Water and Life: Study Notes for General Biology

Water and Life

Introduction

Water is a fundamental molecule for life, with unique properties that arise from its ability to form hydrogen bonds. These properties influence biological systems at molecular, cellular, and ecological levels. Understanding water's behavior is essential for studying biology.

Properties of Water

Hydrogen Bonding

Hydrogen bonds form between water molecules due to the polarity of the water molecule, where the oxygen atom is slightly negative and the hydrogen atoms are slightly positive.

• Hydrogen bond: A weak interaction between a hydrogen atom covalently bonded to an electronegative atom (such as oxygen) and another electronegative atom.

• Polarity: Water is a polar molecule, meaning it has a partial positive charge on one side and a partial negative charge on the other.

Emergent Properties of Water

Water exhibits several emergent properties due to hydrogen bonding, which are crucial for supporting life.

• Cohesive Behavior: Water molecules stick to each other due to hydrogen bonding.

  • Cohesion: Attraction between water molecules; responsible for phenomena like surface tension.

  • Adhesion: Attraction between water molecules and other substances; important for capillary action.

  • Surface Tension: The measure of how difficult it is to break the surface of a liquid; water has a high surface tension.

• Moderation of Temperature: Water can absorb or release large amounts of heat with only a slight change in its own temperature.

  • High Specific Heat: The amount of heat needed to raise the temperature of 1 gram of water by 1°C is high.

  • Heat of Vaporization: Water requires a lot of energy to change from liquid to gas.

  • Evaporative Cooling: As water evaporates, it removes heat, cooling the surface.

• Expansion Upon Freezing: Water expands as it freezes, making ice less dense than liquid water. This allows ice to float, insulating aquatic environments.

• Versatility as a Solvent: Water is known as the "universal solvent" because it can dissolve many substances, especially polar and ionic compounds.

  • Hydration Shell: Water molecules surround and separate ions or polar molecules in solution.

  • Protein Solubilization: Water helps dissolve proteins and other biological macromolecules.

Acids, Bases, and pH

Water Dissociation

Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-), which is fundamental to acid-base chemistry.

• Dissociation Equation: $\mathrm{H_2O} \rightleftharpoons \mathrm{H^+} + \mathrm{OH^-}$

• Acid: A substance that increases the concentration of H+ ions in solution.

• Base: A substance that reduces the concentration of H+ ions, often by increasing OH- ions.

• pH Scale: Measures the concentration of H+ ions in solution. $\mathrm{pH} = -\log_{10}[\mathrm{H^+}]$

• Buffer: A substance that minimizes changes in pH by accepting or donating H+ ions.

Hydrophilic and Hydrophobic Substances

Definitions and Examples

Substances interact with water differently based on their chemical properties.

• Hydrophilic: "Water-loving" substances that dissolve easily in water; typically polar or ionic compounds.

• Hydrophobic: "Water-fearing" substances that do not dissolve in water; typically nonpolar molecules like oils.

• Example: Salt (NaCl) is hydrophilic, while oil is hydrophobic.

Key Terms and Phrases

Definitions

• Acid: Substance that donates H+ ions.

• Base: Substance that accepts H+ ions or donates OH- ions.

• Aqueous Solution: Solution in which water is the solvent.

• Adhesion: Attraction between different substances.

• Cohesion: Attraction between like molecules.

• Buffer: Compound that stabilizes pH.

• Calorie: Unit of energy; amount needed to raise 1g of water by 1°C.

• Hydrogen Ion (H+): Proton released in water.

• Hydroxide Ion (OH-): Ion formed when water dissociates.

• Hydrophilic: Water-attracting.

• Hydrophobic: Water-repelling.

• Specific Heat: Heat required to change temperature.

• Surface Tension: Resistance of liquid surface to rupture.

• Solvent: Substance that dissolves another.

• Solute: Substance dissolved in a solvent.

• Evaporative Cooling: Cooling effect due to evaporation.

Comparison Table: Hydrophilic vs. Hydrophobic Substances

Property	Hydrophilic	Hydrophobic
Interaction with Water	Dissolves easily	Does not dissolve
Chemical Nature	Polar or ionic	Nonpolar
Examples	Salt, sugar	Oil, wax

Summary

Water's unique properties, including cohesion, adhesion, high specific heat, expansion upon freezing, and its role as a universal solvent, are essential for life. Understanding acids, bases, pH, and the distinction between hydrophilic and hydrophobic substances is fundamental in biology.