Water and Life: Properties and Biological Importance

Water and Life

The Molecule That Supports All of Life

Water is essential for all known forms of life and serves as the biological medium on Earth. Its unique molecular structure and properties make it indispensable for biological processes.

• Water is the only common substance that exists in the natural environment in all three physical states: solid, liquid, and gas.

• The structure of the water molecule allows it to interact with other molecules, facilitating various chemical reactions.

• Water’s unique emergent properties help make Earth suitable for life.

Structure and Bonding in Water

Polar Covalent Bonds and Hydrogen Bonding

The water molecule (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom. The electrons in these bonds are not shared equally, resulting in polarity.

• Polar covalent bonds in water cause electrons to spend more time near the oxygen atom than the hydrogen atoms.

• This unequal sharing makes water a polar molecule, with a partial negative charge near the oxygen and a partial positive charge near the hydrogens.

• Polarity allows water molecules to form hydrogen bonds with each other, which are weak interactions but collectively significant.

Example: The diagram shows water molecules forming hydrogen bonds, with the oxygen atom (red) attracting the hydrogen atoms (white) of neighboring molecules.

Emergent Properties of Water

Four Properties That Facilitate Life

Water’s molecular structure gives rise to four key properties that are crucial for supporting life on Earth.

• Cohesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion and Adhesion

Cohesion of Water Molecules

Cohesion refers to the attraction between molecules of the same substance. In water, hydrogen bonds hold the molecules together, resulting in high cohesion.

• Cohesion helps transport water against gravity in plants, allowing water to move from roots to leaves.

• Adhesion is the attraction between different substances, such as water and plant cell walls, aiding in water movement through plant tissues.

Example: Water moves upward in plants due to cohesion among water molecules and adhesion to cell walls.

Surface Tension

Surface tension is a measure of how difficult it is to break the surface of a liquid. Water has an unusually high surface tension due to hydrogen bonding.

• Hydrogen bonds at the air-water interface and with water below create a strong surface layer.

• This property allows small insects to walk on water and droplets to form.

Additional info:

• Further emergent properties such as temperature moderation, expansion upon freezing, and solvent versatility are covered in subsequent notes or textbook sections.