Properties of Water and Their Biological Significance

Properties of Water

Introduction

Water is a unique molecule essential for life, exhibiting several properties that support biological systems. Its molecular structure and interactions give rise to characteristics such as cohesion, adhesion, high specific heat, and solvent abilities, all of which are crucial for living organisms.

Key Properties of Water

• Cohesion: The attraction between water molecules due to hydrogen bonding. This property allows water to form droplets and contributes to surface tension.

• Adhesion: The attraction between water molecules and other substances, enabling capillary action in plants.

• Surface Tension: The result of cohesive forces at the surface of water, allowing it to resist external force and enabling small organisms to move across its surface.

• High Specific Heat: Water can absorb or release large amounts of heat with little change in its own temperature. This property helps stabilize environmental and body temperatures.

• High Heat of Vaporization: A significant amount of energy is required to convert water from liquid to gas, which allows for effective cooling through evaporation (e.g., sweating).

• Universal Solvent: Water's polarity enables it to dissolve a wide variety of substances, making it an excellent medium for chemical reactions in cells.

• Density of Ice: Ice is less dense than liquid water due to the hydrogen-bonded lattice structure, causing ice to float and insulate aquatic environments.

Structure and Polarity of Water

• Polar Molecule: Water (H2O) has a bent shape with an angle of about 104.5°, resulting in a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

• Hydrogen Bonds: Weak attractions between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another. These bonds are responsible for many of water's unique properties.

Hydrogen Bonding

• Definition: A hydrogen bond is a weak attraction between a hydrogen atom covalently bonded to an electronegative atom (like oxygen) and another electronegative atom.

• Biological Importance: Hydrogen bonds stabilize the structure of proteins and nucleic acids and are crucial for the properties of water.

• Example: The double helix structure of DNA is maintained by hydrogen bonds between complementary base pairs.

Water as a Solvent

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

• Hydrophilic Substances: Molecules that dissolve easily in water (e.g., salts, sugars) due to their polarity or charge.

• Hydrophobic Substances: Nonpolar molecules (e.g., oils, fats) that do not dissolve in water.

• Example: Table salt (NaCl) dissolves in water as the positive and negative ions are surrounded by water molecules (hydration shell).

Acids, Bases, and pH

• Acid: A substance that increases the hydrogen ion (H+) concentration in a solution.

• Base: A substance that reduces the hydrogen ion concentration, often by accepting H+ or releasing OH- ions.

• pH Scale: Measures the concentration of H+ ions in a solution. Defined as $pH = -\log[H^+]$.

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

• Example: Blood contains buffers to maintain a stable pH despite metabolic changes.

Biological Significance of Water's Properties

• Temperature Regulation: High specific heat and heat of vaporization help organisms maintain stable internal temperatures.

• Transport: Cohesion and adhesion facilitate the movement of water (and dissolved nutrients) in plants via capillary action.

• Cellular Environment: Water's solvent properties enable biochemical reactions and transport of substances within cells.

• Climate Moderation: Large bodies of water moderate Earth's climate by absorbing and releasing heat slowly.

Tables

Comparison of Hydrophilic and Hydrophobic Substances

Property	Hydrophilic	Hydrophobic
Polarity	Polar or charged	Nonpolar
Solubility in Water	High	Low
Examples	Salts, sugars	Oils, fats

Key Terms and Definitions

Term	Definition
Cohesion	Attraction between molecules of the same substance
Adhesion	Attraction between molecules of different substances
Surface Tension	The measure of how difficult it is to stretch or break the surface of a liquid
Specific Heat	The amount of heat required to raise the temperature of 1 gram of a substance by 1°C
Heat of Vaporization	The amount of energy required to convert 1 gram of a liquid into a gas
Buffer	A substance that minimizes changes in pH

Additional Info

• Hydrogen bonds are weaker than covalent bonds but collectively provide significant stability to water's structure.

• Water's high heat capacity is due to the energy required to break hydrogen bonds before temperature can increase.

• Acid rain and ocean acidification are environmental issues related to changes in water's pH due to human activity.

• Cell membranes are composed primarily of hydrophobic molecules (phospholipids) to create a barrier between the cell and its environment.