Water and Life: Properties and Importance of Water in Biology

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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 it essential for biological processes and the maintenance of life on Earth.

Water makes life possible on Earth by providing a medium for chemical reactions and supporting cellular structure.
Water is the only common substance that exists naturally in all three physical states (solid, liquid, gas) in Earth's environment.
Emergent properties of water contribute to Earth's suitability for life, including cohesion, temperature moderation, expansion upon freezing, and versatility as a solvent.
The structure of the water molecule allows it to interact with other molecules through hydrogen bonding.

Structure and Polarity of Water

Polar Covalent Bonds and Hydrogen Bonding

The water molecule (H2O) has a unique structure that leads to its polarity and ability to form hydrogen bonds.

Polar covalent bonds: In water, electrons spend more time near the oxygen atom than the hydrogen atoms, creating a partial negative charge (δ-) on oxygen and partial positive charges (δ+) on hydrogens.
Polar molecule: Water has an uneven distribution of charge, making it a polar molecule.
Hydrogen bonds: Weak attractions form between the partially positive hydrogen of one water molecule and the partially negative oxygen of another. These are called hydrogen bonds.

Example: The polarity of water allows it to form up to four hydrogen bonds with neighboring water molecules, leading to a highly cohesive liquid.

Emergent Properties of Water

1. Cohesion and Adhesion

Cohesion and adhesion are two key properties of water that result from hydrogen bonding.

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, making it difficult to break the surface due to the collective strength of hydrogen bonds at the surface.

Example: Water droplets form beads on a waxed car hood due to high surface tension.

2. Moderation of Temperature

Water can absorb or release large amounts of heat with only slight changes in its own temperature, helping to stabilize environmental and organismal temperatures.

High specific heat: Water's specific heat is high because much of the energy is used to break hydrogen bonds before increasing molecular motion (temperature).
Heat absorption and release: Heat is absorbed when hydrogen bonds break and released when they form.

Example: Coastal areas have milder climates than inland areas due to water's ability to moderate temperature.

3. Expansion Upon Freezing

Water is less dense as a solid (ice) than as a liquid, which is unusual among substances.

Hydrogen bonds in ice: In solid form, water molecules are held in a rigid lattice by stable hydrogen bonds, keeping them farther apart than in liquid water.
Ice floats: Because ice is less dense than liquid water, it floats, insulating the water below and enabling aquatic life to survive in cold climates.

Example: Lakes and ponds do not freeze solid in winter, allowing organisms to live beneath the ice.

4. Versatility as a Solvent

Water is known as the "universal solvent" due to its ability to dissolve a wide variety of substances.

Solution: A homogeneous mixture of two or more substances.
Solvent: The dissolving agent (water in aqueous solutions).
Solute: The substance that is dissolved.
Polarity: Water's polarity allows it to surround and dissolve ionic and other polar molecules.
Hydrophilic substances: Substances with an affinity for water (e.g., salts, acids, bases, carbohydrates).
Hydrophobic substances: Substances that do not interact well with water (e.g., lipids, hydrocarbons).

Example: Table salt (NaCl) dissolves readily in water due to the attraction between water molecules and the ions.

Concentration in Aqueous Solutions

Molecular Mass and Molarity

Understanding the concentration of solutes in water is essential for studying biological reactions.

Molecular mass: The sum of the masses of all atoms in a molecule (measured in Daltons).
Mole: A unit representing 6.02 × 1023 molecules (Avogadro's number).
Molarity (M): The number of moles of solute per liter of solution.

Formula:

Acids, Bases, and pH

Acidic and Basic Conditions Affect Organisms

The concentration of hydrogen ions (H+) and hydroxide ions (OH-) in water determines its acidity or basicity, which is critical for biological systems.

Acid: A substance that increases the H+ concentration of a solution.
Base: A substance that reduces the H+ concentration, often by increasing OH- concentration.
Hydronium ion (H3O+): Formed when a water molecule gains a proton.
Hydroxide ion (OH-): Formed when a water molecule loses a proton.

Water dissociation:

The pH Scale

The pH scale measures the concentration of hydrogen ions in a solution, indicating its acidity or basicity.

pH: Defined as the negative logarithm of the hydrogen ion concentration.

Formula:

In pure water, M, so (neutral).
Acidic solutions have pH < 7; basic solutions have pH > 7.

Examples of pH Values

Substance	pH
Lemon juice, vinegar	2-3
Cola, wine	3
Human blood, tears	7.4
Household ammonia	11-12
Household bleach	12-13

pH Calculations and Changes

Each unit change in pH represents a tenfold change in [H+].
For example, increasing [H+] by a factor of 1,000 lowers the pH by 3 units.

Example: If a solution's pH changes from 8 to 5, [H+] increases by 1,000 times.

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