Chapter 3: Water and Life

Introduction

Water is essential for all known forms of life. Its unique chemical and physical properties make Earth habitable and support biological processes. This chapter explores the molecular structure of water, its emergent properties, and its critical role in biological systems.

Polar Covalent Bonds and Hydrogen Bonding

Structure of Water Molecules

Water (H2O) is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen atoms. Oxygen is more electronegative, resulting in a partial negative charge (δ-) near the oxygen and partial positive charges (δ+) near the hydrogens. This polarity enables water molecules to form hydrogen bonds with each other.

• Polar covalent bond: A covalent bond in which electrons are shared unequally between atoms.

• Hydrogen bond: A weak attraction between the hydrogen atom of one water molecule and the oxygen atom of another.

Emergent Properties of Water

Overview of Water's Properties

Four key properties of water contribute to Earth's suitability for life:

• Cohesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion and Adhesion

Cohesion refers to the attraction between water molecules due to hydrogen bonding, resulting in high surface tension. Adhesion is the attraction between water molecules and other substances, such as plant cell walls, aiding in water transport against gravity in plants.

• Surface tension: The measure of how difficult it is to stretch or break the surface of a liquid.

• Example: Some insects can walk on water due to surface tension.

Moderation of Temperature

Water moderates temperature by absorbing heat from warmer air and releasing heat to cooler air. It can absorb or release large amounts of heat with only slight changes in its own temperature due to its high specific heat.

• Kinetic energy: The energy of motion.

• Thermal energy: The kinetic energy associated with the random movement of atoms or molecules.

• Specific heat: The amount of heat required to change the temperature of 1 g of a substance by 1°C.

• Calorie (cal): The amount of heat needed to raise 1 g of water by 1°C.

• Joule (J): SI unit of energy; .

Water's high specific heat is due to hydrogen bonding: heat is absorbed to break bonds and released when bonds form. This property stabilizes temperatures in organisms and environments.

Evaporative Cooling

Evaporation is the transformation of a substance from liquid to gas. Heat of vaporization is the heat required for 1 g of liquid to become gas. As water evaporates, the surface cools—a process called evaporative cooling—which helps regulate temperature in organisms and bodies of water.

Expansion Upon Freezing

Water is less dense as a solid (ice) than as a liquid. At 0°C, water molecules form a crystalline lattice, keeping them farther apart and making ice about 10% less dense than liquid water. This allows ice to float, insulating aquatic life below.

• Importance: If ice sank, bodies of water would freeze solid, making life impossible.

Effects of Climate Change on the Arctic

Global warming is reducing sea ice, threatening species that depend on ice platforms for survival, such as polar bears and seals.

Water: The Solvent of Life

A solution is a homogeneous mixture of substances. The solvent dissolves the solute. An aqueous solution uses water as the solvent. Water's polarity allows it to dissolve ionic compounds and polar molecules, forming hydration shells around ions.

• Hydrophilic: Substances with an affinity for water.

• Hydrophobic: Substances that repel water, such as oils.

Solute Concentration in Aqueous Solutions

Chemical reactions in organisms often occur in aqueous solutions. Molecular mass is the sum of the masses of all atoms in a molecule. The mole is a unit for counting molecules, with Avogadro's number ( molecules/mol). Molarity (M) is the number of moles of solute per liter of solution.

Possible Evolution of Life on Other Planets

Biologists search for extraterrestrial life by looking for water, as it is essential for life as we know it. Evidence of water has been found on Mars and in the atmospheres of some exoplanets.

Acidic and Basic Conditions Affect Living Organisms

Dissociation of Water

Water molecules can dissociate into hydronium ions (H3O+) and hydroxide ions (OH-). This process is rare but significant for biological systems.

• Acid: Increases the concentration of H+ in a solution.

• Base: Reduces the concentration of H+ in a solution.

The pH Scale

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

• Neutral solution: pH = 7

• Acidic solution: pH < 7

• Basic solution: pH > 7

Buffers

Buffers are substances that minimize changes in pH by accepting or donating H+ ions. Most buffers consist of a weak acid and its corresponding base. They are crucial for maintaining stable pH in biological systems.

Acidification: A Threat to Our Oceans

Human activities, such as burning fossil fuels, increase atmospheric CO2, which dissolves in oceans and forms carbonic acid. This process, called ocean acidification, reduces carbonate ion concentration, threatening organisms that rely on calcium carbonate for their skeletons and shells, such as corals.

Summary Table: Properties of Water