Water: The Molecule That Supports Life

Introduction

Water is the essential biological medium on Earth, making up 70–95% of most cells and surrounding all living organisms. Its abundance and unique properties are fundamental to the habitability of our planet and the survival of life.

The Structure and Polarity of Water

Polarity and Hydrogen Bonding

The water molecule (H2O) is polar, with a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms. This polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.

• Polarity: The unequal sharing of electrons between oxygen and hydrogen creates a dipole moment.

• Hydrogen Bond: A weak bond formed between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another.

• Example: The structure of ice and liquid water is maintained by extensive hydrogen bonding.

Emergent Properties of Water

Overview

Water exhibits four emergent properties that are critical for supporting life on Earth:

• Cohesive and Adhesive Behavior

• Ability to Moderate Temperature

• Expansion Upon Freezing

• Versatility as a Solvent

Cohesion and Adhesion

Cohesion refers to the attraction between water molecules due to hydrogen bonding, which helps water move against gravity in plants. Adhesion is the attraction between water molecules and other substances, such as plant cell walls.

• Cohesion: Responsible for surface tension, allowing small insects to walk on water.

• Adhesion: Helps water "climb" up plant vessels (capillary action).

• Example: Water droplets forming beads on a leaf due to cohesive forces.

Ability to Moderate Temperature

Water can absorb or release large amounts of heat with only a slight change in its own temperature, due to its high specific heat.

• Specific Heat: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

• Water's Specific Heat: 1 cal/g/°C, much higher than many other substances.

• Heat Absorption and Release: Heat is absorbed when hydrogen bonds break and released when they form.

• Example: Coastal areas have milder climates due to water's temperature-buffering effect.

Evaporative Cooling: As water evaporates, the surface cools, helping organisms regulate temperature (e.g., sweating in humans).

Expansion Upon Freezing

Unlike most substances, water expands when it freezes. Ice is less dense than liquid water because its hydrogen bonds are more "ordered," causing it to float.

• Density: Water is most dense at 4°C; ice floats on liquid water.

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

• Example: Aquatic life survives under the ice in winter.

Versatility as a Solvent

Water is known as the "universal solvent" because its polarity allows it to dissolve many substances, especially ionic and polar compounds.

• 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: Sphere of water molecules surrounding each dissolved ion.

• Example: Table salt (NaCl) dissolving in water.

Hydrophilic and Hydrophobic Substances

Definitions and Examples

• Hydrophilic: Substances with an affinity for water (e.g., salts, sugars, proteins with polar regions).

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

• Example: Cell membranes are composed of hydrophobic lipid bilayers.

Acids, Bases, and pH

Acid-Base Chemistry in Aqueous Solutions

Water can dissociate into hydronium ions (H3O+, often written as H+) and hydroxide ions (OH-).

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

• Base: Substance that reduces the hydrogen ion concentration.

The product of the concentrations of H+ and OH- in water at 25°C is constant:

The pH of a solution is defined as:

• Neutral Solution: , so

• Acidic Solution: pH < 7

• Basic Solution: pH > 7

• Most biological fluids: pH 6–8

Buffers

Buffers are substances that minimize changes in concentrations of H+ and OH- in a solution. Most buffers consist of an acid-base pair that reversibly combines with hydrogen ions.

• Example: The bicarbonate buffer system in human blood helps maintain pH near 7.4.

Environmental Impact: Acid Precipitation and Water Quality

Acid Precipitation

Acid precipitation (rain, snow, or fog with pH < 5.6) is caused by pollutants reacting with water in the atmosphere. It can damage aquatic life, soil chemistry, and forests.

• Human Impact: Burning fossil fuels releases CO2 and other pollutants, contributing to acid rain and ocean acidification.

• Greenhouse Effect: Increased CO2 leads to global warming and affects the ability of corals to form reefs.

Summary Table: Properties of Water and Their Biological Importance

Key Terms and Definitions

• Solution: Homogeneous mixture of two or more substances

• Solvent: The dissolving agent of a solution

• Solute: The substance that is dissolved

• Hydrophilic: Water-attracting (polar or charged) substance

• Hydrophobic: Water-repelling (nonpolar) substance

• Acid: Increases H+ concentration

• Base: Decreases H+ concentration

• pH: Measure of hydrogen ion concentration;

• Buffer: Substance that minimizes pH changes

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