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

Water and Life

Introduction

Water is fundamental to all known forms of life. Its unique chemical and physical properties make it the biological medium on Earth, supporting the processes necessary for life. This chapter explores the structure of water, its emergent properties, and its critical roles in biological systems.

The Molecule That Supports All of Life

Importance of Water

• Essential for Life: All living organisms require water more than any other substance.

• Universal Presence: Water is the only common substance that exists naturally on Earth in all three physical states: solid, liquid, and gas.

• Biological Medium: Most biochemical reactions occur in aqueous environments.

Structure of the Water Molecule

• Polar Covalent Bonds: In a water molecule (H2O), the oxygen atom is more electronegative than hydrogen, causing electrons to spend more time near the oxygen. This results in a partial negative charge (δ-) on oxygen and partial positive charges (δ+) on hydrogens.

• Polarity: Water is a polar molecule because its overall charge is unevenly distributed.

• Hydrogen Bonding: The polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.

Example: The interaction between two water molecules involves a hydrogen bond between the hydrogen atom of one molecule and the oxygen atom of another.

Emergent Properties of Water

Overview

Water exhibits several emergent properties that make it suitable for supporting life. These properties arise from the structure and interactions of water molecules.

• Cohesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion and Adhesion

• Cohesion: Hydrogen bonds hold water molecules together, contributing to the transport of water against gravity in plants.

• Adhesion: The attraction between water molecules and other substances (e.g., plant cell walls) helps counteract gravity.

• Surface Tension: Water has a high surface tension due to hydrogen bonding, making it difficult to break the surface of a liquid.

Example: The ability of some insects, such as the Asian House Gecko, to walk on water is due to water's high surface tension.

Moderation of Temperature

• High Specific Heat: Water can absorb or release a large amount of heat with only a slight change in its own temperature.

• Definition: Specific heat is the amount of heat that must be absorbed or lost for 1 gram of a substance to change its temperature by 1°C.

• Value for Water: 1 calorie per gram per degree Celsius ($1\ \text{cal}/(\text{g} \cdot ^\circ\text{C})$).

• Biological Significance: High specific heat helps organisms maintain stable internal temperatures and stabilizes climate in large bodies of water.

Example: Coastal cities experience milder temperatures due to the high specific heat of the ocean.

Heat of Vaporization and Evaporative Cooling

• Heat of Vaporization: The amount of heat a liquid must absorb for 1 gram to be converted to gas.

• Water's Value: Water has a high heat of vaporization due to strong hydrogen bonds.

• Evaporative Cooling: As water evaporates, the surface cools, helping organisms regulate temperature.

Example: Sweating in humans cools the body as water evaporates from the skin.

Expansion Upon Freezing

• Ice Floats: Hydrogen bonds in ice are more ordered, making ice less dense than liquid water.

• Biological Importance: If ice sank, bodies of water would freeze solid, making life impossible in aquatic environments.

Water: The Solvent of Life

Solutions, Solvents, and Solutes

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

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

• Solute: The substance that is dissolved.

• Aqueous Solution: A solution in which water is the solvent.

Water as a Versatile Solvent

• Polarity: Water's polarity allows it to dissolve ionic compounds and other polar molecules.

• Hydration Shell: When an ionic compound dissolves, each ion is surrounded by a sphere of water molecules.

• Large Molecules: Water can dissolve large polar molecules, such as proteins, if they have ionic or polar regions.

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Substances with an affinity for water ("water-loving").

• Hydrophobic: Substances that do not have an affinity for water ("water-fearing"), often due to nonpolar bonds (e.g., oils).

Acidic and Basic Conditions Affect Living Organisms

Dissociation of Water Molecules

• Ionization: A hydrogen atom in a hydrogen bond between two water molecules can shift, leaving its electron behind and becoming a hydrogen ion ($\text{H}^+$).

• Hydronium Ion: The molecule with the extra proton is a hydronium ion ($\text{H}_3\text{O}^+$), often represented as $\text{H}^+$.

• Hydroxide Ion: The molecule that lost the proton is a hydroxide ion ($\text{OH}^-$).

• Dynamic Equilibrium: Water molecules dissociate and reform at equal rates.

Acids, Bases, and the pH Scale

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

• Base: A substance that reduces the hydrogen ion concentration.

• pH Scale: Measures the concentration of $\text{H}^+$ in a solution, ranging from 0 (most acidic) to 14 (most basic). Pure water has a pH of 7 (neutral).

pH Value	Example	Acidic/Basic
1	Battery acid	Strongly acidic
2	Gastric juice	Acidic
3	Vinegar, wine, cola	Acidic
7	Pure water	Neutral
12	Household ammonia	Basic
13	Household bleach	Strongly basic

Buffers

• Definition: Buffers are substances that minimize changes in concentrations of $\text{H}^+$ and $\text{OH}^-$ in a solution.

• Mechanism: Most buffers consist of an acid-base pair that reversibly combines with $\text{H}^+$.

• Biological Importance: Buffers help maintain a stable pH in biological fluids, which is critical for proper cellular function.

Homeostasis and Water

Definition and Relevance

• Homeostasis: The tendency toward a relatively stable equilibrium between interdependent elements, maintained by physiological processes.

• Biological Context: In biology, homeostasis refers to the steady internal, physical, and chemical conditions maintained by living systems.

• Variables: Includes temperature, fluid balance, and pH, all of which are regulated within certain limits.

• Role of Water: Water's properties are essential for maintaining homeostasis in organisms.

Additional info: Homeostatic mechanisms involve feedback systems that detect and respond to changes in the internal environment, often using water as a medium for transport and chemical reactions.