Chapter 3: Water and Life – Properties, Functions, and Biological Importance

Water and Life

The Molecule That Supports All of Life

Water is essential for all known forms of life and possesses unique properties that make Earth habitable. Its molecular structure and interactions underlie its critical biological roles.

• Water is the only common substance to exist naturally in all three physical states: solid, liquid, and gas.

• Its emergent properties arise from its molecular structure and hydrogen bonding.

• The ability of water molecules to interact with other molecules is fundamental to biological processes.

Concept 3.1: Polar Covalent Bonds and Hydrogen Bonding

Polarity and Molecular Interactions

The water molecule (H2O) is polar due to the unequal sharing of electrons between oxygen and hydrogen atoms, resulting in partial charges.

• Electrons spend more time near the oxygen atom (more electronegative) than the hydrogen atoms.

• This creates a polar molecule with an uneven charge distribution.

• Polarity enables water molecules to form hydrogen bonds with each other.

Definition: Electronegativity is the tendency of an atom to attract electrons in a covalent bond. Oxygen's higher electronegativity compared to hydrogen leads to water's polarity.

Example: Hydrogen bonds between water molecules are responsible for many of water's unique properties.

Concept 3.2: Four Emergent Properties of Water

Properties Facilitating Life

Water's structure and hydrogen bonding result in four key properties that support 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 enables water transport against gravity in plants.

• Adhesion helps water stick to plant cell walls, aiding upward movement.

Example: Water moves from roots to leaves in trees due to cohesion and adhesion.

Surface Tension

Surface tension is the measure of how difficult it is to break the surface of a liquid. Water's high surface tension is due to hydrogen bonding at the air-water interface.

• Allows small insects, like water striders, to walk on water.

Moderation of Temperature

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

• Kinetic energy is the energy of motion.

• Thermal energy is the kinetic energy associated with random motion of atoms or molecules.

• Temperature is the average kinetic energy of molecules.

• Heat is thermal energy transferred between bodies.

Units: Calorie (cal) is the amount of heat needed to raise 1 g of water by 1°C. Kilocalorie (kcal) = 1,000 cal. Joule (J) is another energy unit: 1 J = 0.239 cal.

Water's High Specific Heat

Specific heat is the amount of heat required to change 1 g of a substance by 1°C. Water's specific heat is 1 cal/(g·°C), which is unusually high.

• Hydrogen bonds absorb heat when breaking and release heat when forming.

• Minimizes temperature fluctuations, supporting life.

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

Evaporative Cooling

Evaporation is the transformation from liquid to gas. Heat of vaporization is the heat required for 1 g of liquid to become gas.

• Evaporative cooling occurs as the surface cools when high-energy molecules leave as vapor.

• Helps regulate temperature in organisms and environments.

Floating of Ice on Liquid Water

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

• Water is densest at 4°C.

• If ice sank, aquatic life would be impossible as bodies of water would freeze solid.

Water: The Solvent of Life

Solutions, Solvents, and Solutes

A solution is a homogeneous mixture of substances. The solvent dissolves the solute. An aqueous solution uses water as the solvent.

• Water's polarity makes it a versatile solvent.

• Ionic compounds dissolve as ions are surrounded by a hydration shell of water molecules.

• Polar molecules, including proteins, can dissolve if they have ionic and polar regions.

Hydrophilic and Hydrophobic Substances

• Hydrophilic substances have an affinity for water.

• Hydrophobic substances do not; they have nonpolar bonds (e.g., oils).

• Hydrophobic molecules are major components of cell membranes.

Solute Concentration in Aqueous Solutions

Most biological reactions occur in aqueous solutions. Concentration calculations are essential for experiments.

• Molecular mass is the sum of all atomic masses in a molecule.

• Mole (mol) is a unit for counting molecules: $1 ext{ mol} = 6.02 imes 10^{23}$ molecules (Avogadro's number).

• Molarity (M) is moles of solute per liter of solution: $M = ext{[Concentration]}$

Concept 3.3: Acidic and Basic Conditions Affect Living Organisms

Acids, Bases, and pH

Water molecules can dissociate, forming hydrogen ions (H+) and hydroxide ions (OH-).

• Acid: Increases H+ concentration.

• Base: Reduces H+ concentration (increases OH-).

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

The pH Scale

The pH of a solution is the negative logarithm of H+ concentration.

• In pure water at 25°C: $[H^+][OH^-] = 10^{-14}$

• $ext{pH} = - ext{log} [H^+]$

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

• Acidic: pH < 7; Basic: pH > 7

• Most biological fluids: pH 6–8

Buffers

Buffers are substances that minimize changes in pH by reversibly binding H+ or OH-. Most buffers consist of a weak acid and its corresponding base.

Acidification and Environmental Impact

Human activities, such as burning fossil fuels, increase CO2 in the atmosphere. About 25% of this CO2 is absorbed by oceans, forming carbonic acid and leading to ocean acidification.

• Acidification reduces carbonate ions, which are essential for marine organisms to build calcium carbonate structures.

Summary 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	Helps water move up plant vessels
Surface Tension	Difficulty in breaking water's surface	Allows small organisms to walk on water
High Specific Heat	Resists temperature change	Stabilizes climate and organism temperature
Heat of Vaporization	High energy needed for evaporation	Evaporative cooling in organisms
Expansion upon Freezing	Ice is less dense than liquid water	Prevents bodies of water from freezing solid
Versatility as Solvent	Dissolves many substances	Facilitates chemical reactions in cells

Additional info: These notes expand on the original slides and text, providing definitions, equations, and examples for a comprehensive understanding of water's role in biology.