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

Chapter 3: Carbon and the Molecular Diversity of Life

Key Concepts

• Carbon atoms can form diverse molecules by bonding to four other atoms

• Macromolecules are polymers, built from monomers

• Carbohydrates serve as fuel and building material

• Lipids are a diverse group of hydrophobic molecules

• Proteins include a diversity of structures, resulting in a wide range of functions

• Nucleic acids store, transmit, and help express hereditary information

• Genomics and proteomics have transformed biological inquiry and applications

Carbon: The Backbone of Biological Molecules

Carbon Atoms and Molecular Diversity

Carbon is a key element in organic molecules due to its ability to form four covalent bonds, allowing for a variety of stable and complex structures. This versatility underlies the diversity of organic compounds found in living organisms.

• Organic compounds: Molecules containing carbon atoms bonded to other elements, typically hydrogen, oxygen, and nitrogen.

• Electron configuration: Carbon has 4 valence electrons, allowing it to form 4 covalent bonds with other atoms.

• Common bonding partners: Hydrogen, oxygen, nitrogen, and other carbons.

• Carbon skeletons: Can vary in length, branching, double bond position, and presence of rings, contributing to molecular diversity.

Hydrocarbons and Isomers

• Hydrocarbons: Organic molecules consisting only of carbon and hydrogen. They are generally hydrophobic due to nonpolar C-H bonds. Examples: petroleum, fats.

• Isomers: Compounds with the same molecular formula but different structures and properties.

Term	Definition	Example
Isotope	Different atomic forms of the same element	Carbon-12, Carbon-14
Isomer	Compounds with the same number of atoms of the same elements but different structures and properties	Pentane, isopentane; butane, isobutane

• Pharmacological effects: Isomers can have different biological activities (e.g., S-ibuprofen is effective, R-ibuprofen is not).

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

• Hydroxyl group (-OH): Found in alcohols; makes molecules polar.

• Carbonyl group (C=O): Found in sugars; can be at the end (aldehyde) or within (ketone) a carbon skeleton.

• Carboxyl group (-COOH): Acts as an acid; found in amino acids and fatty acids.

• Amino group (-NH2): Acts as a base; found in amino acids.

• Sulfhydryl group (-SH): Can form disulfide bonds; found in some amino acids.

• Phosphate group (-PO4): Involved in energy transfer (e.g., ATP).

• Methyl group (-CH3): Affects gene expression.

Structure	Hydroxyl	Carbonyl	Carboxyl	Amino	Sulfhydryl	Phosphate	Methyl
Example	Alcohols	Sugars	Amino acids, fatty acids	Amino acids	Cysteine	ATP	Methylated DNA

Macromolecules: Polymers Built from Monomers

Polymers and Monomers

Most biological macromolecules are polymers, long chains made by linking together smaller units called monomers.

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

• Monomer: The repeating subunit that serves as the building block of a polymer.

• Major classes of macromolecules: Carbohydrates, lipids, proteins, nucleic acids.

Polymerization Reactions

• Dehydration synthesis: Monomers are joined together by covalent bonds, with the removal of a water molecule for each bond formed.

• Hydrolysis: Polymers are broken down into monomers by the addition of water.

General equation for dehydration synthesis:

$C_6H_{12}O_6 + C_6H_{12}O_6 \rightarrow C_{12}H_{22}O_{11} + H_2O$

Carbohydrates: Fuel and Building Material

Monosaccharides

Carbohydrates are sugars and polymers of sugars. The simplest carbohydrates are monosaccharides, or simple sugars.

• Monosaccharide formula: $(CH_2O)_n$

• Common monosaccharides: Glucose, fructose, galactose

• Functional group: Carbonyl group (C=O)

• Classification: By number of carbons (triose, pentose, hexose)

Disaccharides and Glycosidic Linkages

• Disaccharide: Two monosaccharides joined by a glycosidic linkage (covalent bond formed by dehydration reaction).

• Common disaccharides: Sucrose, lactose, maltose

• Glycosidic linkage: Covalent bond between two monosaccharides.

• 1-4 glycosidic linkage: Bond between the 1st carbon of one glucose and the 4th carbon of another.

Polysaccharides

Polysaccharides are large macromolecules formed from monosaccharides. They serve as storage or structural materials.

Type of Polysaccharide	Examples
Storage	Starch (plants), glycogen (animals)
Structural	Cellulose (plants), chitin (arthropods, fungi)

• Starch: Storage polysaccharide in plants; composed of glucose monomers in alpha configuration.

• Glycogen: Storage polysaccharide in animals; highly branched.

• Cellulose: Structural polysaccharide in plant cell walls; composed of glucose monomers in beta configuration. Most animals cannot digest cellulose, but some microorganisms can.

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

Summary Table: Key Carbohydrates

Carbohydrate	Description
Starch	Plant storage polysaccharide; alpha 1-4 glycosidic linkages
Glycogen	Animal storage polysaccharide; highly branched
Cellulose	Plant structural polysaccharide; beta 1-4 glycosidic linkages
Glucose	Monosaccharide; main energy source
Fructose	Monosaccharide; "fruit sugar"
Lactose	Disaccharide; "milk sugar"

Additional info:

• Enzymes are required to hydrolyze polysaccharides; not all organisms have the necessary enzymes (e.g., humans cannot digest cellulose).

• Carbohydrates play roles in cell recognition and signaling, in addition to energy storage and structural support.