Water and Life: Properties, Structure, and Biological Importance

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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 supporting life.

Structure of Water

Polar Covalent Bonds and Hydrogen Bonding

The water molecule (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom. The electrons in these bonds are shared unequally, resulting in a polar molecule.

Polar covalent bonds: Electrons spend more time near the oxygen atom, giving it a partial negative charge (δ−), while hydrogen atoms have a partial positive charge (δ+).
Polarity: The uneven distribution of charge makes water a polar molecule.
Hydrogen bonding: The polarity allows water molecules to form weak attractions, called hydrogen bonds, with each other.
Example: Hydrogen bonds are responsible for many of water’s unique properties, such as cohesion and high specific heat.

Emergent Properties of Water

Overview of Properties

Water exhibits several 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 refers to the attraction between water molecules due to hydrogen bonding, while adhesion is the attraction between water molecules and other substances.

Cohesion: Responsible for high surface tension, making it difficult to break the surface of water.
Adhesion: Helps water move against gravity in plants by sticking to cell walls.
Example: Water transport in plants relies on both cohesion and adhesion to move water from roots to leaves.

Moderation of Temperature

Water can absorb or release large amounts of heat with only slight changes in its own temperature, helping to stabilize environments.

Kinetic energy: The energy of motion; thermal energy is the kinetic energy associated with random movement of atoms or molecules.
Heat: Thermal energy transferred 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 .
High specific heat: Water resists temperature changes due to hydrogen bonding. Heat is absorbed when bonds break and released when they form.
Example: Coastal areas experience milder climates due to water’s ability to moderate air temperature.

Evaporative Cooling

Evaporation is the transformation of a substance from liquid to gas. As water evaporates, the surface cools, stabilizing temperatures in organisms and bodies of water.

Heat of vaporization: The amount of heat a liquid must absorb for 1 g to be converted to gas.
Evaporative cooling: Water molecules with the highest kinetic energy leave as gas, cooling the remaining liquid.
Example: Sweating in humans helps regulate body temperature.

Expansion Upon Freezing

Water is less dense as a solid (ice) than as a liquid, allowing ice to float.

Crystalline structure: At 0°C, hydrogen bonds keep water molecules far enough apart to make ice about 10% less dense than liquid water.
Biological importance: Floating ice insulates water below, protecting aquatic life in cold climates.
Example: Arctic sea ice provides a platform for species such as Phoca hispida (ringed seals).

Water as a Solvent

Versatility as a Solvent

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

Solution: A homogeneous mixture of substances.
Solvent: The dissolving agent (water in aqueous solutions).
Solute: The substance dissolved.
Hydration shell: When ionic compounds dissolve, each ion is surrounded by water molecules.
Nonionic polar molecules: Large polar molecules, such as proteins, can dissolve in water if they have ionic and polar regions.

Hydrophilic and Hydrophobic Substances

Hydrophilic: Substances with an affinity for water (often polar or ionic).
Hydrophobic: Substances that do not interact with water (often nonpolar, such as oils).
Example: Hydrophobic molecules are major components of cell membranes.

Solute Concentration in Aqueous Solutions

Molecular Mass and Moles

Molecular mass: The sum of the masses of all atoms in a molecule.
Mole: molecules (Avogadro’s number).
Dalton: Unit of atomic mass; daltons = 1 g.
Molarity (M): Number of moles of solute per liter of solution.

Acids, Bases, and pH

Dissociation of Water

Water molecules can dissociate into hydrogen ions (H+) and hydroxide ions (OH−).

Hydronium ion (H3O+): Formed when a water molecule gains a proton.
Hydroxide ion (OH−): Formed when a water molecule loses a proton.
Dynamic equilibrium: Water molecules dissociate and reform at equal rates.

Acids and Bases

Acid: Substance that increases H+ concentration in solution.
Base: Substance that reduces H+ concentration.
Strong acids/bases: Dissociate completely in water.
Weak acids/bases: Reversibly release and accept H+ ions.

pH Scale

Definition: pH is the negative logarithm of the H+ concentration.

Formula:

Neutral solution: pH = 7
Acidic solution: pH < 7
Basic solution: pH > 7
Biological fluids: Most have pH values between 6 and 8.

Buffers

Buffers are substances that minimize changes in concentrations of H+ and OH− in a solution.

Composition: Usually consist of a weak acid and its corresponding base.
Function: Combine reversibly with H+ ions to maintain pH stability.
Example: Blood contains buffers to maintain pH near 7.4.

Ocean Acidification

Impact of Human Activities

Burning fossil fuels releases CO2, which is absorbed by oceans and forms carbonic acid, leading to ocean acidification.

Process: CO2 + H2O → H2CO3 (carbonic acid)
Effect: Increased H+ ions combine with carbonate, reducing its availability for marine organisms that build shells and coral reefs.
Ecological impact: Ocean acidification threatens marine ecosystems, especially coral reefs.

Possible Evolution of Life on Other Planets

Search for Water

Astrobiologists focus on planets with evidence of water as potential sites for life. Mars, for example, has shown signs of liquid water.

Example: Dark streaks on Mars suggest the presence of liquid water.

Appendix: Key Table

Comparison of Water’s Properties

Property	Description	Biological Importance
Cohesion	Hydrogen bonds hold water molecules together	Transport of water in plants
Adhesion	Water molecules stick to other substances	Helps counter gravity in plants
High Specific Heat	Resists temperature change	Stabilizes climate and organism temperature
Expansion upon Freezing	Ice is less dense than liquid water	Insulates aquatic life
Versatility as Solvent	Dissolves many substances	Facilitates chemical reactions in cells