Water and Life: Properties, Chemistry, 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 and the maintenance of life on Earth. This chapter explores the molecular structure of water, its emergent properties, and its role in biological systems.

Properties of Water

Polar Covalent Bonds and Hydrogen Bonding

The structure of water molecules and their ability to form hydrogen bonds are fundamental to water's distinctive properties.

• Polar covalent bonds: In a water molecule (H2O), electrons are shared unequally between oxygen and hydrogen, making oxygen slightly negative and hydrogen slightly positive.

• Electronegativity: Oxygen is more electronegative than hydrogen, attracting electrons closer to itself.

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

• Hydrogen bonds: The polarity allows water molecules to form hydrogen bonds with each other, leading to many of water's unique properties.

• Example: Hydrogen bonds are responsible for water's high surface tension and its ability to moderate temperature.

Four Emergent Properties of Water

Water exhibits four key properties that contribute to Earth's suitability for life:

• Cohesive behavior: Water molecules stick together due to hydrogen bonding, resulting in high surface tension.

• Ability to moderate temperature: Water absorbs and releases heat with minimal temperature change, due to its high specific heat.

• Expansion upon freezing: Ice is less dense than liquid water, allowing it to float and insulate aquatic environments.

• Versatility as a solvent: Water dissolves many substances, making it the solvent of life.

Cohesion and Adhesion

Cohesion of Water Molecules

Cohesion refers to the attraction between water molecules, which is critical for processes such as water transport in plants.

• Cohesion: Hydrogen bonds hold water molecules together.

• Surface tension: Cohesion results in high surface tension, making it difficult to break the surface of water.

Adhesion

Adhesion is the attraction between water molecules and other substances.

• Adhesion: Water molecules adhere to surfaces, such as plant cell walls, helping counteract gravity during water transport.

Temperature Moderation

Water's High Specific Heat

Water resists temperature changes due to its high specific heat, which is the amount of heat required to change the temperature of 1 gram of a substance by 1°C.

• Specific heat of water: $1 \text{ cal} / (\text{g} \cdot ^\circ\text{C})$

• Hydrogen bonding: Heat is absorbed when hydrogen bonds break and released when they form.

• Biological significance: Large bodies of water stabilize climate and temperature, supporting life.

Evaporative Cooling

Evaporation allows water to cool surfaces, stabilizing temperatures in organisms and environments.

• Heat of vaporization: The amount of heat required for 1 gram of liquid to become gas.

• Evaporative cooling: As water evaporates, the remaining surface cools.

• Example: Sweating in humans helps regulate body temperature.

Expansion Upon Freezing

Ice Floats on Liquid Water

Unlike most substances, water expands upon freezing, making ice less dense than liquid water.

• Crystalline lattice: At 0°C, water molecules form a rigid structure due to hydrogen bonding.

• Biological importance: Floating ice insulates water below, allowing aquatic life to survive in cold climates.

Water as a Solvent

The Solvent of Life

Water's polarity makes it an excellent solvent for many substances.

• Solution: A homogeneous mixture of substances.

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

• Solute: The substance dissolved.

• Aqueous solution: Water is the solvent.

• Hydration shell: Water molecules surround ions and polar molecules, facilitating dissolution.

Hydrophilic and Hydrophobic Substances

Water interacts differently with various substances based on their polarity.

• Hydrophilic: Substances that have an affinity for water (ionic or polar compounds).

• Hydrophobic: Substances that repel water (nonpolar compounds, such as oils).

• Example: Cell membranes contain hydrophobic molecules that help maintain structure.

Chemical Calculations in Aqueous Solutions

Molarity and Molecular Mass

Chemical reactions in biology often occur in aqueous solutions, requiring calculations of concentration and molecular mass.

• Mole: 1 mole = $6.02 \times 10^{23}$ molecules (Avogadro's number).

• Molecular mass: Sum of atomic masses in a molecule (g/mol).

• Molarity (M): $\text{Molarity} = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Example: To make a 1 M NaCl solution, dissolve 58 g of NaCl in 1 L of water.

Acidic and Basic Conditions

Acids, Bases, and pH

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

• Acid: Increases H+ concentration; pH < 7.

• Base: Reduces H+ concentration; pH > 7.

• pH scale: Measures acidity/basicity; $\text{pH} = -\log [\text{H}^+]$

• Neutral solution: [H+] = [OH-] = $1 \times 10^{-7}$ M; pH = 7.

pOH and Calculations

• Relationship: $\text{pH} + \text{pOH} = 14$

• Example: For a 0.0235 M HCl solution, $\text{pH} = -\log(0.0235) = 1.629$; $\text{pOH} = 14 - 1.629 = 12.371$

Buffers

Buffers help maintain stable pH in biological systems.

• Buffer: A solution containing a weak acid and its corresponding base, minimizing changes in H+ and OH- concentrations.

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

Ocean Acidification

Causes and Effects

Excessive carbon dioxide (CO2) emissions are absorbed by oceans, leading to acidification and threatening marine life.

• Process: CO2 dissolves in seawater, forming carbonic acid.

• Impact: Increased H+ ions combine with carbonate ions, reducing carbonate availability for organisms that need it for shells and skeletons.

• Environmental significance: Ocean acidification disrupts marine ecosystems and biodiversity.

Summary Table: Water's Properties and Biological Importance

Property	Description	Biological Importance
Cohesion	Water molecules stick together via hydrogen bonds	Enables transport in plants, high surface tension
Temperature Moderation	High specific heat and evaporative cooling	Stabilizes climate, regulates organism temperature
Expansion Upon Freezing	Ice is less dense than liquid water	Insulates aquatic environments, supports life
Versatility as a Solvent	Dissolves ionic and polar substances	Facilitates biochemical reactions

Key Equations

• $\text{Molarity} = \frac{\text{moles of solute}}{\text{liters of solution}}$

• $\text{pH} = -\log [\text{H}^+]$

• $\text{pOH} = 14 - \text{pH}$

• $[\text{H}^+][\text{OH}^-] = 1 \times 10^{-14}$

Conclusion

Understanding water's properties and its role in biological systems is fundamental to the study of biology. Water's unique characteristics support life, regulate environmental conditions, and drive essential chemical reactions.