Water and Life: Structure, Properties, and Biological Importance

Chapter 3: Water and Life

Learning Objectives

This chapter explores the molecular structure of water, its unique properties, and the critical role it plays in supporting life. Students should be able to:

• Explain how hydrogen bonding results from polar covalent bonds.

• Identify four properties of water important for life and describe how they result from hydrogen bonding.

• Differentiate between an acid and a base; define pH and describe its effects on biological processes.

Polar Covalent Bonds and Hydrogen Bonding in Water

Structure of a Water Molecule

Water's molecular structure underlies its unique chemical behavior and biological significance.

• V-shaped (bent) molecule: Water (H2O) consists of two hydrogen atoms and one oxygen atom.

• Single polar covalent bonds: Each hydrogen atom is joined to oxygen by a polar covalent bond.

• Electronegativity difference: Oxygen is more electronegative than hydrogen, pulling shared electrons closer to itself.

Polarity and Partial Charges

The unequal sharing of electrons in water's covalent bonds creates a polar molecule with distinct charge regions.

• Polar molecule: Opposite ends of the molecule have opposite charges.

• Partial charges: Oxygen atom has a partial negative charge (δ-), hydrogen atoms have partial positive charges (δ+).

Hydrogen Bonding

Hydrogen bonds are weak attractions between the partial positive hydrogen of one water molecule and the partial negative oxygen of another.

• Hydrogen bonds: Water molecules can form hydrogen bonds with each other, leading to many of water's unique properties.

• Example: The δ+ hydrogen of one water molecule is attracted to the δ- oxygen of another.

Emergent Properties of Water

Four Properties Contributing to Earth's Suitability for Life

Hydrogen bonding gives rise to four key properties of water that support life on Earth:

• Cohesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion and Surface Tension

Cohesion and surface tension are direct results of hydrogen bonding between water molecules.

• Cohesion: Hydrogen bonds hold water molecules together, resulting in the sticking together of water molecules.

• Surface tension: Cohesion creates high surface tension, making it difficult to stretch or break the surface of water.

Role in Plants: Transport Against Gravity

Water's cohesive and adhesive properties enable the upward movement of water and nutrients in plants.

• Upward transport: Cohesion helps water move upward through plant vessels, supporting nutrient transport from roots to leaves.

• Adhesion: Water molecules stick to the walls of plant cells, resisting the downward pull of gravity.

• Combined effect: Cohesion and adhesion together help water climb upward in plant tissues, essential for plant survival.

• Example: Evaporation from leaves pulls water upward, while adhesion to cell walls and cohesion between water molecules maintain the column of water.

Additional info:

• Expansion upon freezing: Water expands as it freezes due to the formation of a crystalline structure stabilized by hydrogen bonds, making ice less dense than liquid water. This property allows ice to float, insulating aquatic environments.

• Ability to moderate temperature: Water has a high specific heat capacity, meaning it can absorb or release large amounts of heat with minimal temperature change. This stabilizes climates and internal environments of organisms.

• Versatility as a solvent: Water's polarity allows it to dissolve a wide variety of substances, making it an excellent medium for chemical reactions in biological systems.

Visual Representation

Water Molecule and Hydrogen Bonding

The provided images illustrate the bent shape of water, the distribution of partial charges, and the formation of hydrogen bonds between molecules. In plants, diagrams show how cohesion and adhesion facilitate water transport against gravity.

Summary Table: Properties of Water

Property	Description	Biological Importance
Cohesion	Water molecules stick together via hydrogen bonds	Enables transport of water in plants
Surface Tension	Difficulty in breaking the surface of water	Allows small organisms to move on water surface
Adhesion	Water molecules stick to other surfaces	Helps water move upward in plant tissues
Expansion upon Freezing	Water becomes less dense as a solid	Ice floats, insulating aquatic life
High Specific Heat	Resists temperature change	Stabilizes climate and organismal temperature
Versatility as a Solvent	Dissolves many substances	Facilitates biochemical reactions

Key Terms

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

• Hydrogen bond: A weak attraction between a hydrogen atom with a partial positive charge and another atom with a partial negative charge.

• Cohesion: The tendency of molecules of the same kind to stick together.

• Adhesion: The tendency of different kinds of molecules to stick together.

• Surface tension: The measure of how difficult it is to stretch or break the surface of a liquid.

Example Application

• Water transport in plants: Cohesion and adhesion allow water to move from roots to leaves, supporting photosynthesis and nutrient distribution.

• Surface tension: Water striders and other insects can walk on water due to high surface tension.