Water and Life: Properties and Biological Importance (Chapter 3 Study Notes)

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Water and Life

Introduction

Water is a fundamental molecule that supports all known forms of life. Its unique chemical and physical properties make Earth habitable and enable biological processes essential for living organisms. This chapter explores the structure of water, its emergent properties, and its critical roles in biological systems.

The Water Molecule: Structure and Polarity

Structure of Water (H2O)

Water is a polar molecule: It consists of two hydrogen atoms covalently bonded to one oxygen atom.
Polarity: Oxygen is more electronegative than hydrogen, so electrons spend more time near the oxygen atom, giving it a partial negative charge (δ−) and the hydrogens partial positive charges (δ+).
Shape: The molecule has a bent shape, with an angle of about 104.5°, contributing to its polarity.

Definition: Polar molecule – a molecule with an uneven distribution of charges, resulting in regions of partial positive and negative charge.

Hydrogen Bonding

Hydrogen bonds are weak attractions between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of another.
Each water molecule can form up to four hydrogen bonds with neighboring water molecules.
Hydrogen bonding is responsible for many of water’s unique properties.

Emergent Properties of Water

Overview

Water exhibits several emergent properties that are essential for life:

Cohesive behavior
Ability to moderate temperature
Expansion upon freezing
Versatility as a solvent

Cohesion and Adhesion

Cohesion: The attraction between water molecules due to hydrogen bonding. This property helps water move upward against gravity in plants (capillary action).
Adhesion: The attraction between water molecules and other substances, such as plant cell walls, aiding in water transport.
Surface tension: Water has a high surface tension because of the collective strength of hydrogen bonds at the surface.

Example: Water droplets forming beads on a surface and the ability of some insects to walk on water are due to surface tension.

Moderation of Temperature

Water can absorb or release large amounts of heat with only a slight change in its own temperature.
Specific heat: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C. Water’s high specific heat is due to hydrogen bonding.
When hydrogen bonds break, heat is absorbed; when they form, heat is released.

Example: Coastal areas have milder climates than inland areas because water moderates temperature fluctuations.

Expansion Upon Freezing

In liquid water, molecules are close together, but in ice, hydrogen bonds stabilize and keep molecules farther apart.
This makes ice less dense than liquid water, so ice floats.
Biological significance: Floating ice insulates the water below, allowing aquatic life to survive in cold climates.

Versatility as a Solvent

Water’s polarity allows it to dissolve many substances, making it a universal solvent.
Solution: A homogeneous mixture of two or more substances.
Solvent: The dissolving agent (water in aqueous solutions).
Solute: The substance that is dissolved.
Water dissolves ionic compounds and polar molecules by surrounding them and separating their ions or molecules.

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

Hydrophilic and Hydrophobic Substances

Hydrophilic: Substances with an affinity for water (e.g., salts, sugars, acids, bases).
Hydrophobic: Substances that do not have an affinity for water (e.g., oils, lipids).

Concentration of Solutions

Molecular Mass and Moles

Molecular mass: The sum of the masses of all atoms in a molecule.
Mole (mol): A unit representing 6.02 × 1023 molecules (Avogadro’s number).
1 mole of a substance has a mass in grams equal to its molecular mass in daltons.

Molarity

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

Formula:

Acids, Bases, and pH

Acids and Bases

Acid: A substance that increases the hydrogen ion (H+) concentration of a solution.
Base: A substance that reduces the hydrogen ion concentration, often by accepting H+ or releasing OH−.

Dissociation of Water

Occasionally, a hydrogen atom in a water molecule shifts to another water molecule, forming a hydronium ion (H3O+) and a hydroxide ion (OH−).
In pure water, the concentrations of H+ and OH− are equal.

Equation:

pH Scale

The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration:

In neutral water, [H+] = 10−7 M, so pH = 7.
Acidic solutions have pH < 7; basic solutions have pH > 7.

Common Substances and Their pH Values

Substance	pH
Lemon juice	2
Vinegar, wine, cola	3
Rainwater	5-6
Pure water (neutral)	7
Human blood, tears	7.4
Milk of magnesia	10.5
Household ammonia	12

pH Calculations and Changes

Each pH unit represents a tenfold difference in H+ concentration.
For example, a solution with pH 5 has 10 times more H+ than a solution with pH 6.
Increasing [H+] by a factor of 1,000 lowers the pH by 3 units.

Example: If a solution starts at pH 8 and [H+] increases by 1,000 times, the new pH is 5.

Summary Table: Properties of Water

Property	Description	Biological Importance
Cohesion	Hydrogen bonds hold water molecules together	Helps transport water in plants
Adhesion	Attraction between water and other substances	Assists water movement in plant tissues
High specific heat	Water resists temperature change	Stabilizes climate and organismal temperature
Expansion upon freezing	Ice is less dense than liquid water	Insulates aquatic environments
Versatile solvent	Dissolves many substances	Facilitates chemical reactions in cells

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the summary tables and pH calculation examples.