Water and Life: Properties, Functions, and Biological Importance

Chapter 3: 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, from cellular metabolism to ecosystem regulation. This chapter explores the structure of water, its emergent properties, and its critical roles in living systems.

Structure and Polarity of Water

Molecular Structure of Water

• Water (H2O) is a polar molecule, consisting of two hydrogen atoms covalently bonded to one oxygen atom.

• The oxygen atom is more electronegative, creating a partial negative charge near the oxygen and partial positive charges near the hydrogens.

• This polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.

Example: The bent shape of the water molecule (about 104.5° angle) contributes to its polarity.

Emergent Properties of Water

Cohesion and Adhesion

• Cohesion: The attraction between water molecules due to hydrogen bonding. This property allows water to form droplets and contributes to surface tension.

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

• Surface Tension: A measure of how difficult it is to stretch or break the surface of a liquid. Water has a high surface tension due to cohesive forces.

Example: Water striders can walk on water due to surface tension.

Transport in Plants: Cohesion-Tension Mechanism

• Cohesion helps the transport of water against gravity in plants.

• Adhesion between water and plant cell walls aids in water movement.

• Transpiration (evaporation of water from leaves) pulls water upward from roots through xylem vessels.

Example: Water moves from roots to leaves in tall trees via the cohesion-tension mechanism.

Moderation of Temperature by Water

• Water absorbs heat from warmer air and releases stored heat to cooler air, moderating temperature fluctuations.

• Specific Heat: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C. Water has a high specific heat (1 cal/g/°C), which helps stabilize temperatures in organisms and environments.

• Heat of Vaporization: The quantity of heat a liquid must absorb for 1 gram to be converted to gas. Water's high heat of vaporization helps moderate Earth's climate.

Example: Coastal areas have milder climates due to the high specific heat of water.

Evaporative Cooling

• As water evaporates, the surface of the remaining liquid cools down. This is because the molecules with the highest kinetic energy leave as gas.

• Evaporative cooling helps organisms regulate body temperature (e.g., sweating, transpiration in plants).

Example: Dogs pant to cool themselves through evaporative cooling.

Expansion Upon Freezing

• Water is less dense as a solid than as a liquid because hydrogen bonds stabilize and keep water molecules further apart in ice.

• This property allows ice to float on liquid water, insulating aquatic life in winter.

Example: Lakes do not freeze solid in winter, allowing fish to survive beneath the ice.

Water as a Solvent

Solvent of Life

• Water is a versatile solvent due to its polarity, which allows it to dissolve many ionic and polar substances.

• Solution: A liquid that is a homogeneous mixture of two or more substances.

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

• Solute: The substance that is dissolved.

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

Hydrophilic and Hydrophobic Substances

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

• Hydrophobic: Substances that do not have an affinity for water (e.g., oils, fats).

Example: Cell membranes are composed of hydrophobic lipid bilayers that separate aqueous environments.

Acids, Bases, and pH

Dissociation of Water Molecules

• Occasionally, a hydrogen atom in a water molecule shifts to another water molecule, forming a hydronium ion (H3O+) and a hydroxide ion (OH-).

• This process is called dissociation and is reversible.

Acids and Bases

• Acid: A substance that increases the hydrogen ion concentration of a solution.

• Base: A substance that reduces the hydrogen ion concentration, often by accepting H+ or releasing OH-.

pH Scale

• The pH scale measures the concentration of hydrogen ions in a solution.

• pH is defined as: $\text{pH} = -\log_{10}[H^+]$

• Each unit change in pH represents a tenfold change in [H+].

• Pure water has a pH of 7 (neutral).

Buffers

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

• They help maintain stable pH in biological systems.

Example: Blood contains bicarbonate buffer to maintain pH near 7.4.

Acidification: A Threat to the Oceans

Ocean Acidification

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

• This process lowers ocean pH, a phenomenon known as ocean acidification.

• Acidification reduces carbonate ion concentration, affecting marine organisms that build shells and skeletons from calcium carbonate (CaCO3).

Example: Coral reefs are threatened by ocean acidification, which impairs their ability to form skeletons.

Summary Table: Properties of Water

Property	Description	Biological Importance
Cohesion	Hydrogen bonds hold water molecules together	Helps transport water in plants
Adhesion	Attraction between water and other substances	Assists movement of water in plant vessels
High Specific Heat	Water resists temperature change	Stabilizes climate and organismal temperature
High Heat of Vaporization	Requires much energy to evaporate	Evaporative cooling, climate moderation
Expansion Upon Freezing	Ice is less dense than liquid water	Insulates aquatic life in winter
Versatile Solvent	Dissolves many substances	Facilitates chemical reactions in cells

Possible Existence of Life on Other Planets

• Scientists search for water as a key indicator of potential life on other planets.

• Evidence of water has been found on Mars and some moons in the solar system.

Example: The presence of water ice on Mars suggests the possibility of past or present life.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology textbooks.