2.2 Water - Biology 2e | OpenStax

Introduction to Water

• Water is essential to life, comprising 60-70% of the human body.
• It is vital for cellular chemistry and metabolism.
• Special properties include high heat capacity, vaporization, ability to dissolve polar molecules, cohesion, adhesion, and pH regulation.

Water's Polarity

• Water is composed of polar molecules (H₂O) where oxygen is more electronegative than hydrogen.
• This creates a slightly negative charge on oxygen and a slightly positive charge on hydrogen.
• Polarity leads to hydrogen bonding between water molecules and with other polar molecules.
• Hydrophilic substances interact with water, while hydrophobic substances do not.

States of Water: Gas, Liquid, and Solid

• Hydrogen bonds are crucial in the liquid state, breaking and forming as molecules move.
• In gas form, energy breaks bonds, allowing molecules to escape as vapor.
• Ice forms a crystalline structure because of hydrogen bonds, making it less dense than liquid water.
• Ice floats, providing an insulating layer for aquatic life in cold environments.

High Heat Capacity

• Water has the highest specific heat capacity of any liquid, allowing it to absorb and release heat slowly.
• This property helps regulate temperature in organisms and environments.

Heat of Vaporization

• High energy is required for water to transition from liquid to gas.
• Evaporation cools organisms by absorbing heat from the body, as seen in sweat.

Solvent Properties

• Water's polarity allows it to dissolve ionic and polar substances (solvent).
• It forms hydration shells around ions, facilitating dissolution.

Cohesive and Adhesive Properties

• Cohesion: Water molecules stick together, creating surface tension.
• Adhesion: Water interacts with other substances, seen in capillary action.
• These properties enable water transport in plants and allow insects to walk on water.

pH, Buffers, Acids, and Bases

• pH measures the acidity/basicity of a solution.
• Pure water has a neutral pH of 7.
• Acids increase H+ concentration, lowering pH.
• Bases increase OH- concentration, raising pH.
• Buffers maintain pH stability in organisms by absorbing excess H+ or OH-.

Buffers in Biological Systems

• Buffers like carbonic acid and bicarbonate maintain blood pH.
• They moderate pH changes by converting between carbon dioxide and bicarbonate.

Conclusion

• Water's unique properties are essential for life on Earth, affecting everything from cellular processes to maintaining ecosystems.