Water and Life: Properties 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 significance for living organisms.

Polar Covalent Bonds and Hydrogen Bonding in Water

Structure of the Water Molecule

• Water (H2O) is a polar molecule:

  • Oxygen is more electronegative than hydrogen, so it pulls shared electrons closer, giving oxygen a partial negative charge (δ−) and hydrogen atoms partial positive charges (δ+).

  • This uneven distribution of charge makes water a polar molecule.

• Hydrogen bonds form between the partially positive hydrogen of one water molecule and the partially negative oxygen of another.

Example: The polarity of water allows it to form hydrogen bonds, which are responsible for many of its unique properties.

Emergent Properties of Water

Overview

Water exhibits several emergent properties that are critical for life:

• Cohesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion and Adhesion

• Cohesion is the attraction between water molecules due to hydrogen bonding.

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

• Adhesion is the attraction between water molecules and other substances (e.g., plant cell walls).

• Cohesion and adhesion together enable the transport of water and dissolved nutrients against gravity in plants (capillary action).

Example: Water moves upward through plant vessels due to cohesion and adhesion, helping plants transport nutrients from roots to leaves.

Moderation of 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 is the amount of heat required to change the temperature of 1 g of a substance by 1°C.

• For water, the specific heat is 1 cal/(g·°C).

• Hydrogen bonding is responsible: heat is absorbed to break bonds and released when bonds form.

• This property helps stabilize temperatures in organisms and environments.

Example: Coastal areas have milder climates because large bodies of water absorb and release heat slowly.

Evaporative Cooling

• Evaporation is the transformation of a substance from liquid to gas.

• Heat of vaporization is the amount of heat required to convert 1 g of liquid to gas.

• As water evaporates, the surface cools, a process called evaporative cooling.

• This helps regulate temperature in organisms (e.g., sweating) and bodies of water.

Example: Sweating cools the body as water evaporates from the skin.

Expansion Upon Freezing

• Water is less dense as a solid (ice) than as a liquid.

• At 0°C, water molecules form a crystalline lattice, keeping them farther apart than in liquid water.

• This makes ice about 10% less dense than liquid water, allowing it to float.

• Floating ice insulates the water below, protecting aquatic life in cold climates.

Example: Lakes and ponds do not freeze solid in winter, allowing organisms to survive beneath the ice.

Water: The Solvent of Life

Solutions and Solubility

• A solution is a homogeneous mixture of two or more substances.

• The solvent is the dissolving agent; the solute is the substance dissolved.

• An aqueous solution is one in which water is the solvent.

• Water's polarity allows it to dissolve many substances, making it a versatile solvent.

• When ionic compounds dissolve, each ion is surrounded by a hydration shell of water molecules.

• Water can also dissolve large polar molecules, such as proteins, if they have 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 have an affinity for water (e.g., salts, sugars).

• Hydrophobic substances do not have an affinity for water, often because they are nonpolar (e.g., oils).

• Hydrophobic molecules are important in the formation of cell membranes.

Example: Oil droplets do not mix with water due to their hydrophobic nature.

Concentration of Solutes in Aqueous Solutions

• Most chemical reactions in organisms occur in aqueous solutions.

• Concentration is often measured in moles (mol), where 1 mol = $6.022 \times 10^{23}$ molecules (Avogadro's number).

• To prepare solutions, mass is used to calculate the number of solute molecules.

Water and the Search for Life Beyond Earth

• Astrobiologists search for extraterrestrial life by looking for planets with water.

• Evidence of water vapor has been found on some exoplanets and on Mars within our solar system.

Example: The presence of water on Mars is a key factor in the search for past or present life.

Table: Comparison of Water's States and Properties

State	Arrangement of Molecules	Density	Biological Significance
Liquid	Molecules close together, can slip past each other	High	Supports life, allows transport of nutrients
Solid (Ice)	Molecules locked in a crystalline lattice, farther apart	Lower (about 10% less than liquid)	Floats, insulates water below, protects aquatic life
Gas (Vapor)	Molecules far apart, move freely	Very low	Evaporative cooling, water cycle

Key Equations and Units

• 1 calorie (cal) = amount of heat to raise 1 g of water by 1°C

• 1 kilocalorie (kcal) = 1,000 cal

• 1 joule (J) = 0.239 cal; 1 cal = 4.184 J

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