Water, Carbon, and the Molecules of Life: Study Notes for General Biology

Water, Carbon, and the Molecules of Life

Introduction

This section explores the fundamental roles of water and carbon in biological systems, focusing on their chemical properties and significance for life. Understanding these molecules is essential for grasping the molecular basis of life processes.

Water

Importance of Water in Biology

• Universal Medium: Water is the most common medium for life on Earth and possibly elsewhere in the universe.

• Biological Prevalence: Organisms are composed mostly of water, and cellular processes occur in aqueous environments.

• Cellular Context: Cells are surrounded by water, and their internal environment is primarily aqueous.

Chemical Structure of Water

• Polar Covalent Bonds: Each water molecule (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom. The electrons are shared unequally, making the molecule polar.

• Hydrogen Bonding: The polarity of water molecules allows them to form hydrogen bonds with each other, which are weak individually but strong collectively.

• Partial Charges: The oxygen atom carries a partial negative charge (δ−), while the hydrogen atoms carry partial positive charges (δ+).

Emergent Properties of Water

Water exhibits several unique properties that arise from its structure and hydrogen bonding:

• Cohesive Behavior: Water molecules stick to each other due to hydrogen bonding, resulting in high surface tension. This allows certain organisms, like water striders, to walk on water.

• Ability to Moderate Temperature: Water can absorb or release large amounts of heat with only a slight change in its own temperature, due to its high specific heat capacity. This property helps stabilize temperatures in organisms and environments.

• Expansion Upon Freezing: As water freezes, hydrogen bonds stabilize and keep molecules further apart, making ice less dense than liquid water. This is why ice floats, providing insulation for aquatic life in cold climates.

• Versatility as a Solvent: Water is known as the "universal solvent" because it can dissolve a wide variety of substances, especially ionic and polar compounds, facilitating chemical reactions in cells.

Key Terms

• Solvent: The dissolving agent of a solution (e.g., water in an aqueous solution).

• Solute: The substance that is dissolved in a solution.

• Aqueous Solution: A solution in which water is the solvent.

Example

• Polar bears rely on the unique properties of water and ice for their habitat, as shown by their dependence on floating ice in the Arctic.