Water and Life: Properties, Structure, 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 Earth habitable and support biological processes. This chapter explores the molecular structure of water, its emergent properties, and its critical role in biological systems.

Polar Covalent Bonds and Hydrogen Bonding in Water

Structure of the Water Molecule

The water molecule (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom. The electrons in these bonds are shared unequally, making water a polar molecule with 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, which are weak attractions between the partially positive hydrogen of one molecule and the partially negative oxygen of another.

Emergent Properties of Water

Overview of Water's Properties

Four key properties of water contribute to Earth's suitability for life:

• Cohesive 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, resulting in high surface tension. Adhesion is the attraction between water molecules and other substances, such as plant cell walls, which helps counteract gravity and enables water transport in plants.

Moderation of Temperature

Water moderates temperature by absorbing heat from warmer air and releasing heat to cooler air. It can absorb or release large amounts of heat with only slight changes in its own temperature due to its high specific heat. This property is a result of hydrogen bonding: heat is absorbed to break bonds and released when bonds form. Water's high specific heat stabilizes temperatures in organisms and environments, especially in coastal regions.

Evaporative Cooling

Evaporation is the transformation of a substance from liquid to gas. As water evaporates, the surface cools, a process called evaporative cooling. This helps regulate temperature in organisms and bodies of water. The heat of vaporization is the amount of heat required for 1 g of liquid to become gas.

Expansion Upon Freezing

Water is less dense as a solid than as a liquid because, at 0°C, water molecules form a crystalline lattice held by hydrogen bonds, keeping them farther apart. This makes ice float on liquid water, insulating aquatic life in cold climates. If ice sank, bodies of water would freeze solid, making life impossible.

Water: The Solvent of Life

Water's polarity makes it an excellent solvent. In an aqueous solution, water dissolves ionic compounds by surrounding each ion with a hydration shell. Water can also dissolve polar molecules and even large molecules like proteins if they have ionic or polar regions.

Hydrophilic and Hydrophobic Substances

Hydrophilic substances have an affinity for water, while hydrophobic substances do not. Oils are hydrophobic due to nonpolar bonds and are major components of cell membranes.

Solute Concentration in Aqueous Solutions

Chemical reactions in organisms often occur in aqueous solutions. Molecular mass is the sum of the masses of all atoms in a molecule. The mole is a unit for counting molecules, with Avogadro’s number ($6.02 \times 10^{23}$ molecules/mole). Molarity (M) is the number of moles of solute per liter of solution.

Water and the Search for Life

Biologists search for life on other planets by looking for water. Evidence of water has been found on Mars and in the atmospheres of some exoplanets.

Acidic and Basic Conditions Affect Living Organisms

Dissociation of Water Molecules

Water molecules can dissociate into hydronium ions (H3O+) and hydroxide ions (OH-). This process is rare but significant for biological systems. The concentrations of these ions are equal in pure water.

Acids, Bases, and the pH Scale

Acids increase the concentration of H+ in a solution, while bases reduce it. The pH scale measures the acidity or basicity of a solution, defined as $pH = -\log_{10}[H^+]$. At 25°C, $[H^+][OH^-] = 10^{-14}$ M2. Acidic solutions have pH < 7, basic solutions have pH > 7, and neutral solutions have pH = 7.

Buffers

Buffers are substances that minimize changes in H+ and OH- concentrations. Most buffers consist of a weak acid and its corresponding base, which combine reversibly with hydrogen ions. Buffers are crucial for maintaining stable pH in biological systems.

Acidification: A Threat to Our Oceans

Ocean Acidification

Human activities, such as burning fossil fuels, increase atmospheric CO2, about 25% of which is absorbed by oceans. Dissolved CO2 forms carbonic acid, lowering ocean pH in a process called ocean acidification. This reduces carbonate ion concentration, which is necessary for marine organisms to produce calcium carbonate for shells and skeletons.

Environmental Impact and Future Outlook

Ocean acidification threatens marine biodiversity, especially organisms that rely on calcification. However, increased scientific understanding and informed action can help protect water resources and maintain ecological balance.