Chapter 3: Water and Life – Properties, Functions, and Biological Importance

Water and Life

Introduction

Water is essential for all known forms of life. Its unique chemical and physical properties make it indispensable for biological processes, supporting life at molecular, cellular, and ecological levels.

Properties of Water Molecule

Polarity and Hydrogen Bonding

• Polarity: Water (H2O) is a polar molecule due to the difference in electronegativity between oxygen (3.5) and hydrogen (2.1), resulting in a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

• Hydrogen Bonds: Water molecules readily form hydrogen bonds, which are weak compared to covalent bonds but collectively strong, leading to significant cohesion among water molecules.

Example: The polarity of water allows it to dissolve many substances, making it a universal solvent.

Emergent Properties of Water

Four Key Properties Facilitating Life

• Cohesive Behavior: Water molecules stick together due to hydrogen bonding, aiding in the transport of water against gravity in plants (cohesion and adhesion).

• 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.

• Expansion Upon Freezing: Ice is less dense than liquid water because hydrogen bonds in ice are more ordered, causing it to float.

• Versatility as a Solvent: Water dissolves a wide variety of substances due to its polarity.

Cohesion, Adhesion, and Surface Tension

Cohesion and Adhesion

• Cohesion: The attraction between water molecules, which helps transport water in plants.

• Adhesion: The attraction between water molecules and other substances, such as plant cell walls.

Surface Tension

• Surface Tension: A measure of how hard it is to break the surface of a liquid. Water has high surface tension due to hydrogen bonding at the air-water interface.

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

Temperature, Heat, and Specific Heat

Definitions and Units

• Specific Heat: The amount of heat required to raise or lower 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.

• Kilocalorie (kcal): 1 kcal = 1,000 cal; used in food energy measurements.

• Joule (J): 1 J = 0.239 cal; 1 cal = 4.184 J.

Moderation of Temperature by Water

• Water's high specific heat is due to hydrogen bonding: heat is absorbed when bonds break and released when bonds form.

• This property minimizes temperature fluctuations, creating stable environments for life.

Equation: $q = m imes c imes riangle T$ where $q$ = heat absorbed or released, $m$ = mass, $c$ = specific heat, $riangle T$ = temperature change.

Expansion Upon Freezing

• Ice floats on liquid water because hydrogen bonds in ice are more ordered, making ice less dense than liquid water.

• This property insulates bodies of water, protecting aquatic life in cold climates.

Water: 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 in the solvent.

• Aqueous Solution: A solution where water is the solvent.

Water as a Versatile Solvent

• Water dissolves ionic compounds by surrounding each ion with a hydration shell.

• It can also dissolve polar molecules and even large molecules like proteins if they have ionic or polar regions.

Example: Table salt (NaCl) dissolves in water as Na+ and Cl- ions become surrounded by water molecules.

Acidic and Basic Conditions

Dissociation of Water

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

• The concentration of these ions determines the pH of a solution.

Equation: $2H_2O ightleftharpoons H_3O^+ + OH^-$

Acids and Bases

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

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

• Strong acids and bases: Dissociate completely in water.

• Weak acids and bases: Reversibly release and accept hydrogen ions, affecting the balance of H+ and OH-.

pH Scale

• The pH scale measures the concentration of H+ ions in a solution, ranging from 0 (most acidic) to 14 (most basic).

• Pure water has a neutral pH of 7, where [H+] = [OH-].

Equation: $pH = -\log_{10}[H^+]$

Buffers

Role and Importance

• Buffers: Substances that minimize changes in concentrations of H+ and OH- in a solution.

• Most buffers consist of a weak acid and its corresponding base, which combine reversibly with H+ ions.

• Buffers are crucial for maintaining stable pH in biological systems, such as blood (normal pH = 7.4).

Example: If blood pH drops below 7.35, it can cause illness; below 7.0, it is life-threatening.

Summary Table: Key Properties and Functions of Water

Property	Biological Importance
Cohesion & Adhesion	Transport of water in plants, surface tension
High Specific Heat	Stabilizes temperature in organisms and environments
Expansion Upon Freezing	Ice floats, insulating aquatic habitats
Versatile Solvent	Dissolves a wide range of substances, facilitating biochemical reactions
Buffering Capacity	Maintains stable pH in cells and body fluids

Additional info: The notes above integrate and expand upon the provided slides and text, ensuring a comprehensive, self-contained summary suitable for college-level General Biology students.