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

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Water and Life

Introduction

Water is essential for all known forms of life. Its unique chemical structure and 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 their significance for living organisms.

Structure of Water and Hydrogen Bonding

Polar Covalent Bonds in Water Molecules

Water (H2O) is a polar molecule, meaning it has regions of partial positive and negative charge due to unequal sharing of electrons in its covalent bonds.

Polar covalent bonds: The electrons spend more time near the oxygen atom than the hydrogen atoms, giving oxygen a partial negative charge (δ-) and hydrogens partial positive charges (δ+).
Polarity: This uneven distribution of charge makes water a polar molecule.
Hydrogen bonding: The polarity allows water molecules to form hydrogen bonds with each other, where the partially positive hydrogen of one molecule is attracted to the partially negative oxygen of another.

Example: In ice, water molecules are held in a rigid lattice by hydrogen bonds, while in liquid water, these bonds constantly break and reform, allowing molecules to move closer together.

Emergent Properties of Water

Overview of Four Key Properties

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

Cohesive behavior
Ability to moderate temperature
Expansion upon freezing
Versatility as a solvent

Cohesion and Adhesion

Cohesion and adhesion are two related properties that arise from hydrogen bonding.

Cohesion: The attraction between water molecules due to hydrogen bonding. This results in high surface tension, making it difficult to stretch or break the surface of water.
Adhesion: The attraction between water molecules and other substances, such as the walls of plant cells. Adhesion helps counteract gravity during water transport in plants.

Example: Water droplets form beads on a surface due to cohesion. In plants, cohesion and adhesion enable the upward movement of water from roots to leaves (capillary action).

Moderation of Temperature

Water moderates temperature by absorbing and releasing heat with only slight changes in its own temperature.

Kinetic energy: The energy of motion; in molecules, this is called thermal energy.
Heat: The transfer of thermal energy from one body to another.
Specific heat: The amount of heat required to raise the temperature of 1 g of a substance by 1°C. For water, this is 1 cal/(g·°C).
High specific heat: Water resists temperature changes due to hydrogen bonding. Heat is absorbed to break hydrogen bonds and released when they form.

Example: Large bodies of water stabilize climate by absorbing heat during the day and releasing it at night, moderating coastal temperatures.

Evaporative Cooling

Evaporation is the transformation of a substance from liquid to gas. As water evaporates, the surface cools, a process known as evaporative cooling.

Heat of vaporization: The amount of heat required to convert 1 g of liquid to gas. For water, this is high due to hydrogen bonding.
Biological significance: Evaporative cooling helps organisms regulate body temperature (e.g., sweating, transpiration in plants).

Expansion Upon Freezing

Water is less dense as a solid (ice) than as a liquid, which is unusual among substances.

Crystalline structure: At 0°C, water molecules form a lattice held by hydrogen bonds, keeping them farther apart than in liquid water.
Density: Ice is about 10% less dense than liquid water, so it floats.
Ecological importance: Floating ice insulates the water below, allowing aquatic life to survive in cold climates.

Example: If ice sank, lakes and oceans would freeze solid, making life impossible in many environments.

Water as the Solvent of Life

Water's polarity makes it an excellent solvent, capable of dissolving a wide variety of substances.

Solution: A homogeneous mixture of two or more substances.
Solvent: The dissolving agent (water in aqueous solutions).
Solute: The substance being dissolved.
Hydration shell: When ionic compounds dissolve, water molecules surround each ion, forming a hydration shell.
Nonionic solutes: Water can also dissolve polar molecules and large molecules (e.g., proteins) with ionic or polar regions.

Example: Table salt (NaCl) dissolves in water as Na+ and Cl- ions become surrounded by water molecules.

Hydrophilic and Hydrophobic Substances

Hydrophilic: Substances with an affinity for water (e.g., salts, sugars, proteins).
Hydrophobic: Substances that do not interact with water, usually nonpolar (e.g., oils, fats). Hydrophobic molecules are major components of cell membranes.

Solute Concentration in Aqueous Solutions

Calculating Concentrations

Molecular mass: The sum of the masses of all atoms in a molecule.
Mole: A unit representing 6.02 × 1023 molecules (Avogadro's number).
Molarity (M): The number of moles of solute per liter of solution.

Example: To make a 1 M solution of glucose, dissolve 1 mole of glucose in enough water to make 1 liter of solution.

Acidic and Basic Conditions

Acids, Bases, and the pH Scale

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

Acid: Increases the concentration of H+ in a solution.
Base: Reduces the concentration of H+ (either by accepting H+ or producing OH-).
Strong acids/bases: Dissociate completely in water.
Weak acids/bases: Reversibly release and accept H+.

pH Scale: Measures the concentration of H+ in a solution.

The product of [H+] and [OH-] in water at 25°C is constant:
pH is defined as:
Pure water has [H+] = M, so pH = 7 (neutral).

Buffers

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

Most buffers consist of a weak acid and its corresponding base.
They help maintain stable pH in biological systems, which is crucial for cellular function.

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

Acidification: A Threat to Our Oceans

Ocean Acidification

Human activities, such as burning fossil fuels, increase atmospheric CO2, a portion of which is absorbed by oceans, forming carbonic acid and lowering ocean pH.

Lower pH reduces carbonate ion concentration, which is necessary for marine organisms (e.g., corals) to form calcium carbonate skeletons.
Ocean acidification threatens marine ecosystems and biodiversity.

Possible Evolution of Life on Other Planets

Astrobiologists search for extraterrestrial life by looking for planets with water, as its unique properties are considered essential for life as we know it.

Evidence of water has been found on Mars and some exoplanets, raising the possibility of life beyond Earth.