines its valence—the number of covalent bonds it can form.

	Hydrogen	Oxygen	Nitrogen	Carbon
Lewis dot structure	H·	·O··	·N··	·C··
Electron distribution diagram	1 shell, 1 electron	2 shells, 6 electrons	2 shells, 7 electrons	2 shells, 4 electrons in outer shell
Electrons needed to fill valence shell	1	2	3	4
Valence (number of bonds)	1	2	3	4

Hydrocarbons

Definition and Properties

• Hydrocarbons are organic molecules consisting only of carbon and hydrogen.

• They are nonpolar and hydrophobic (do not dissolve in water).

• Hydrocarbons store large amounts of energy (e.g., in fossil fuels and lipids).

• They serve as the backbone for adding functional groups, which modify their properties.

Isomers

Types of Isomers

• Isomers are compounds with the same molecular formula but different structures and properties.

• Structural isomers: Differ in the covalent arrangement of their atoms.

• Cis-trans (geometric) isomers: Have the same covalent bonds but differ in spatial arrangement around a double bond.

• Enantiomers: Are mirror images of each other and cannot be superimposed.

Example: The difference between cis and trans isomers affects the physical and biological properties of molecules, such as in fatty acids.

Functional Groups

Importance in Biological Molecules

• Functional groups are specific groups of atoms attached to carbon skeletons that determine the chemical behavior of molecules.

• Small differences in functional groups can lead to significant biological effects (e.g., testosterone vs. estradiol).

Major Functional Groups in Biology

Functional Group	Structure	Properties	Example/Function
Hydroxyl	-OH	Polar, forms hydrogen bonds, increases solubility	Alcohols (e.g., ethanol)
Carbonyl	=O	Found in sugars, can form structural isomers	Aldehydes, ketones
Carboxyl	-COOH	Acts as an acid, donates H+	Amino acids, fatty acids
Amino	-NH2	Acts as a base, picks up H+	Amino acids
Sulfhydryl	-SH	Forms disulfide bonds, stabilizes protein structure	Cysteine (amino acid)
Phosphate	-PO4	Transfers energy, part of ATP and DNA backbone	ATP, nucleic acids
Methyl	-CH3	Affects gene expression, hydrophobic	Methylated DNA

Summary

• Carbon's versatility as a backbone allows for the formation of a vast array of biological molecules.

• Functional groups attached to carbon skeletons determine the chemical behavior and function of organic molecules.

• Even small modifications in functional groups can result in significant changes in molecular function and biological activity.

Example Application

• Testosterone vs. Estradiol: These hormones differ only in their functional groups, yet have dramatically different effects in the body.

• Hydroxyl and Carboxyl Groups: Adding a hydroxyl group to a hydrocarbon increases its solubility in water, while adding a carboxyl group makes it acidic.