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

Water and Life

Introduction

Water is essential for all known forms of life. Its unique chemical and physical properties arise from its molecular structure and hydrogen bonding, making it a critical component in biological systems. This chapter explores the structure of water, its interactions, and its roles in living organisms.

Properties of Water

Orbital Hybridization in Oxygen

• Oxygen atom in water undergoes sp3 hybridization, resulting in a tetrahedral arrangement of orbitals.

• Oxygen has a valence of 2, allowing it to form two covalent bonds with hydrogen atoms, creating the H2O molecule.

• This geometry leads to a bent molecular shape and a polar molecule.

Polarity and Hydrogen Bonding

• Polarity: Water is a polar molecule due to the difference in electronegativity between oxygen and hydrogen, resulting in partial positive (δ+) and negative (δ-) charges.

• Hydrogen bonds: The polarity allows water molecules to form hydrogen bonds with each other, accounting for many of water’s unique properties.

• Each water molecule can form up to four hydrogen bonds.

• Example: Hydrogen bonds are responsible for water’s high boiling point and surface tension.

Hydrophilic and Hydrophobic Substances

Hydrophilic Substances

• Definition: Substances that have an affinity for water ("water-loving").

• Examples: Ionic compounds (e.g., NaCl), polar molecules (e.g., glucose, proteins).

• Water molecules surround and separate ions or polar molecules, dissolving them.

• Application: Table salt (NaCl) dissolves in water as water molecules surround and pull apart the Na+ and Cl- ions.

Hydrophobic Substances

• Definition: Substances that do not have an affinity for water ("water-fearing").

• Examples: Nonpolar molecules (e.g., oils, fats).

• These substances do not dissolve in water and tend to aggregate due to hydrophobic interactions.

• Oils form droplets in water, stabilized by van der Waals interactions.

Cohesion, Adhesion, and Water Transport

Cohesion and Adhesion

• Cohesion: The attraction between water molecules due to hydrogen bonding.

• Adhesion: The attraction between water molecules and other polar substances (e.g., cellulose in plant cell walls).

• These properties enable water to move upward through plant vessels (xylem and phloem) against gravity.

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

Thermal Properties of Water

Thermal Energy and Temperature

• Thermal energy: The total kinetic energy of molecules in a substance.

• Temperature: The average kinetic energy of molecules.

• Water’s high specific heat means it can absorb or release large amounts of heat with only a slight change in its own temperature.

• Specific heat of water: 1 cal/g/°C

• Heat of vaporization: 580 cal/g at 25°C

• These properties make water an excellent temperature buffer in biological systems.

Evaporative Cooling

• When water evaporates, the molecules with the highest kinetic energy leave first, lowering the average kinetic energy (temperature) of the remaining liquid.

• This process helps organisms regulate temperature (e.g., sweating in humans).

Density and States of Water

Density of Ice vs. Liquid Water

• Unlike most substances, water is less dense as a solid (ice) than as a liquid.

• Hydrogen bonds in ice are stable and hold water molecules apart, creating an open lattice structure.

• This property allows ice to float, insulating aquatic life in cold environments.

Acids, Bases, and pH

Water Dissociation and pH

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

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

• In pure water, [H+] = [OH-] = $1 \times 10^{-7}$ M, which is neutral.

• pH: The negative logarithm of the hydrogen ion concentration:

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

• The pH scale is logarithmic; a difference of 1 pH unit represents a tenfold difference in [H+].

• Physiological pH is typically around 7.4.

The pH Scale

pH	Example
1	Battery acid
2	Gastric juice, lemon juice
7	Pure water, human blood
12	Household bleach
14	Oven cleaner

Buffers

Definition and Function

• Buffer: A solution that minimizes changes in pH when acids or bases are added.

• Consists of a weak acid and its conjugate base.

• Buffers maintain stable pH in biological fluids (e.g., blood, cytoplasm).

• Follow LeChatelier’s Principle: the system shifts to counteract changes in [H+].

Phosphate Buffer System

• Important in maintaining intracellular pH.

• Components: H3PO4 (phosphoric acid), H2PO4- (dihydrogen phosphate), HPO42- (monohydrogen phosphate), PO43- (phosphate ion).

• Acts by picking up or releasing H+ as needed to maintain pH near 7.4.

Bicarbonate Buffer System

• Maintains blood pH.

• Components: H2CO3 (carbonic acid), HCO3- (bicarbonate ion).

• Helps neutralize excess acids or bases in the bloodstream.

Summary Table: Key Properties of Water

Property	Biological Importance
Cohesion/Adhesion	Water transport in plants
High Specific Heat	Temperature regulation in organisms
High Heat of Vaporization	Evaporative cooling (sweating, transpiration)
Lower Density of Ice	Insulation of aquatic environments
Versatile Solvent	Dissolves nutrients and waste products

Additional info: These notes expand on the provided slides and handwritten content, adding definitions, examples, and context for clarity and completeness. All equations are provided in LaTeX format as required.