Chapter 3: Water and Life – Properties, Biological Importance, and Chemical Context

Water and Life

Overview

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 their significance for living organisms and ecosystems.

Basic Requirements of Life and Major Molecules

Key Biological Molecules

• Water (H2O): The most abundant molecule in living organisms, crucial for chemical reactions, temperature regulation, and transport.

• Macromolecules: Proteins, nucleic acids, carbohydrates, and lipids, all of which rely on water for structure and function.

Emergent Properties of Water

1. Cohesive Behavior

• Cohesion: Hydrogen bonds hold water molecules together, resulting in high surface tension.

• Surface Tension: The measure of how difficult it is to stretch or break the surface of a liquid. Enables phenomena such as water striders walking on water.

• Adhesion: Attraction between water molecules and other substances (e.g., plant cell walls), aiding in water transport against gravity in plants.

• Example: Water transport in plants is possible due to cohesion (between water molecules) and adhesion (between water and cell walls), facilitating the upward movement of water from roots to leaves.

2. Ability to Moderate Temperature

• High Specific Heat: Water can absorb or release a large amount of heat with only a slight change in its own temperature.

• Definition: The specific heat of water is 1 cal/(g·°C).

• Hydrogen Bonding: Heat is absorbed when hydrogen bonds break and released when they form, minimizing temperature fluctuations.

• High Heat of Vaporization: Large amounts of energy are required to convert water from liquid to gas, allowing for evaporative cooling.

• Evaporative Cooling: As water evaporates, the surface cools, stabilizing temperatures in organisms and environments (e.g., sweating in mammals, cooling of lakes).

• Example: Elephants spray water on themselves; as it evaporates, it cools their skin.

3. Expansion Upon Freezing

• Ice Floats: Hydrogen bonds in ice are more ordered, making ice less dense than liquid water.

• Density: Water reaches its greatest density at 4°C; ice is about 10% less dense than liquid water.

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

• Example: Melting sea ice threatens species like Phoca hispida (ringed seals) that depend on ice platforms.

4. Versatility as a Solvent

• Solution: A homogeneous mixture of substances.

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

• Solute: The substance dissolved.

• Aqueous Solution: Solution where water is the solvent.

• Hydration Shells: Water molecules surround and dissolve ionic and polar compounds by forming hydrogen bonds.

• Hydrophilic Substances: Have an affinity for water (e.g., salts, sugars).

• Hydrophobic Substances: Repel water (e.g., oils, major components of cell membranes).

Solute Concentration in Aqueous Solutions

• Molecular Mass: Sum of all atomic masses in a molecule.

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

• Molarity (M): Number of moles of solute per liter of solution.

Acidic and Basic Conditions Affect Living Organisms

Water Dissociation and pH

• A hydrogen atom in a hydrogen bond between two water molecules can shift, forming a hydronium ion ($\mathrm{H_3O^+}$) and a hydroxide ion ($\mathrm{OH^-}$).

• In pure water, $[\mathrm{H^+}] = [\mathrm{OH^-}]$.

• Acids increase $[\mathrm{H^+}]$; bases decrease $[\mathrm{H^+}]$.

The pH Scale

• pH is defined as the negative logarithm of the hydrogen ion concentration:

$\mathrm{pH} = -\log [\mathrm{H^+}]$

• At 25°C, $[\mathrm{H^+}][\mathrm{OH^-}] = 10^{-14}$.

• Neutral solution: $[\mathrm{H^+}] = 10^{-7}$, so pH = 7.

• Acidic solutions: pH < 7; Basic solutions: pH > 7.

Buffers and Biological Importance

• Buffers: Substances that minimize changes in pH by accepting or donating H+ ions.

• Example: The carbonic acid-bicarbonate buffer system in blood:

$\mathrm{CO_2} + \mathrm{H_2O} \rightleftharpoons \mathrm{H_2CO_3} \rightleftharpoons \mathrm{HCO_3^-} + \mathrm{H^+}$

• Maintains blood pH near 7.4.

Environmental Impact: Ocean Acidification

• Burning fossil fuels increases atmospheric CO2, which dissolves in oceans, forming carbonic acid and lowering pH (ocean acidification).

• Acidification reduces carbonate ion availability, threatening marine organisms that build shells from calcium carbonate (e.g., corals).

Summary Table: Four Emergent Properties of Water

Property	Description	Biological Importance
Cohesion/Adhesion	Hydrogen bonds hold water molecules together and to other substances	Water transport in plants, surface tension
Moderation of Temperature	High specific heat and heat of vaporization	Stable environments, evaporative cooling
Expansion Upon Freezing	Ice is less dense than liquid water	Insulates aquatic life, prevents bodies of water from freezing solid
Versatility as a Solvent	Dissolves ionic and polar substances	Facilitates chemical reactions, nutrient transport