Chapter 3: Water and Life – Properties, Structure, and Biological Importance

Chapter 3: Water and Life

Learning Objectives

• Explain how hydrogen bonding results from polar covalent bonds in water molecules.

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

• Differentiate between an acid and a base; define pH and describe how it affects biological processes.

Polar Covalent Bonds and Hydrogen Bonding in Water

Structure of a Water Molecule

Water (H2O) is a simple molecule with unique properties due to its structure and bonding:

• It has a V-shaped (bent) geometry.

• Composed of two hydrogen (H) atoms and one oxygen (O) atom.

• Each hydrogen is joined to oxygen by a single polar covalent bond.

Electronegativity Difference

• Oxygen is more electronegative than hydrogen, meaning it attracts shared electrons more strongly.

Polarity of the Water Molecule

• Water is a polar molecule due to unequal sharing of electrons.

• This results in partial charges: oxygen has a partial negative charge (δ-), hydrogens have partial positive charges (δ+).

Hydrogen Bonding

• Water molecules can form hydrogen bonds with each other.

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

Example: The diagram shows water molecules forming hydrogen bonds, with partial charges indicated.

Emergent Properties of Water Essential for Life

Overview of Four Key Properties

Water's unique properties, resulting from hydrogen bonding, make it vital for life:

• Cohesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion and Adhesion

Cohesion of Water Molecules

• Hydrogen bonds hold water molecules together, a phenomenon called cohesion.

• Cohesion is responsible for water's high surface tension, making it difficult to stretch or break the surface.

Surface Tension

• Surface tension is a measure of how hard it is to break the surface of a liquid.

• Water's high surface tension allows small insects to walk on its surface.

Role in Plants – Transport Against Gravity

• Cohesion helps water move upward through plant vessels, supporting the transport of water and nutrients from roots to leaves, even against gravity.

Adhesion to Other Surfaces

• Adhesion is the attraction between different substances, such as water sticking to plant cell walls.

• Adhesion helps water resist the downward pull of gravity, aiding upward movement in plants.

Example: In plants, both cohesion and adhesion enable water to travel from roots to leaves, as shown in the diagram of water movement in a tree.

Additional info:

• These properties are critical for processes such as transpiration in plants and maintaining stable environments for aquatic life.