Chapter 3: Water and Life

Introduction

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

Concept 3.1: Polar Covalent Bonds and Hydrogen Bonding in Water

Structure and Polarity of Water

• Water Molecule Structure: A water molecule (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom.

• Polar Covalent Bonds: Electrons spend more time near the oxygen atom, making it partially negative (δ-) and the hydrogens partially positive (δ+).

• Polarity: Water is a polar molecule with an uneven charge distribution.

• Hydrogen Bonding: The polarity allows water molecules to form hydrogen bonds with each other, which are weak individually but strong collectively.

Example: Hydrogen bonds are responsible for many of water’s unique properties, such as high surface tension and cohesion.

Concept 3.2: Emergent Properties of Water

Four Key Properties

• Cohesive Behavior: Water molecules stick together due to hydrogen bonding, resulting in high surface tension.

• Ability to Moderate Temperature: Water can absorb or release large amounts of heat with only slight temperature changes due to its high specific heat.

• Expansion Upon Freezing: Water is less dense as a solid (ice) than as a liquid, allowing ice to float.

• Versatility as a Solvent: Water’s polarity enables it to dissolve many substances, making it the “universal solvent.”

Cohesion and Adhesion

• Cohesion: Hydrogen bonds hold water molecules together, aiding in the transport of water and nutrients in plants.

• Surface Tension: The measure of how difficult it is to break the surface of a liquid; water has a high surface tension.

• Adhesion: Attraction between water molecules and other substances (e.g., plant cell walls), helping counteract gravity in plants.

Moderation of Temperature

• Kinetic Energy: The energy of motion; thermal energy is the kinetic energy of molecules.

• Temperature: Represents the average kinetic energy of molecules.

• Heat: Thermal energy in transfer from one body to another.

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

• Water’s High Specific Heat:

• Hydrogen bonds absorb heat when breaking and release heat when forming, minimizing temperature fluctuations.

Example: Large bodies of water moderate coastal climates by absorbing heat during the day and releasing it at night.

Evaporative Cooling

• Evaporation: Transformation from liquid to gas.

• Heat of Vaporization: The heat required for 1 g of liquid to become gas.

• Evaporative Cooling: As water evaporates, the surface cools, stabilizing temperatures in organisms and environments.

Expansion Upon Freezing

• At 0°C, water molecules form a crystalline lattice, making ice less dense than liquid water.

• Ice floats, insulating bodies of water and protecting aquatic life in winter.

Concept 3.2: Water as the Solvent of Life

Solutions and Solubility

• Solution: A homogeneous mixture of substances.

• Solvent: The dissolving agent (water in aqueous solutions).

• Solute: The substance dissolved.

• Aqueous Solution: Water is the solvent.

• Hydration Shell: Sphere of water molecules surrounding dissolved ions.

• Water dissolves ionic and polar molecules, including large molecules like proteins if they have polar regions.

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Substances with an affinity for water (e.g., salts, sugars).

• Hydrophobic: Substances that repel water (e.g., oils, fats), often forming the basis of cell membranes.

Concept 3.3: Acidic and Basic Conditions Affect Living Organisms

Dissociation of Water

• Water molecules can dissociate into hydrogen ions (H+) and hydroxide ions (OH-).

• Hydronium Ion: H3O+ (often represented as H+).

• In pure water, [H+] = [OH-].

Acids, Bases, and the pH Scale

• Acid: Increases H+ concentration in solution.

• Base: Reduces H+ concentration, often by increasing OH-.

• Strong Acids/Bases: Dissociate completely in water.

• Weak Acids/Bases: Reversibly release and accept H+.

• pH Scale: Measures acidity/basicity;

• Acidic solutions: pH < 7; Basic solutions: pH > 7; Most biological fluids: pH 6–8.

Buffers

• Buffers minimize changes in [H+] and [OH-] in a solution.

• Usually consist of a weak acid and its corresponding base.

• Help maintain stable pH in biological systems (e.g., blood).

Acidification and Ocean Health

• Burning fossil fuels increases atmospheric CO2, some of which dissolves in oceans, forming carbonic acid (H2CO3).

• This process, called ocean acidification, threatens marine life by lowering ocean pH.