Ch 3 Outline Water: Structure, Properties, and Its Role in Biological Systems

Chapter 3

Water: Structure and Bonding

Polar Covalent Bonds in Water

Water (H2O) is a polar molecule, meaning it has regions of partial positive and negative charge due to unequal sharing of electrons in its covalent bonds. This polarity is fundamental to water's unique properties.

• Polar Covalent Bond: A type of chemical bond where electrons are shared unequally between atoms, resulting in partial charges. In water, oxygen is more electronegative than hydrogen, so electrons spend more time near the oxygen atom.

• Hydrogen Bond: An attraction between the partial positive charge of hydrogen in one water molecule and the partial negative charge of oxygen in another.

• In the water molecule: The electrons of the polar covalent bonds spend more time near the oxygen atom, giving it a partial negative charge (δ-) and the hydrogens a partial positive charge (δ+).

Example: The diagram shows water molecules with oxygen (O) and hydrogen (H) atoms. The bonds within a molecule are polar covalent, while the dotted lines between molecules represent hydrogen bonds.

Emergent Properties of Water

Water is Sticky: Cohesion and Adhesion

Water molecules exhibit strong cohesive and adhesive properties due to hydrogen bonding. These properties are essential for many biological processes.

• Cohesion: The attraction between water molecules due to hydrogen bonding. This allows water to form droplets and move as a column in plant vessels.

• Adhesion: The attraction between water molecules and other substances. This helps water "stick" to surfaces, aiding in processes like capillary action.

• Biological Benefit: Cohesion and adhesion enable the transport of water and nutrients in plants (e.g., xylem transport).

Example: Water moving up a plant stem against gravity due to cohesive and adhesive forces.

Thermal Properties of Water

Water has unique thermal properties that help regulate temperature in organisms and environments.

• Kinetic Energy: The energy of motion in molecules.

• Thermal Energy: The total kinetic energy of particles in a substance.

• Temperature: A measure of the average kinetic energy of particles.

• Heat: The transfer of thermal energy from one body to another.

• Specific Heat: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C. Water has a high specific heat, meaning it resists temperature changes.

• Heat of Vaporization: The amount of energy needed to convert 1 gram of liquid into vapor. Water's high heat of vaporization allows for effective cooling.

• Evaporative Cooling: As water evaporates, it removes heat, cooling the surface (e.g., sweating in humans).

Example: Sweating helps regulate body temperature through evaporative cooling.

Ice is Less Dense Than Liquid Water

Unlike most substances, water expands and becomes less dense when it freezes. This property is crucial for aquatic life.

• Cause: Hydrogen bonds stabilize and keep water molecules further apart in ice than in liquid water.

• Biological Benefit: Ice floats on water, insulating aquatic environments and allowing life to persist beneath the surface during cold periods.

Example: Lakes and ponds freeze from the top down, protecting organisms below.

Water is a Great Solvent

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

• Solvent: The substance that dissolves the solute (e.g., water).

• Solute: The substance being dissolved (e.g., salt).

• Solution: A homogeneous mixture of solvent and solute.

• Types of Molecules Dissolved: Water dissolves ionic compounds (e.g., NaCl) and polar molecules (e.g., sugars) due to its polarity.

• Biological Benefit: Water facilitates chemical reactions and transport of nutrients and waste in living organisms.

Example: Blood plasma is mostly water, dissolving salts, nutrients, and gases for transport.

Acids, Bases, and Buffers

pH

pH is a measure of the hydrogen ion concentration in a solution, indicating its acidity or basicity.

• Definition: $pH = -\log_{10}[H^+]$

• Scale: Ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

Acids

Acids are substances that increase the concentration of hydrogen ions (H+) in a solution.

• Properties: Taste sour, can donate protons (H+).

• Example: Hydrochloric acid (HCl).

Bases

Bases are substances that decrease the concentration of hydrogen ions, often by accepting H+ or releasing hydroxide ions (OH-).

• Properties: Taste bitter, feel slippery, can accept protons or release OH-.

• Example: Sodium hydroxide (NaOH).

Buffers

Buffers are solutions that resist changes in pH when acids or bases are added. They are vital for maintaining stable conditions in biological systems.

• Function: Buffers absorb excess H+ or OH- to maintain pH.

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

Example: Carbonic acid (H2CO3) helps maintain blood pH around 7.4. If pH drops to 7.17, the buffer system works to restore normal pH, protecting against harmful changes.

Comparison Table: Water's Properties and Biological Importance

Property	Description	Biological Importance
Cohesion	Water molecules stick to each other	Enables transport in plants
Adhesion	Water molecules stick to other surfaces	Assists capillary action
High Specific Heat	Resists temperature change	Stabilizes climate and body temperature
Ice is Less Dense	Ice floats on water	Insulates aquatic life
Solvent Ability	Dissolves many substances	Facilitates biochemical reactions

Additional info: Academic context and definitions have been expanded for clarity and completeness.