Chapter 2: The Chemistry of the Cell – Key Principles and Molecular Foundations

The Chemistry of the Cell

Overview

This chapter introduces the fundamental chemical principles that underlie cell structure and function. Five key principles are highlighted: the characteristics of water, the characteristics of carbon, selectively permeable membranes, synthesis by polymerization of small molecules, and self-assembly. These principles form the basis for understanding the molecular complexity and organization of living cells.

Five Principles Important to Cell Biology

• Characteristics of Water

• Characteristics of Carbon

• Selectively Permeable Membranes

• Synthesis by Polymerization of Small Molecules

• Self-Assembly

Characteristics of Carbon

Carbon: Life’s Most Essential Building Block

• Versatility: Carbon atoms have four valence electrons, allowing them to form up to four single covalent bonds with other atoms.

• Diversity of Structures: Carbon can form chains, branches, and rings, resulting in a limitless array of molecular structures.

• Stability: Carbon-carbon (C–C) bonds are strong and stable, making them ideal for the backbone of biological molecules.

Example: Hydrocarbons (e.g., methane, ethane, benzene) and their derivatives form the basis of many biomolecules.

Some Biologically Important Atoms and Molecules

• Major Atoms: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N)

• Simple Molecules: Methane (CH4), Ammonia (NH3), Water (H2O), Carbon dioxide (CO2), Ethylene (C2H4), Acetylene (C2H2)

Stability of Carbon-Containing Molecules

Bond Energies and Life

• Covalent Bonds: High bond energies (e.g., C–C bond ≈ 83 kcal/mol) provide stability to organic molecules.

• Hydrogen Bonds: Weaker than covalent bonds but crucial for molecular interactions and structure.

• Bond Energy Equation:

$\text{Bond energy} = \text{amount of energy required to break 1 mole (6 \times 10^{23}) of such bonds}$

Table: Length and Strength of Some Chemical Bonds

Bond Type	Length (nm)	Strength (kcal/mol)
Covalent (C–C)	0.154	83
Noncovalent (Hydrogen bond)	0.17	1–5
Noncovalent (Ionic bond in water)	0.25	3
Van der Waals	0.35	0.1

Additional info: In aqueous solution, ionic bonds are 10–100x weaker than covalent bonds due to hydration shells.

Hydrocarbons and Functional Groups

Hydrocarbons

• Composed only of carbon and hydrogen (e.g., ethane, propane, benzene).

• Nonpolar and hydrophobic.

Functional Groups

• Negatively Charged: Carboxyl (–COO–), Phosphate (–PO42–)

• Positively Charged: Amino (–NH3+)

• Neutral but Polar: Hydroxyl (–OH), Sulfhydryl (–SH), Carbonyl (–CO), Aldehyde (–CHO)

Example: Amino acids contain both amino and carboxyl groups, contributing to their chemical reactivity.

Biologically Important Functional Groups

• Hydroxyl group (–OH)

• Carboxyl group (–COOH)

• Amino group (–NH2)

• Phosphate group (–PO4)

• Sulfhydryl group (–SH)

• Disulfide group (–S–S–)

• Carbonyl group (–CO)

• Aldehyde group (–CHO)

The Chemistry of Life is Water Chemistry

Importance of Water

• All living organisms depend on water for survival.

• The unique structure of water underlies its remarkable properties.

Water’s Polarity and Hydrogen Bonding

• Water is a polar molecule with a bent shape (104.5° bond angle).

• Polarity allows water molecules to form hydrogen bonds with each other.

• Hydrogen bonding is responsible for many of water’s unique properties.

Properties of Water

• Cohesiveness: Water molecules stick together via hydrogen bonds.

• High Surface Tension: Water forms droplets and supports small objects.

• High Specific Heat: Water can absorb large amounts of heat with little temperature change.

• High Heat of Vaporization: Large amounts of energy are required to convert water from liquid to gas.

• Expansion Upon Freezing: Water expands as it freezes, making ice less dense than liquid water.

• Efficient Solvent: Water dissolves many ionic and polar substances.

Selectively Permeable Membranes

Importance and Structure

• Physical Barrier: Separates the cell from its environment.

• Selective Permeability: Regulates the entry and exit of substances.

• Phospholipid Bilayer: The fundamental structure of cellular membranes.

Permeability: Small, nonpolar molecules (e.g., O2, CO2) cross easily; ions and large polar molecules require transport proteins.

Macromolecule Biosynthesis

Pathways and Processes

• Inorganic Precursors: H2O, CO2, O2, N2, PO43–

• Small Organic Molecules: Monosaccharides, fatty acids, amino acids, nucleotides

• Macromolecules: Polysaccharides, lipids, proteins, nucleic acids

Steps in Polymerization:

1. Monomer Activation: Monomers are activated by coupling to a carrier molecule (often using ATP).

2. Monomer Condensation: Activated monomers are joined, releasing water (dehydration synthesis).

3. Polymerization: The growing polymer chain is elongated by the sequential addition of monomers.

Equation for Dehydration Synthesis:

$\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Enzyme}} \text{Polymer} + \text{H}_2\text{O}$

Macromolecule Degradation

• Hydrolysis: The process of breaking down polymers into monomers by adding water.

Equation for Hydrolysis:

$\text{Polymer} + \text{H}_2\text{O} \xrightarrow{\text{Enzyme}} \text{Monomer}_1 + \text{Monomer}_2$

The Importance of Self-Assembly

• After synthesis, macromolecules spontaneously assemble into higher-order structures.

• Principle of Self-Assembly: The information required for folding and assembly is inherent in the polymer’s sequence.

• Strict Self-Assembly: Occurs without assistance.

• Assisted Self-Assembly: Requires helper proteins (e.g., chaperones).

Example: Protein folding into functional three-dimensional structures.

Biologically Important Macromolecular Polymers

	Proteins	Nucleic Acids	Polysaccharides
General Function	Various (structural, enzymatic, hormonal, etc.)	Informational (DNA, RNA)	Storage (starch, glycogen), Structural (cellulose, chitin)
Type of Monomer	Amino acids	Nucleotides	Monosaccharides
Number of Different Monomers	20	4	One or a few