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

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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 and the maintenance of life on Earth. This chapter explores the molecular structure of water, its emergent properties, and its role in supporting life.

Structure and Polarity of Water

Polar Covalent Bonds and Hydrogen Bonding

The water molecule (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom. The unequal sharing of electrons creates a polar molecule, with distinct partial charges at each end.

Polar Molecule: Oxygen is more electronegative than hydrogen, resulting in a partial negative charge (δ−) near the oxygen and partial positive charges (δ+) near the hydrogens.
Hydrogen Bonds: Weak attractions form between the partial positive hydrogen of one water molecule and the partial negative oxygen of another. These are called hydrogen bonds.
Example: Hydrogen bonding is responsible for many of water’s unique properties, such as cohesion and high specific heat.

Emergent Properties of Water

1. Cohesive and Adhesive Behavior

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

Cohesion: Hydrogen bonds hold water molecules together, resulting in high surface tension.
Adhesion: Water molecules can also adhere to other surfaces, such as plant cell walls, aiding in water transport.
Example: Cohesion and adhesion enable water to move upward against gravity in plants (capillary action).

2. Ability to Moderate Temperature

Water can absorb or release large amounts of heat with only slight changes in its own temperature, helping to stabilize environmental and organismal temperatures.

Specific Heat: The amount of heat required to raise the temperature of 1 g of water by 1°C is 1 cal/(g·°C).
High Specific Heat: Water resists temperature changes due to hydrogen bonding. Heat is absorbed when bonds break and released when bonds form.
Evaporative Cooling: As water evaporates, the surface cools because the molecules with the highest kinetic energy leave as gas.
Example: Large bodies of water moderate coastal climates; sweating cools the body.

3. Expansion Upon Freezing

Unlike most substances, water becomes less dense as it freezes, allowing ice to float on liquid water.

Crystalline Lattice: At 0°C, hydrogen bonds lock water molecules into a lattice, spacing them farther apart than in liquid water.
Density: Ice is about 10% less dense than liquid water.
Biological Importance: Floating ice insulates water below, protecting aquatic life in cold climates.
Example: Polar habitats depend on ice cover for survival of species such as seals and polar bears.

4. Versatility as a Solvent

Water’s polarity makes it an excellent solvent, capable of dissolving a wide variety of substances.

Solution: A homogeneous mixture of substances.
Solvent: The dissolving agent (water in aqueous solutions).
Solute: The substance dissolved.
Hydration Shell: Water molecules surround ions or polar molecules, separating and shielding them.
Hydrophilic vs. Hydrophobic: Hydrophilic substances have an affinity for water; hydrophobic substances do not and are typically nonpolar (e.g., oils).
Example: Water dissolves salts, sugars, and proteins with polar or ionic regions.

Acidic and Basic Conditions

Dissociation of Water and pH

Water molecules can dissociate into hydrogen ions (H+) and hydroxide ions (OH−), affecting the pH of solutions.

Dynamic Equilibrium: Water molecules dissociate and reform at equal rates.
Acids: Substances that increase H+ concentration.
Bases: Substances that reduce H+ concentration.
pH Scale: Measures H+ concentration; defined as .
Neutral Solution: M, so .
Acidic: pH < 7; Basic: pH > 7.
Buffers: Substances that minimize changes in pH by accepting or donating H+.
Example: Blood contains buffers to maintain pH near 7.4.

Environmental Impact: Ocean Acidification

Human Activities and Water Quality

Burning fossil fuels releases CO2, which dissolves in oceans, forming carbonic acid and lowering pH (ocean acidification).

Carbonate Availability: Acidification reduces carbonate ions needed for marine organisms to build shells and skeletons.
Ecological Consequences: Threatens coral reefs and other marine life.
Example: Coral bleaching and loss of biodiversity in affected areas.

Quantitative Aspects of Solutions

Molecular Mass, Moles, and Molarity

Understanding solution concentration is essential for biological experiments and processes.

Molecular Mass: Sum of atomic masses in a molecule.
Mole: 1 mole = molecules (Avogadro’s number).
Molarity (M): Number of moles of solute per liter of solution.
Example: Preparing a 1 M NaCl solution involves dissolving 1 mole of NaCl in 1 liter of water.

Table: Comparison of Water’s States

State	Molecular Arrangement	Density	Biological Impact
Liquid	Molecules slip close together	High	Supports aquatic life, enables transport
Solid (Ice)	Molecules locked in lattice, farther apart	Lower (about 10% less than liquid)	Floats, insulates water below, protects organisms

Summary

Water’s molecular structure and emergent properties are fundamental to life. Its ability to moderate temperature, dissolve substances, and support biological processes underlies the diversity and resilience of life on Earth. Understanding these properties is essential for studying biology and addressing environmental challenges.