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 search for life on other planets often focuses on the presence of water.

• Water's abundance: Most cells are surrounded by water, and cells themselves are about 70–95% water.

• Role in chemical reactions: Water is a key participant in many chemical reactions necessary for life.

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

• Life's origin: Life is believed to have originated in water, and it remains vital for all living organisms.

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

Structure and Polarity of Water

Water molecules are polar, with oxygen being more electronegative than hydrogen, resulting in unequal sharing of electrons and partial charges.

• Polar covalent bonds: Electrons are shared unequally between oxygen and hydrogen.

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

• Dynamic bonds: Hydrogen bonds are transient, constantly forming and breaking.

Example: Water's ability to dissolve many substances is due to its polarity and hydrogen bonding.

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

Organisms Depend on the Cohesion of Water Molecules

Cohesion and adhesion are critical for water transport in plants and for maintaining biological structures.

• Cohesion: Water molecules stick together due to hydrogen bonding.

• Adhesion: Water molecules also stick to other substances, aiding in capillary action.

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

Example: Water transport in plants relies on cohesion and adhesion to move water from roots to leaves.

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.

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

Water's High Specific Heat

• Definition: Specific heat is the amount of heat required to raise the temperature of 1 g of a substance by 1°C.

• Water's value: 1 cal/g/°C

• Biological significance: Stabilizes temperature in organisms and environments.

Water's High Heat of Vaporization and Evaporative Cooling

• Heat of vaporization: Quantity of heat needed to convert 1 g of liquid to gas.

• Evaporative cooling: Surface cooling due to evaporation, important for temperature regulation.

Floating of Ice on Liquid Water

Ice is less dense than liquid water due to hydrogen bonding, allowing it to float and insulate aquatic environments.

• Density: Water expands as it freezes, making ice less dense than liquid water.

• Ecological impact: Floating ice insulates water below, protecting aquatic life.

Water: The Solvent of Life

Water as a Solvent

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

• Solution: Homogeneous mixture of two or more substances.

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

• Solute: The substance dissolved.

• Hydrophilic substances: Attracted to water, dissolve easily.

• Hydrophobic substances: Repel water, do not dissolve easily.

Example: Salt (NaCl) dissolves in water as ions are surrounded by water molecules.

Molecular Mass and Moles

• Mole: Standard unit for measuring molecules, 1 mole = $6.02 \times 10^{23}$ molecules (Avogadro's number).

• Molarity: Number of moles of solute per liter of solution.

Concept 3.3: Acidic and Basic Conditions Affect Living Organisms

Acids, Bases, and pH

Acids and bases alter the concentration of hydrogen ions (H+) and hydroxide ions (OH-) in solution, affecting biological processes.

• Acid: Increases H+ concentration.

• Base: Reduces H+ concentration, often by increasing OH-.

• pH scale: Measures H+ concentration; $\text{pH} = -\log[\text{H}^+]$

• Neutral solution: pH = 7; acidic < 7; basic > 7.

Example: Human blood is slightly basic, with a pH of about 7.4.

Buffers

Buffers help maintain stable pH in biological systems by absorbing excess H+ or OH-.

• Function: Minimize changes in pH.

• Example: Carbonic acid-bicarbonate buffer system in blood.

Acidification of Oceans and Environmental Impact

Increased CO2 absorption by oceans leads to acidification, affecting marine life and global ecosystems.

• CO2 absorption: Oceans absorb about 25% of human-generated CO2.

• Formation of carbonic acid: CO2 reacts with water to form H2CO3, lowering pH.

• Impact: Acidification threatens marine organisms, especially those with calcium carbonate shells.

Table: Key Properties of Water

Property	Description	Biological Importance
Cohesion	Hydrogen bonds hold water molecules together	Transport of water in plants
Adhesion	Water molecules stick to other substances	Capillary action in plants
High Specific Heat	Water resists temperature change	Stabilizes climate and organismal temperature
High Heat of Vaporization	Requires much energy to evaporate	Evaporative cooling
Expansion upon Freezing	Ice is less dense than liquid water	Insulates aquatic environments
Versatility as a Solvent	Dissolves many substances	Facilitates chemical reactions

Additional info: These notes expand on the original outline by providing definitions, examples, equations, and a summary table for key water properties relevant to biology.