Water and Life: Properties and Biological Importance

Chapter 3: Water and Life

Overview: The Molecule That Supports Life

Water is the essential biological medium on Earth, playing a critical role in the survival and function of all living organisms. Its abundance and unique properties make Earth habitable and support cellular processes.

• Water is the most required substance for all living organisms.

• Cells are composed of 70–95% water, and are surrounded by water in their environment.

• Earth's habitability is largely due to the presence and properties of water.

Structure and Polarity of Water Molecules

The water molecule (H2O) is a polar molecule, meaning it has regions of partial positive and negative charge due to the unequal sharing of electrons between oxygen and hydrogen atoms.

• Polarity: Oxygen is more electronegative than hydrogen, resulting in a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

• Hydrogen bonds: The polarity allows water molecules to form hydrogen bonds with each other, which are weak interactions but collectively strong in large numbers.

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

Unique Chemical Properties of Water

Water's polar structure gives rise to several chemical properties that are essential for life. Four key properties facilitate a life-supporting environment:

• Cohesive behavior: Water molecules stick together due to hydrogen bonding.

• Ability to moderate temperature: Water can absorb and release heat with minimal temperature change.

• Expansion upon freezing: Water becomes less dense as a solid, allowing ice to float.

• Versatility as a solvent: Water can dissolve a wide variety of substances due to its polarity.

Cohesion and Adhesion of Water Molecules

Cohesion and adhesion are two important properties of water that result from hydrogen bonding and polarity.

• Cohesion: The attraction between water molecules due to hydrogen bonds. This property allows water to move as a continuous column in plant vessels.

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

• Example: Transpiration in plants relies on both cohesion and adhesion to transport water from roots to leaves.

Transpiration in Plants

Transpiration is the process by which water moves up through plants, from roots to leaves, and evaporates into the atmosphere. This movement is facilitated by both cohesion and adhesion.

• Water-conducting cells: Specialized cells in plants allow water to move upward.

• Cohesion: Water molecules stick together, forming a continuous column.

• Adhesion: Water molecules adhere to the walls of the conducting cells, helping counteract gravity.

• Example: The diagram illustrates water movement in a tree, showing the roles of cohesion and adhesion in transpiration.

Additional info:

• Surface tension: Water's cohesion results in high surface tension, making it difficult to break the surface of a liquid.

• Biological significance: These properties are crucial for processes such as nutrient transport in plants and temperature regulation in organisms.