Chapter 3: Water and Life

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

Water's unique properties arise from its molecular structure and the interactions between its molecules. The polar covalent bonds and hydrogen bonding are fundamental to understanding water's behavior in biological systems.

• Polar Covalent Bonds: In a water molecule (H2O), oxygen is more electronegative than hydrogen, resulting in unequal sharing of electrons. This creates a partial negative charge (δ-) on the oxygen atom and partial positive charges (δ+) on the hydrogen atoms.

• Hydrogen Bonds: The partial charges allow water molecules to form hydrogen bonds with each other, where the hydrogen atom of one molecule is attracted to the oxygen atom of another.

• Structure of Water Molecules: Each water molecule can form up to four hydrogen bonds with neighboring water molecules, leading to a highly cohesive structure.

Example: In liquid water, the hydrogen bonds are constantly forming and breaking, allowing for fluidity, while in ice, the hydrogen bonds are more stable, creating a crystalline structure.

Concept 3.2: Emergent Properties of Water and Their Biological Significance

Water exhibits several emergent properties that are essential for life on Earth. These properties arise from the collective behavior of water molecules and their ability to form hydrogen bonds.

• Cohesion and Adhesion: Cohesion refers to the attraction between water molecules, while adhesion is the attraction between water molecules and other substances. These properties enable processes such as the transport of water in plants through capillary action.

• Moderation of Temperature: Water has a high specific heat capacity, meaning it can absorb or release large amounts of heat with only a slight change in its own temperature. This helps stabilize temperatures in organisms and environments.

• Expansion Upon Freezing: Water is less dense as a solid (ice) than as a liquid, allowing ice to float. This insulates bodies of water and protects aquatic life in cold climates.

• Versatility as a Solvent: Water is known as the "universal solvent" because it can dissolve a wide variety of substances, facilitating chemical reactions in living organisms.

• Hydrophilic vs. Hydrophobic Substances: Hydrophilic substances have an affinity for water and dissolve easily (e.g., salts, sugars), while hydrophobic substances repel water (e.g., oils, fats).

• Solution, Solvent, and Solute: A solution is a homogeneous mixture of two or more substances. The solvent is the dissolving agent (often water), and the solute is the substance being dissolved.

Example: Water moves from the roots to the leaves of a tree due to cohesion (water molecules sticking together) and adhesion (water molecules sticking to the walls of xylem vessels).

Example: To make a 1 molar (1 M) solution of ethyl alcohol, dissolve 1 mole of ethyl alcohol in enough water to make 1 liter of solution.

Concept 3.3: Acids, Bases, and the pH Scale in Biological Systems

The balance of acids and bases in aqueous solutions is crucial for biological processes. The pH scale measures the concentration of hydrogen ions, affecting enzyme activity and cellular function.

• Dissociation of Water: Water molecules can dissociate into hydrogen ions (H+) and hydroxide ions (OH-):

• Acids and Bases: An acid increases the concentration of H+ in a solution, while a base reduces the H+ concentration, often by increasing OH- concentration.

• pH Scale: The pH scale ranges from 0 to 14 and is defined as:

  • pH 7 is neutral (pure water).

  • pH < 7 is acidic; pH > 7 is basic (alkaline).

• Relationship Between H+ and OH-: In pure water at 25°C:

• Effect of Acids and Bases: Acids increase [H+], lowering pH; bases decrease [H+] (or increase [OH-]), raising pH.

• Biological Importance: Most biological processes occur within a narrow pH range; significant deviations can be harmful to cells and organisms.

Example: Adding hydrochloric acid (HCl) to water increases [H+] and lowers the pH, making the solution more acidic.

Example: If [H+] increases from M to M, the pH decreases from 7 to 6.