Chapter 3: Water and Life – Key Properties and Biological Importance

Water and Life

The Molecule That Supports All of Life

Water is essential for life on Earth, existing uniquely in all three physical states (solid, liquid, gas) in the natural environment. Its molecular structure and emergent properties make Earth suitable for life.

• Water's unique properties allow it to interact with other molecules and support biological processes.

• It is the only common substance to exist naturally in all three states of matter.

Concept 3.1: Polar Covalent Bonds and Hydrogen Bonding

Polarity and Hydrogen Bonds

The water molecule (H2O) is polar due to unequal sharing of electrons in its covalent bonds, resulting in partial charges.

• Polar covalent bonds: Electrons spend more time near oxygen than hydrogen.

• Polarity: Water has an uneven charge distribution, making it a polar molecule.

• Hydrogen bonds: Polarity allows water molecules to form hydrogen bonds with each other.

Example: Intramolecular bonds (within a molecule) are covalent, while intermolecular bonds (between molecules) are hydrogen bonds.

Concept 3.2: Four Emergent Properties of Water

Properties Facilitating Life

Water's emergent properties are crucial for supporting life on Earth.

• 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 and other substances.

• Cohesion: Helps transport water against gravity in plants.

• Adhesion: Facilitates movement of water through plant cell walls.

Example: Water transport in plants relies on both cohesion and adhesion.

Surface Tension

Surface tension is the difficulty of breaking the surface of a liquid. Water's high surface tension is due to hydrogen bonding at the air-water interface.

• Surface tension: Allows small organisms to move on water surfaces.

Moderation of Temperature

Water moderates temperature by absorbing and releasing heat with minimal temperature change.

• High specific heat: Water can absorb or release large amounts of heat with little temperature change.

• Heat capacity: Water's high heat capacity stabilizes environmental and organismal temperatures.

Definitions:

• Kinetic energy: Energy of motion.

• Thermal energy: Kinetic energy associated with random motion of atoms/molecules.

• Calorie (cal): Amount of heat required to raise 1 g of water by 1°C.

• Kilocalorie (kcal): 1,000 cal; used in food energy.

Water's High Specific Heat

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

• Water's specific heat: 1 cal/(g·°C)

• Hydrogen bonding is responsible for water's high specific heat.

Equation:

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

Evaporative Cooling

Evaporation transforms liquid to gas, cooling the remaining surface.

• Heat of vaporization: Heat required for 1 g of liquid to become gas.

• Evaporative cooling: Stabilizes temperatures in organisms and bodies of water.

Expansion Upon Freezing

Ice floats because hydrogen bonds in ice are more ordered, making it less dense than liquid water.

• Water reaches maximum density at 4°C.

• If ice sank, aquatic life would be impossible.

Effects of Climate Change

Loss of ice due to global warming threatens species dependent on icy environments.

• 99% of climate scientists agree human activity is causing climate change.

Water: The Solvent of Life

Solutions and Solubility

Water is a versatile solvent due to its polarity.

• Solution: Homogeneous mixture of substances.

• Solvent: Dissolving agent.

• Solute: Substance dissolved.

• Aqueous solution: Water is the solvent.

Hydration Shells

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

Solubility of Polar and Nonpolar Molecules

• Water dissolves nonionic polar molecules and large polar molecules (e.g., proteins) if they have ionic/polar regions.

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Affinity for water.

• Hydrophobic: No affinity for water; major component of cell membranes.

Possible Evolution of Life on Other Planets

Search for extraterrestrial life focuses on planets with water. Mars and other locations in our solar system have water.

Concept 3.3: Acidic and Basic Conditions Affect Living Organisms

Dissociation of Water

Water molecules can dissociate, affecting biological systems.

• Hydrogen atom can shift, forming hydrogen ion (H+) and hydroxide ion (OH-).

• Extra proton forms hydronium ion (H3O+).

Equation:

• $2\text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{OH}^-$

Acids and Bases

• Acid: Increases H+ concentration.

• Base: Reduces H+ concentration.

• Strong acids/bases dissociate completely; weak acids/bases reversibly release/accept H+.

The pH Scale

pH measures the concentration of hydrogen ions in solution.

• Equation: $[\text{H}^+][\text{OH}^-] = 10^{-14}$

• Equation: $\text{pH} = -\log [\text{H}^+]$

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

Acidic solutions: pH < 7 Basic solutions: pH > 7 Most biological fluids: pH 6–8

Solution Type	pH Range	Examples
Acidic	< 7	Battery acid, gastric juice, cola
Neutral	7	Pure water, human blood
Basic	> 7	Milk of magnesia, household ammonia, bleach

Buffers

Buffers maintain stable pH in biological systems by minimizing changes in H+ and OH- concentrations.

• Contain weak acid and its corresponding base.

• Combine reversibly with H+ ions.

Acidification and Ocean Health

Human activities, such as burning fossil fuels, increase CO2 in the atmosphere, leading to ocean acidification and threatening marine life.

• CO2 forms carbonic acid in seawater.

• About 25% of human-generated CO2 is absorbed by oceans.

Additional info: The notes are based on Campbell Biology, Chapter 3, and are suitable for General Biology college students.