Chapter 3: Water and Life – Properties and Biological Importance of Water

Water and Life

Introduction

Water is essential for all known forms of life. Its unique physical and chemical properties make it indispensable for biological processes and the maintenance of life on Earth. This chapter explores the molecular structure of water, its emergent properties, and its significance for living organisms and ecosystems.

Structure of Water

Molecular Structure and Polarity

Water (H2O) is a simple molecule composed of two hydrogen atoms covalently bonded to one oxygen atom. However, its molecular geometry and the difference in electronegativity between hydrogen and oxygen give rise to several important properties.

• Polar Molecule: Water is a polar molecule. The oxygen atom is more electronegative than hydrogen, resulting in a partial negative charge (δ-) near the oxygen and partial positive charges (δ+) near the hydrogens.

• Hydrogen Bonding: The polarity of water molecules allows them to form hydrogen bonds—weak attractions between the partially positive hydrogen of one molecule and the partially negative oxygen of another.

• Example: In liquid water, each molecule can form up to four hydrogen bonds with neighboring water molecules, creating a dynamic network.

Emergent Properties of Water

Overview

Water exhibits several emergent properties that are critical for life. These properties arise from the collective behavior of water molecules and their ability to form hydrogen bonds.

• Cohesive Behavior: Water molecules stick together due to hydrogen bonding, resulting in high surface tension and the ability to move as a column in plant vessels.

• Ability to Moderate Temperature: Water can absorb or release large amounts of heat with only a slight change in its own temperature, due to its high specific heat capacity.

• Expansion Upon Freezing: Unlike most substances, water expands as it freezes. Ice is less dense than liquid water, allowing it to float and insulate aquatic environments.

• Versatility as a Solvent: Water's polarity makes it an excellent solvent, capable of dissolving a wide variety of ionic and polar substances.

Cohesion and Adhesion

Cohesion refers to the attraction between water molecules, while adhesion is the attraction between water molecules and other substances.

• Cohesion: Responsible for surface tension and the formation of droplets.

• Adhesion: Helps water climb up plant vessels (capillary action).

• Example: Water transport in plants relies on both cohesion (holding water molecules together) and adhesion (sticking to cell walls).

Moderation of Temperature

Water stabilizes temperatures in organisms and environments due to its high specific heat and high heat of vaporization.

• Specific Heat: The amount of heat required to raise the temperature of 1 gram of water by 1°C is high, allowing water to buffer temperature changes.

• Evaporative Cooling: As water evaporates, it removes heat from surfaces, cooling organisms (e.g., sweating in humans).

• Example: Coastal regions experience milder climates due to the moderating effect of large bodies of water.

Expansion Upon Freezing

When water freezes, its molecules form a crystalline lattice that spaces them farther apart than in liquid water.

• Ice Floats: Solid water (ice) is less dense than liquid water, so it floats. This insulates the water below and provides a habitat for certain organisms.

• Example: Arctic sea ice supports species such as ringed seals (Phoca hispida), which rely on the ice for hunting and breeding.

Water as a Solvent

Water's polarity allows it to dissolve many substances, making it the "universal solvent." This property is vital for biological reactions and transport of nutrients and wastes.

• Hydrophilic Substances: Ionic and polar compounds dissolve readily in water.

• Hydrophobic Substances: Nonpolar molecules do not dissolve well in water.

• Example: Table salt (NaCl) dissolves in water as the positive and negative ions are surrounded by water molecules.

Acids, Bases, and the pH Scale

Dissociation of Water

Water molecules can dissociate into hydronium ions (H3O+) and hydroxide ions (OH-):

• Equation:

$2H_2O \rightleftharpoons H_3O^+ + OH^-$

• Concentration: In pure water at 25°C, the concentrations of H+ and OH- are both $1 \times 10^{-7}$ M.

The pH Scale

The pH scale measures the concentration of hydrogen ions in a solution. It is defined as:

• Formula:

$pH = -\log [H^+]$

• Neutral Solution: $[H^+] = 1 \times 10^{-7}$ M, so $pH = 7$.

• Acidic Solution: $pH < 7$ (higher concentration of H+).

• Basic Solution: $pH > 7$ (lower concentration of H+).

Table: pH Values of Common Substances

Substance	pH Value
Battery acid	1
Lemon juice	2
Vinegar	3
Pure water	7
Human blood	7.4
Household ammonia	11
Bleach	13

Water and Climate

Impact of Water on Climate and Life

Water plays a crucial role in regulating Earth's climate and supporting life. Its high heat capacity moderates temperature fluctuations, and its solid form (ice) provides habitats and influences global climate patterns.

• Example: Melting Arctic sea ice affects species such as polar bears and ringed seals, and has broader implications for global climate change.

Summary

Water's unique structure and properties are fundamental to life on Earth. Its ability to form hydrogen bonds leads to cohesion, temperature moderation, expansion upon freezing, and versatility as a solvent. Understanding these properties is essential for studying biological systems and the impact of environmental changes on life.