Water and Life

Introduction

Water is fundamental to all life on Earth, exhibiting unique chemical and physical properties that make it indispensable for biological processes. This chapter explores the molecular structure of water, its emergent properties, and its role in supporting life.

Structure of Water Molecules

Polar Covalent Bonds and Hydrogen Bonding

The water molecule (H2O) consists of two hydrogen atoms covalently bonded to an oxygen atom. The electrons in these bonds spend more time near the oxygen, making water a polar molecule with an uneven charge distribution. This polarity enables water molecules to form hydrogen bonds with each other, which are weak attractions between the partially positive hydrogen and partially negative oxygen atoms of adjacent molecules.

• Polar covalent bond: A bond where electrons are shared unequally, resulting in partial charges.

• Hydrogen bond: A weak bond between the hydrogen atom of one molecule and the oxygen atom of another.

Emergent Properties of Water

Four Properties Facilitating Life

Water's unique properties arise from its molecular structure and hydrogen bonding. These properties are:

• Cohesive behavior: Water molecules stick together, resulting in high surface tension.

• Ability to moderate temperature: Water absorbs and releases heat with minimal temperature change.

• Expansion upon freezing: Ice is less dense than liquid water, allowing it to float.

• Versatility as a solvent: Water dissolves a wide range of substances due to its polarity.

Cohesion and Adhesion

Cohesion refers to the attraction between water molecules, which is responsible for surface tension. Adhesion is the attraction between water and other substances, such as plant cell walls, aiding in the transport of water against gravity in plants.

• Surface tension: The difficulty of stretching or breaking the surface of a liquid.

• Example: Water transport in plants relies on both cohesion and adhesion.

Moderation of Temperature

Water moderates temperature by absorbing heat from warmer air and releasing it to cooler air. Its high specific heat means it resists temperature changes, which is crucial for maintaining stable environments.

• Specific heat: The amount of heat required to change the temperature of 1 g of a substance by 1°C.

• Heat of vaporization: The heat needed for 1 g of liquid to become gas.

• Evaporative cooling: As water evaporates, the surface cools, stabilizing temperatures in organisms and environments.

Expansion Upon Freezing

Water is less dense as a solid than as a liquid due to the formation of a crystalline lattice by hydrogen bonds at 0°C. This allows ice to float, insulating bodies of water and supporting aquatic life.

• Density: Ice is about 10% less dense than liquid water.

• Ecological impact: Floating ice prevents bodies of water from freezing solid.

Water: The Solvent of Life

Water's polarity makes it a versatile solvent. It dissolves ionic compounds by surrounding ions with hydration shells and can also dissolve polar molecules, including large proteins.

• Solution: A homogeneous mixture of substances.

• Solvent: The dissolving agent (water in aqueous solutions).

• Solute: The substance dissolved.

• Hydrophilic: Substances with affinity for water.

• Hydrophobic: Substances that repel water, such as oils.

Solute Concentration in Aqueous Solutions

Moles, Molecular Mass, and Molarity

Chemical reactions in organisms often occur in aqueous solutions. The concentration of solutes is measured using moles, molecular mass, and molarity.

• Molecular mass: The sum of the masses of all atoms in a molecule.

• Mole: A unit representing 6.022 × 1023 molecules (Avogadro’s number).

• Molarity (M): Number of moles of solute per liter of solution.

Acidic and Basic Conditions

Dissociation of Water

Water molecules can dissociate, forming hydronium ions (H3O+) and hydroxide ions (OH-). This process is rare but crucial for biological chemistry.

Acids, Bases, and the pH Scale

An acid increases the concentration of H+ ions, while a base reduces it. The pH scale measures the acidity or basicity of a solution, defined as:

• pH = -log10[H+]

• Acidic solutions: pH < 7

• Basic solutions: pH > 7

• Most biological fluids: pH 6–8

Buffers

Buffers are substances that minimize changes in pH by accepting or donating H+ ions. Most buffers consist of a weak acid and its corresponding base, maintaining pH stability in biological systems.

• Example: Carbonic acid (H2CO3) acts as a buffer in blood.

Acidification: A Threat to Our Oceans

Human Impact and Ocean Acidification

Burning fossil fuels increases atmospheric CO2, a portion of which is absorbed by oceans, forming carbonic acid and lowering pH. This process, known as ocean acidification, reduces carbonate ion concentration, affecting marine organisms that rely on calcium carbonate for their shells and skeletons.

• Carbonate ions: Essential for calcification in corals and other marine life.

• Environmental concern: Acidification threatens biodiversity and ecosystem stability.

Possible Evolution of Life on Other Planets

Water as a Requirement for Life

Biologists searching for extraterrestrial life focus on planets with evidence of water, as it is considered essential for life. Mars, for example, has shown signs of water in its atmosphere and surface features.

Summary Table: Properties of Water