Water and Life: Properties, Chemistry, and Biological Importance

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Chapter 3: Water and Life

Introduction

Water is essential for all known forms of life. Its unique chemical and physical properties make it indispensable for biological processes and the maintenance of life on Earth. This chapter explores the molecular structure of water, its emergent properties, and its role as a solvent, as well as the concepts of acids, bases, and pH in biological systems.

Polar Covalent Bonds and Hydrogen Bonding in Water

Structure and Polarity of Water Molecules

Water Molecule Structure: A water molecule (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom.
Polar Covalent Bonds: The electrons in the O-H bonds are shared unequally, spending more time near the oxygen atom due to its higher electronegativity.
Polarity: This unequal sharing creates a partial negative charge near the oxygen and a partial positive charge near the hydrogens, making water a polar molecule.
Hydrogen Bonding: The polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.

Example: Hydrogen bonds are responsible for water's high boiling point compared to other molecules of similar size.

Emergent Properties of Water

Four Key Properties

Water exhibits four emergent properties that are crucial for life:

Cohesive Behavior: Water molecules stick together due to hydrogen bonding, resulting in high surface tension and the ability to transport water against gravity in plants.
Ability to Moderate Temperature: Water can absorb or release large amounts of heat with only slight changes in its own temperature, due to its high specific heat.
Expansion Upon Freezing: Water is less dense as a solid (ice) than as a liquid, allowing ice to float and insulate aquatic environments.
Versatility as a Solvent: Water's polarity makes it an excellent solvent for ionic and polar substances, facilitating chemical reactions in cells.

Cohesion and Adhesion

Cohesion: The attraction between water molecules due to hydrogen bonding.
Surface Tension: A measure of how difficult it is to stretch or break the surface of a liquid; water has a high surface tension.
Adhesion: The attraction between water molecules and other substances, important for processes like capillary action in plants.

Temperature Moderation

Specific Heat: The amount of heat required to change the temperature of 1 gram of a substance by 1°C. For water, this is 1 cal/(g·°C).
Heat Absorption and Release: Water absorbs heat when hydrogen bonds break and releases heat when they form, buffering temperature changes.
Evaporative Cooling: As water evaporates, the surface cools, helping organisms regulate temperature.

Example: Coastal areas experience milder climates due to the high specific heat of water in oceans and lakes.

Expansion Upon Freezing

Ice Formation: At 0°C, water molecules form a crystalline lattice, making ice less dense than liquid water.
Biological Importance: Ice floating on water insulates aquatic life during cold periods.

Water as the Solvent of Life

Solutions and Solubility

Solution: A homogeneous mixture of two or more substances.
Solvent: The dissolving agent (water in aqueous solutions).
Solute: The substance that is dissolved.
Aqueous Solution: A solution in which water is the solvent.
Hydration Shell: The sphere of water molecules surrounding each dissolved ion.

Hydrophilic vs. Hydrophobic Substances

Hydrophilic: Substances that have an affinity for water (ionic or polar compounds).
Hydrophobic: Substances that repel water (nonpolar compounds, such as oils).

Example: Cell membranes are composed of hydrophobic lipid molecules, which prevent water from freely passing through.

Concentration of Solutes in Aqueous Solutions

Moles and Molarity

Mole (mol): The amount of substance containing 6.02 × 1023 molecules (Avogadro's number).
Molecular Mass: The sum of the atomic masses of all atoms in a molecule (g/mol).
Molarity (M): The number of moles of solute per liter of solution.

Formula:

Example Calculation: To make a 1 M NaCl solution, dissolve 58 g of NaCl (molecular mass = 58 g/mol) in 1 L of water.

Acids, Bases, and pH

Acid-Base Chemistry in Biology

Acid: A substance that increases the hydrogen ion (H+) concentration of a solution (pH < 7).
Base: A substance that reduces the H+ concentration (pH > 7).
pH Scale: Measures the concentration of H+ ions; ranges from 0 (most acidic) to 14 (most basic).
Neutral Solution: [H+] = [OH-], pH = 7.

Formulas:

Example: A solution with [H+] = 1 × 10-3 M has a pH of 3.

Buffers

Definition: Buffers are substances that minimize changes in pH by accepting or donating H+ ions.
Biological Importance: Most biological fluids are buffered to maintain a stable pH, crucial for enzyme function and cellular processes.
Example: The bicarbonate buffer system in blood helps maintain pH near 7.4.

Ocean Acidification

Causes and Consequences

Definition: Ocean acidification refers to the decrease in pH of ocean water due to absorption of excess atmospheric CO2.
Chemical Process: CO2 dissolves in seawater, forming carbonic acid, which dissociates to release H+ ions, lowering pH.
Impact: Increased H+ ions combine with carbonate ions, reducing their availability for marine organisms that build shells and skeletons from calcium carbonate.

Example: Coral reefs are threatened by ocean acidification, which impairs the ability of corals to produce their calcium carbonate skeletons.

Summary Table: Properties of Water

Property	Description	Biological Importance
Cohesion	Hydrogen bonds hold water molecules together	Enables transport of water in plants
High Specific Heat	Water resists temperature changes	Stabilizes climate and organismal temperature
Expansion Upon Freezing	Ice is less dense than liquid water	Insulates aquatic life in winter
Versatility as a Solvent	Dissolves many substances	Facilitates biochemical reactions

Practice Problems and Calculations

Molarity Calculation: Calculate the mass of NaOH needed to make a 0.100 M solution (Na = 23, O = 16, H = 1; molar mass = 40 g/mol).
pH Calculation: What is the pH of a 0.0235 M HCl solution?
Buffer Calculation: If a solution contains 3.60 × 10-3 mol NaOH and 5.95 × 10-4 mol HCl in 1.00 L, calculate the resulting pH.

Additional info: These notes are based on Campbell Biology, Chapter 3, and are suitable for introductory college-level biology courses.