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.

• 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 the processes of life.

Polar Covalent Bonds in Water Molecules Result in Hydrogen Bonding

Structure of a Water Molecule

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

• V-shaped (bent) geometry: Each water molecule consists of two hydrogen (H) atoms and one oxygen (O) atom, arranged in a bent shape.

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

Electronegativity Difference and Polarity

• Oxygen is more electronegative than hydrogen: Oxygen attracts the shared electrons more strongly, resulting in unequal sharing.

• Polarity: Water is a polar molecule, meaning it has regions of partial positive and negative charge.

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

Hydrogen Bonding

• Water molecules can form hydrogen bonds with each other.

• The δ+ hydrogen of one water molecule is attracted to the δ- oxygen of another, creating a network of hydrogen bonds.

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

Four Emergent Properties of Water Contribute to Earth's Suitability for Life

Overview of Water's Properties

Water's unique properties, resulting from hydrogen bonding, are essential for life on Earth. The four key emergent properties are:

• Cohesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion of Water Molecules

Cohesion refers to the attraction between molecules of the same substance. In water, this is primarily due to hydrogen bonding.

• Hydrogen bonds hold water molecules together, resulting in strong cohesion.

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

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: The attraction between different substances, such as water molecules and the walls of plant cells.

• Fighting gravity: Adhesion helps water resist the downward pull of gravity, aiding in the upward movement of water in plant tissues.

Example: In plants, both cohesion and adhesion are essential for water transport from roots to leaves, as illustrated in the diagram of water movement in a tree.

Additional info:

• Capillary action is the combined effect of cohesion and adhesion, allowing water to move through narrow tubes against gravity, such as in plant xylem.