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 physical and chemical 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 cells themselves are composed of 70–95% water.

• Human body composition: Humans are approximately 70% water by mass.

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

• Shape and charge: The shape and charge distribution of the water molecule are crucial to its role in supporting life.

Structure of Water – Hydrogen Bonding

Polarity and Hydrogen Bonds

• Polarity: Oxygen is highly electronegative, while hydrogen is less so. This difference causes water molecules to form polar covalent bonds, resulting in a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

• Hydrogen bonding: The polarity of water molecules allows them to form hydrogen bonds with each other. These are weak bonds individually, but collectively they are strong and require a lot of energy to break.

• Energy dynamics: Hydrogen bonds require significant energy to break and release energy when formed.

Example: The high boiling point of water compared to similar-sized molecules is due to extensive hydrogen bonding.

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

These properties arise from the molecular structure and hydrogen bonding of water.

Cohesion & Adhesion

Interactions Between Water Molecules and Other Substances

• Cohesion: The tendency of water molecules to stick to each other due to hydrogen bonding. This property is responsible for phenomena such as surface tension.

• Adhesion: The attraction between water molecules and different substances (e.g., water and plant cell walls). Adhesion helps water move against gravity in plant vessels (capillary action).

Example: Water transport in plants relies on both cohesion (water molecules sticking together) and adhesion (water molecules sticking to cell walls).

Property	Description	Biological Importance
Cohesion	Hydrogen bonds hold water molecules together	Enables transport of water in plants, surface tension
Adhesion	Attraction between water and other substances	Helps water climb plant cell walls (capillary action)

Additional info: The combination of cohesion and adhesion is essential for the movement of water from roots to leaves in plants, a process known as transpiration.