Water and Life

Introduction

Water is fundamental to all known forms of life. Its unique physical and chemical properties make it indispensable for biological processes, from cellular structure to ecosystem function. This chapter explores the molecular structure of water, its emergent properties, and its role in supporting life on Earth.

Water Properties

Water is Critical for Life

• Polarity: Water is a polar molecule, meaning it has a partial positive charge on one end (hydrogen) and a partial negative charge on the other (oxygen).

• Physical and Chemical Properties: Water exhibits unique behaviors such as cohesion, adhesion, high specific heat, and solvent abilities due to its molecular structure.

• Acid/Base Chemistry: Water can act as both an acid and a base, participating in important chemical reactions in biological systems.

Water Supports All of Life

States of Matter and Emergent Properties

• Three Physical States: Water is the only natural substance that exists in all three physical states—solid, liquid, and gas—under Earth's normal conditions.

• Emergent Properties: The unique properties of water, such as cohesion, temperature moderation, expansion upon freezing, and versatility as a solvent, are essential for life.

• Molecular Structure: The bent shape and polar covalent bonds of water molecules allow them to form hydrogen bonds with each other and with other substances.

Structure and Chemistry of Water

Polarity and Hydrogen Bonding

• Polar Covalent Bonds: In water, electrons are shared unequally between oxygen and hydrogen, making the molecule polar.

• Hydrogen Bonds: The partial charges allow water molecules to form hydrogen bonds with each other, which are weaker than covalent bonds but crucial for water's properties.

• Dynamic Bonding: Hydrogen bonds in liquid water are constantly forming and breaking, giving water its fluidity and high cohesion.

Emergent Properties of Water

1. Cohesion and Adhesion

• Cohesion: The attraction between water molecules due to hydrogen bonding. This property helps transport water against gravity in plants.

• Adhesion: The attraction between water molecules and other substances, such as plant cell walls, aiding in capillary action.

• Surface Tension: Water has a high surface tension, making it difficult to break the surface. This allows small organisms to 'walk' on water.

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

2. Moderation of Temperature

• High Specific Heat: Water can absorb or release a large amount of heat with only a slight change in its own temperature. This stabilizes environmental and organismal temperatures.

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

• Formula: (where = heat absorbed or released, = mass, = specific heat, = temperature change)

• Evaporative Cooling: As water evaporates, it removes heat from surfaces, helping organisms regulate temperature.

• Example: Sweating in humans and transpiration in plants are forms of evaporative cooling.

3. Expansion Upon Freezing

• Lower Density of Ice: Water is less dense as a solid than as a liquid because hydrogen bonds stabilize and keep molecules further apart in ice.

• Biological Importance: Ice floats on liquid water, insulating aquatic life in winter and maintaining stable environments.

4. Versatility as a Solvent

• Universal Solvent: Water dissolves more substances than any other liquid due to its polarity, forming hydration shells around ions and polar molecules.

• Hydrophilic vs. Hydrophobic: Hydrophilic substances have an affinity for water (e.g., salts, sugars), while hydrophobic substances do not (e.g., oils).

• Example: Table salt (NaCl) dissolves in water as Na+ and Cl- ions become surrounded by water molecules.

Acids, Bases, and pH

Dissociation of Water

• Ionization: Water can dissociate into hydronium (H3O+) and hydroxide (OH-) ions.

• Equation:

• Concentration: In pure water at 25°C, M.

Acids and Bases

• Acids: Substances that increase the concentration of H+ ions in solution.

• Bases: Substances that reduce the concentration of H+ ions, often by increasing OH- ions.

• Strong vs. Weak: Strong acids and bases dissociate completely in water; weak acids and bases dissociate partially and reversibly.

• Example: HCl is a strong acid; NaOH is a strong base.

The pH Scale

• Definition: pH is the negative logarithm of the hydrogen ion concentration:

• Range: pH values range from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Relationship: at 25°C.

• Biological Fluids: Most biological fluids have pH values between 6 and 8.

Buffers

• Definition: Buffers are substances that minimize changes in concentrations of H+ and OH- in a solution.

• Mechanism: Buffers accept H+ ions when they are in excess and donate H+ ions when they are depleted.

• Example: The bicarbonate buffer system in blood helps maintain pH homeostasis.

Concentration and Molarity

Calculating Solution Concentrations

• Molarity (M): The number of moles of solute per liter of solution.

• Molecular Weight: The sum of atomic weights of all atoms in a molecule (e.g., sucrose, C12H22O11, has a molecular weight of 342 Da).

• Avogadro's Number: molecules per mole.

• Preparation: To make a 1 M solution, dissolve 1 mole of solute in enough water to make 1 liter of solution.

Solutions, Salts, and Electrolytes

Definitions and Types

• Solvent: The liquid in which a substance dissolves (water is the most common biological solvent).

• Solute: The dissolved substance.

• Solution: A homogeneous mixture of solvent and solute.

• Salts: Ionic compounds formed from the reaction of acids and bases (e.g., NaCl from HCl and NaOH).

• Electrolytes: Substances that dissociate into ions in water and conduct electricity (e.g., salts, acids, bases).

• Nonelectrolytes: Substances that dissolve in water but do not form ions (e.g., sugar).

• Mixtures: Can be heterogeneous (not uniform, e.g., living organisms) or homogeneous (uniform, e.g., salt water).

Table: Comparison of Water's Properties

Summary

Water's unique molecular structure and properties are essential for life. Its ability to moderate temperature, dissolve substances, and participate in acid-base chemistry underpins many biological processes. Understanding water's chemistry is fundamental to the study of biology.