Water and Life

Introduction

Water is fundamental to all known forms of life. Its unique chemical and physical properties make it essential for biological processes and the habitability of Earth. This chapter explores the molecular structure of water, its emergent properties, and the impact of acids and bases on living organisms.

The Molecular Structure of Water

Polarity and Hydrogen Bonding

Water molecules are composed of two hydrogen atoms covalently bonded to one oxygen atom. The oxygen atom is highly electronegative, resulting in an uneven distribution of charge and making water a polar molecule.

• Polarity: The oxygen atom has a partial negative charge (δ−), while the hydrogen atoms have partial positive charges (δ+).

• Hydrogen Bonding: The polarity of water allows each molecule to form hydrogen bonds with up to four neighboring water molecules. These are weak bonds individually, but collectively they give water its unique properties.

Example: In a glass of water, each molecule is constantly forming and breaking hydrogen bonds with its neighbors.

Emergent Properties of Water

Overview

Water exhibits several emergent properties that are critical for life on Earth. These properties arise from the collective behavior of water molecules and their ability to form hydrogen bonds.

• Cohesive Behavior: Water molecules stick together due to hydrogen bonding, resulting in high surface tension and the ability to transport water against gravity in plants.

• Ability to Moderate Temperature: Water can absorb or release large amounts of heat with only a slight change in its own temperature, helping to stabilize environmental and biological temperatures.

• Expansion Upon Freezing: Ice is less dense than liquid water because hydrogen bonds form a crystalline structure that spaces molecules further apart. This allows ice to float, insulating aquatic life in winter.

• Versatility as a Solvent: Water's polarity enables it to dissolve a wide variety of substances, making it the "universal solvent" and facilitating chemical reactions in cells.

Cohesion and Adhesion

• Cohesion: The attraction between water molecules due to hydrogen bonding. This property is responsible for phenomena such as surface tension.

• Adhesion: The attraction between water molecules and other substances, aiding processes like capillary action in plants.

• Example: Water moves upward through plant vessels against gravity due to cohesion and adhesion.

Temperature Moderation

• High Specific Heat: Water can absorb a lot of heat before its temperature rises, due to the energy required to break hydrogen bonds.

• Evaporative Cooling: As water evaporates, it removes heat from surfaces, helping organisms regulate temperature.

• Example: Coastal regions have milder climates due to the heat-buffering effect of large bodies of water.

Expansion Upon Freezing

• Ice Formation: At 0°C, water molecules form a crystalline lattice, making ice less dense than liquid water.

• Biological Importance: Floating ice insulates water below, protecting aquatic life during cold periods.

Water as a Solvent

• Solution: A homogeneous mixture of two or more substances.

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

• Solute: The substance that is dissolved.

• Hydration Shell: When ionic compounds dissolve, water molecules surround each ion, preventing them from re-associating.

• Versatility: Water dissolves ionic and polar molecules, and even large molecules like proteins if they have polar or ionic regions.

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

Acids, Bases, and pH

Acidic and Basic Conditions

The concentration of hydrogen ions (H+) and hydroxide ions (OH−) in a solution determines its acidity or basicity, which can significantly affect biological processes.

• Acid: A substance that increases the H+ concentration of a solution.

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

• Strong Acids/Bases: Dissociate completely in water.

• Weak Acids/Bases: Reversibly release and accept hydrogen ions.

The pH Scale

• Definition: pH is the negative logarithm of the hydrogen ion concentration.

• In pure water at 25°C:

• Neutral solution: , so pH = 7

• Acidic solutions: pH < 7; Basic solutions: pH > 7

• Most biological fluids have pH values between 6 and 8.

Buffers

• Definition: Buffers are substances that minimize changes in pH by accepting or donating H+ ions.

• Composition: Typically consist of a weak acid and its corresponding base.

• Example: The bicarbonate buffer system in blood:

Water Quality and Environmental Impact

Ocean Acidification

• Cause: Increased atmospheric CO2 from human activities dissolves in oceans, forming carbonic acid.

• Process: Carbonic acid dissociates, increasing H+ concentration and lowering pH (ocean acidification).

• Impact: Excess H+ combines with carbonate ions, reducing availability for marine organisms that build shells from calcium carbonate.

• Biological Consequence: Threatens marine life and disrupts oceanic chemical balances.

Summary Table: Properties of Water and Their Biological Importance