Water and the Environment: Properties, Functions, and Biological Importance

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WATER AND THE ENVIRONMENT

CHAPTER 3 Polarity of Water Molecules

Water is a unique molecule with properties essential for life. Its polarity and ability to form hydrogen bonds underlie many of its biological functions.

Polarity: Water molecules are polar because of the unequal sharing of electrons between oxygen and hydrogen atoms, resulting in a partial negative charge near oxygen and a partial positive charge near hydrogen.
Hydrogen Bonding: The polarity of water allows it to form hydrogen bonds, which are weak attractions between the hydrogen atom of one water molecule and the oxygen atom of another.
Example: Water's polarity enables it to dissolve many substances, making it an excellent solvent.

Functions of Water in Earth's Systems

Water's physical and chemical properties contribute to its vital roles in biological and environmental systems.

Cohesion and Adhesion

Cohesion: The attraction between water molecules due to hydrogen bonding, holding molecules together. This property is responsible for surface tension.
Surface Tension: The measure of how difficult it is to stretch or break the surface of a liquid. Water has a high surface tension due to cohesion.
Adhesion: The attraction between water molecules and other substances. Adhesion helps water climb up plant roots and stems (capillary action).
Example: Both cohesion and adhesion help transport water from roots to leaves in plants.

Moderation of Temperature

Kinetic Energy: The energy of motion; molecules move faster at higher temperatures.
Heat: The total kinetic energy due to molecular motion in a body of matter.
Temperature: Measures the average kinetic energy of molecules. Common units include Celsius (°C), Calorie (cal), Kilocalorie (kcal), and Joule (J).
Specific Heat: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C. Water has a high specific heat, meaning it resists temperature changes.
Heat of Vaporization: The amount of heat a liquid must absorb for 1 gram to be converted to gas.
Evaporative Cooling: As water evaporates, the surface cools because the hottest molecules leave first.
Example: Sweating cools the body through evaporative cooling.

Ice Insulates

Water is most dense at 4°C. Ice floats because it is less dense than liquid water, insulating aquatic environments in winter.

Water as a Solvent

Solution: A homogeneous mixture of two or more substances.
Solvent: The dissolving agent (usually water in biological systems).
Solute: The substance that is dissolved.
Hydration Shell: The sphere of water molecules around each dissolved ion.
Hydrophilic Substances: Water-loving; substances that dissolve easily in water.
Hydrophobic Substances: Water-fearing; substances that do not dissolve in water (e.g., oils).
Colloid: A stable suspension of fine particles in a liquid.

Solute Concentration in Aqueous Solutions

Molecular Mass: The sum of the masses of all atoms in a molecule.
Mole: A unit representing 6.022 x 1023 molecules (Avogadro's number).
Molarity: The number of moles of solute per liter of solution.
Example: To make a 1 M solution of NaCl, dissolve 58.44 g (1 mole) of NaCl in 1 liter of water.

Acidic and Basic Conditions That Affect Living Organisms

Water can dissociate into ions, affecting pH and biological processes.

pH – Measure of Acidity

pH: The negative logarithm of the hydrogen ion concentration:
Hydrogen Ion (H+): Increases acidity.
Hydroxide Ion (OH-): Increases basicity.
Acid: Substance that increases H+ concentration.
Base: Substance that reduces H+ concentration.
pH Scale: Ranges from 0 (most acidic) to 14 (most basic); 7 is neutral.
Example: Lemon juice has a pH of about 2 (acidic); household ammonia has a pH of about 11 (basic).

Function of Buffers

Buffers: Substances that minimize changes in concentrations of H+ and OH- in a solution.
Buffers work by accepting H+ when they are in excess and donating H+ when they are depleted.
Example: Blood contains bicarbonate buffer to maintain pH near 7.4.

Acids in Our Environment

Ocean Acidification: The decrease in ocean pH due to absorption of excess atmospheric CO2.
CO2 dissolves in seawater, forming carbonic acid, which lowers pH and affects marine life.
Example: Ocean acidification threatens coral reefs and shell-forming organisms.

Summary Table: Key Properties of Water

Property	Description	Biological Importance
Cohesion	Water molecules stick together	Transport in plants
Adhesion	Water molecules stick to other substances	Capillary action
High Specific Heat	Resists temperature change	Stabilizes climate and body temperature
Solvent Ability	Dissolves many substances	Facilitates chemical reactions
Ice Insulation	Ice floats on water	Protects aquatic life in winter
pH Buffering	Minimizes pH changes	Maintains homeostasis

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