Water and Life: Properties, Functions, and Biological Importance

Chapter 3: Water and Life

Overview: The Molecule That Supports All of Life

Water is essential for all known forms of life, and its unique properties make it indispensable for biological processes. The study of water's role in biology reveals why it is central to the chemistry of life.

• Water as a prerequisite for life: The search for life on other planets often focuses on the presence of water.

• Abundance: Most cells are surrounded by water, and cells themselves are composed of 70–95% water.

• Earth's surface: Three-quarters of Earth's surface is covered by water.

• Water's chemical versatility: Water is involved in many chemical reactions essential for life.

Concept 3.1: Polar Covalent Bonds in Water Molecules Result in Hydrogen Bonding

Structure and Polarity of Water

Water's molecular structure and polarity enable it to form hydrogen bonds, which are responsible for many of its unique properties.

• Polar molecule: Water has a bent shape and an uneven distribution of charge, making it polar.

• Hydrogen bonding: The polarity of water molecules allows them to form hydrogen bonds with each other.

• Cohesion and adhesion: Hydrogen bonds cause water molecules to stick together (cohesion) and to other surfaces (adhesion).

Example: Water droplets forming on a leaf due to cohesion and adhesion.

Concept 3.2: Four Emergent Properties of Water Contribute to Earth's Suitability for Life

Organisms Depend on the Cohesion of Water Molecules

Water's cohesive and adhesive properties are vital for biological processes such as nutrient transport in plants.

• Cohesion: Hydrogen bonds hold water molecules together, aiding in the transport of water against gravity in plants.

• Adhesion: Water molecules adhere to other substances, such as cell walls.

• Surface tension: Water has a high surface tension due to hydrogen bonding, allowing small organisms to move across its surface.

Example: Water striders walking on the surface of a pond.

Water Moderates Temperature on Earth

Water absorbs and releases heat with minimal temperature change, helping to stabilize Earth's climate and organismal environments.

• Specific heat: Water has a high specific heat, meaning it can absorb or release large amounts of heat with little temperature change.

• Heat of vaporization: Water requires significant energy to change from liquid to gas, moderating climate and cooling organisms via evaporation.

• Evaporative cooling: As water evaporates, it removes heat, cooling surfaces such as skin or leaves.

Equation: $q = m c riangle T$ (where $q$ is heat energy, $m$ is mass, $c$ is specific heat, and $riangle T$ is temperature change)

Example: Sweating in humans helps regulate body temperature.

Water Expands Upon Freezing

Unlike most substances, water becomes less dense as it freezes, allowing ice to float and insulate aquatic environments.

• Density: Ice is less dense than liquid water due to the arrangement of hydrogen bonds.

• Environmental impact: Floating ice insulates water below, protecting aquatic life in cold climates.

Example: Lakes and ponds do not freeze solid, allowing fish and other organisms to survive winter.

Water is the Solvent of Life

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

• Solution: A homogeneous mixture of two or more substances.

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

• Solute: The substance dissolved.

• Hydrophilic vs. hydrophobic: Hydrophilic substances dissolve easily in water; hydrophobic substances do not.

Example: Salt (NaCl) dissolving in water to form an aqueous solution.

Concept 3.3: Acidic and Basic Conditions Affect Living Organisms

Acids, Bases, and pH

The concentration of hydrogen ions ($[H^+]$) in a solution determines its acidity or basicity, measured by the pH scale.

• Acid: A substance that increases the hydrogen ion concentration in a solution.

• Base: A substance that reduces the hydrogen ion concentration.

• pH scale: Measures the concentration of $[H^+]$; ranges from 0 (most acidic) to 14 (most basic).

Equation: $pH = - ext{log}_{10}[H^+]$

Example: Pure water has a pH of 7; stomach acid has a pH of 2.

Buffers

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

• Biological importance: Buffers help maintain stable pH in blood and cells, crucial for enzyme function and metabolic processes.

Example: The bicarbonate buffer system in human blood.

Acidification of Oceans and Environmental Impact

Human activities, such as burning fossil fuels, increase atmospheric CO2, which dissolves in oceans and forms carbonic acid, leading to ocean acidification.

• Impact on marine life: Acidification affects organisms that build shells or skeletons from calcium carbonate.

• Global concern: Changes in ocean chemistry threaten biodiversity and ecosystem stability.

Example: Coral reefs are at risk due to decreasing pH levels in ocean water.

Key Terms and Definitions

• Hydrogen bond: A weak bond between two molecules resulting from an electrostatic attraction.

• Cohesion: Attraction between molecules of the same substance.

• Adhesion: Attraction between molecules of different substances.

• Specific heat: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

• Solvent: The substance in which the solute dissolves.

• Buffer: A solution that resists changes in pH when acid or base is added.

Table: Comparison of Water's Properties

Property	Description	Biological Importance
Cohesion	Water molecules stick together via hydrogen bonds	Transport of water in plants
Adhesion	Water molecules stick to other surfaces	Movement of water up plant cell walls
High Specific Heat	Resists temperature change	Stabilizes climate and organismal temperature
Expansion Upon Freezing	Ice is less dense than liquid water	Insulates aquatic environments
Versatility as Solvent	Dissolves many substances	Facilitates chemical reactions in cells

Additional info: These notes expand on the original outline by providing definitions, equations, and examples for each property and concept, ensuring a comprehensive and self-contained study guide for General Biology students.