Chapter 3: Water & Life – Properties and Importance of Water in Biology

Chapter 3: Water & Life

Introduction

Water is a fundamental molecule for 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 Supports All of Life

Importance of Water for Life

• Water makes life possible on Earth: About 70% of the planet's surface is covered by water.

• Cellular environment: Most cells are surrounded by water, and their internal environment is 70–95% water.

• Human body composition: Humans are composed of approximately 70% water.

• Physical states: Water is the only common substance that exists naturally in all three physical states—solid, liquid, and gas—within Earth's environment.

• Shape and charge: The molecular shape and charge distribution of water are crucial for its biological functions.

Structure of Water – Hydrogen Bonding

Chemical Structure and Bonding

• Polarity: Water (H2O) is a polar molecule. Oxygen is highly electronegative, attracting electrons more strongly than hydrogen.

• Polar covalent bonds: The bonds between oxygen and hydrogen in water are polar covalent bonds, resulting in a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

• Hydrogen bonds: Water molecules are attracted to each other by hydrogen bonds, which form between the slightly positive hydrogen of one molecule and the slightly negative oxygen of another.

• Energy of hydrogen bonds: Hydrogen bonds require a significant amount of energy to break and release energy when formed, contributing to water's high specific heat and other properties.

Example: The diagram shows water molecules connected by hydrogen bonds, with regions of partial positive and negative charge.

Emergent Properties of Water

Key Properties Facilitating Life

Water exhibits four emergent properties that are critical for supporting life:

• Cohesive and adhesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion & Adhesion

Definitions and Biological Significance

• Cohesion: The attraction between water molecules due to hydrogen bonding. This property allows water molecules to stick together, contributing to phenomena such as surface tension.

• Adhesion: The attraction between water molecules and different substances, such as the walls of plant cells. Adhesion helps water move against gravity in plant vessels (capillary action).

Example: In plants, cohesion holds water molecules together as they move up through the plant, while adhesion helps water stick to the cell walls, facilitating transport from roots to leaves.

Property	Description	Biological Example
Cohesion	Hydrogen bonds hold water molecules together	Water droplets forming on a leaf
Adhesion	Attraction between water and other substances	Water climbing up plant cell walls

Additional info: The ability of water to form hydrogen bonds underlies all its emergent properties, making it indispensable for life processes such as temperature regulation, nutrient transport, and chemical reactions in cells.