Chapter 3: Water and Its Importance for Life

Overview of Water's Role in Life

• Water is essential for life as we know it, with organisms being composed of over 70% water by mass.
• When exploring other planets or moons for potential life, the presence of water is a primary indicator.
• All living organisms, even terrestrial ones, are dependent on water for cellular functions and nutrient exchange.

Water Chemistry

Polar Nature of Water

• Water is a polar molecule due to its bent shape and the differences in electronegativity between oxygen (high) and hydrogen (low).
• This polarity leads to partial charges: oxygen becomes slightly negative, and hydrogen becomes slightly positive.

Hydrogen Bonds

• Water molecules can form hydrogen bonds with each other, leading to significant cohesive properties.
• Cohesion allows water to stick to itself, while adhesion allows it to stick to other substances.

Key Properties of Water

Cohesion and Adhesion

• Cohesion: Water molecules stick to themselves, forming droplets rather than breaking apart.
• Adhesion: Water molecules stick to other substances, such as metal or paper, leading to phenomena like capillarity.

Capillarity

• Water can climb against gravity, demonstrated by water rising in a straw or soaking into paper towel.
• Critical for plant water transport through thin tubes without external pumps.

High Surface Tension

• Water's cohesive properties create high surface tension, allowing small, light objects (like water striders) to walk on its surface.

High Specific Heat

• Water requires significant energy to change temperatures, providing stability for organisms in aquatic environments.
• Protects organisms from rapid temperature fluctuations, aiding in homeostasis.

Density and Ice Formation

• Water is unique in that ice (solid water) is less dense than liquid water, allowing it to float.
• This property is crucial for aquatic life in winter, as ice forms a protective layer on top of lakes, insulating the water below.

Universal Solvent

• Water is known as the universal solvent due to its ability to dissolve many substances, particularly ionic and polar compounds.
• It does not dissolve nonpolar substances, such as oils.

pH and Water

Water Ionization

• Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-).
• Pure water has a neutral pH of 7, where concentrations of H+ and OH- are equal.

Understanding pH Scale

• pH scale is logarithmic: a change of one unit represents a tenfold change in ion concentration.
• Acids increase H+ concentration (pH < 7), while bases increase OH- concentration (pH > 7).

Importance of pH for Aquatic Organisms

• Aquatic organisms are sensitive to pH changes; shifts can be detrimental to their survival.

Buffers

• Buffers help maintain stable pH levels in organisms, resisting changes when acids or bases are added.
• They can absorb excess H+ or release H+ to balance pH.
• Example: Carbonate ions (CO3^2-) and bicarbonate (HCO3^-) help regulate pH in blood.

Conclusion

• Water's unique properties make it integral to life and critical for maintaining homeostasis in organisms.