Chapter 4: The Chemistry of Carbon – Foundation of Biological Molecules

Chapter 4: Carbon – The Backbone of Life

Introduction to Carbon in Biology

Carbon is a fundamental element in biological systems, forming the backbone of the major macromolecules that constitute living organisms. Its unique chemical properties allow it to form a vast array of complex and diverse molecules essential for life.

• Key Role: Besides water, living organisms are primarily composed of carbon-based compounds.

• Major Biological Molecules: Proteins, DNA, carbohydrates, and lipids are all built from carbon compounds.

• Versatility: Carbon's ability to form large, complex, and varied molecules is unparalleled among elements.

Organic Chemistry

Definition and Scope

Organic chemistry is the study of compounds that contain carbon. These compounds can range from simple molecules to colossal macromolecules.

• Organic Compounds: Molecules containing carbon, often in combination with hydrogen, oxygen, nitrogen, sulfur, and phosphorus.

• Uniformity: The overall percentages of the major elements of life (C, H, O, N, S, P) are quite uniform among organisms.

Valence and Bonding

Carbon has four valence electrons, allowing it to form four covalent bonds with a variety of atoms. This property enables the construction of an inexhaustible variety of organic molecules.

• Valence Electrons: The number of electrons in the outermost shell that can participate in bonding.

• Covalent Bonds: Strong chemical bonds formed by the sharing of electron pairs between atoms.

Valence of Major Elements in Biology

Element	Valence	Example
Hydrogen (H)	1	H2
Oxygen (O)	2	O2
Nitrogen (N)	3	N2
Carbon (C)	4	CH4 (methane)

Formation of Bonds with Carbon

Tetrahedral Geometry and Double Bonds

In molecules with multiple carbons, each carbon bonded to four other atoms adopts a tetrahedral shape. When two carbon atoms are joined by a double bond, the atoms joined to the carbons are in the same plane as the carbons.

• Tetrahedral Shape: The spatial arrangement of bonds around a carbon atom with four single bonds.

• Planar Geometry: Occurs when carbons are joined by double bonds.

Variation in Carbon Skeletons

Carbon Chains and Their Diversity

Carbon chains form the skeletons of most organic molecules. These chains can vary in length, branching, double bond position, and the presence of rings, contributing to molecular diversity.

• Length: Carbon chains can be short or long.

• Branching: Chains may be unbranched or branched.

• Double Bond Position: Double bonds can occur at different positions along the chain.

• Rings: Some carbon skeletons form closed rings (e.g., cyclohexane, benzene).

Diversity of Carbon-Based Molecules

Hydrocarbons

Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen. They are hydrophobic and can store large amounts of energy, as seen in fats.

• Hydrophobic: Do not dissolve in water.

• Energy Storage: Hydrocarbons can undergo reactions that release significant energy.

Isomers

Types of Isomers

Isomers are compounds with the same molecular formula but different structures and properties. The three main types are:

• Structural Isomers: Differ in the covalent arrangements of their atoms.

• Cis-Trans (Geometric) Isomers: Have the same covalent bonds but differ in spatial arrangement around a double bond.

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

Comparison of Isomer Types

Type	Definition	Example
Structural	Different covalent arrangement	Pentane vs. 2-methylbutane
Cis-Trans	Different spatial arrangement around double bond	Cis-2-butene vs. trans-2-butene
Enantiomers	Mirror images, non-superimposable	L-alanine vs. D-alanine

Biological Importance: Enantiomers are significant in biology because usually only one isomer is biologically active. This is crucial in pharmaceuticals, where different enantiomers can have different effects.

Functional Groups

Definition and Importance

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The number and arrangement of functional groups give each molecule its unique properties.

• Role: Functional groups are the components of organic molecules most commonly involved in chemical reactions.

• Example: Estradiol and testosterone are both steroids with a common carbon skeleton but differ in the functional groups attached, resulting in different biological activities.

The Seven Most Important Functional Groups in Biology

Functional Group	Structure	Compound Name	Example	Properties
Hydroxyl	-OH	Alcohol	Ethanol	Polar, forms hydrogen bonds, increases solubility in water
Methyl	-CH3	Methylated compound	5-methylcytosine	Affects gene expression, non-polar
Carbonyl	-C=O	Aldehyde or Ketone	Acetone (ketone), Propanal (aldehyde)	Found in sugars, increases reactivity
Carboxyl	-COOH	Carboxylic acid	Acetic acid	Acts as an acid, donates H+
Amino	-NH2	Amine	Glycine	Acts as a base, picks up H+
Sulfhydryl	-SH	Thiol	Cysteine	Forms disulfide bonds, stabilizes proteins
Phosphate	-OPO32-	Organic phosphate	Glycerol phosphate	Contributes negative charge, can release energy

ATP: An Important Source of Energy for Cells

Structure and Function of ATP

Adenosine triphosphate (ATP) is a key organic phosphate that stores and transfers energy within cells. ATP consists of adenosine attached to a string of three phosphate groups. The hydrolysis of ATP releases energy that is used by the cell for various functions.

• Reaction: ATP reacts with water to form ADP (adenosine diphosphate) and inorganic phosphate, releasing energy.

Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Summary Table: Key Terms and Concepts

Term	Definition
Hydrocarbon	Organic molecule consisting only of carbon and hydrogen
Isomer	Compounds with the same molecular formula but different structures
Functional Group	Specific group of atoms responsible for characteristic reactions
ATP	Energy-carrying molecule in cells

Learning Objectives

• Explain why carbon is important in living organisms, including its structure and bonding capabilities.

• Distinguish between different types of isomers: structural, cis-trans, and enantiomers.

• Identify and describe the seven functional groups most important to the chemistry of life.

• Write out the reaction with ATP that releases energy.

Additional info: The notes above expand on the provided slides and text, adding definitions, examples, and context for clarity and completeness as a study guide for General Biology students.