Rucete ✏ AP Biology In a Nutshell
1. Water
This chapter introduces the unique properties of water that make it essential for life, including its molecular structure, hydrogen bonding, specific heat, solvent properties, and the concept of pH in biological systems.
Water and the Importance of Hydrogen Bonds
• Water is a polar molecule due to unequal sharing of electrons between hydrogen and oxygen atoms.
• The oxygen atom has a partial negative charge, while hydrogen atoms have partial positive charges.
• This polarity allows water molecules to form hydrogen bonds with each other.
• Hydrogen bonds give water several life-supporting properties:
– Cohesion and adhesion: Water molecules stick to each other and to other polar substances, aiding in surface tension and capillary action in plants.
– High specific heat: Water resists temperature change, stabilizing climates and body temperatures through evaporative cooling.
– Climate moderation: Large bodies of water absorb and release heat slowly, buffering temperature extremes.
– Expansion upon freezing: Ice is less dense than liquid water due to hydrogen bond structure, allowing it to float and insulate aquatic environments in winter.
– Excellent solvent: Water dissolves many polar and ionic substances, making it essential for biological reactions and transport of nutrients and waste.
pH
• pH measures the concentration of hydrogen ions (H⁺) in a solution.
• The scale ranges from 0 to 14: – pH < 7 is acidic – pH = 7 is neutral – pH > 7 is basic
• The scale is logarithmic: each unit change represents a 10-fold difference in [H⁺].
• Pure water dissociates into equal concentrations of H⁺ and OH⁻, resulting in pH 7.
• Acids increase [H⁺]; bases increase [OH⁻].
• Biological systems rely on buffers to maintain pH stability.
• Example: The carbonic acid–bicarbonate buffer in blood helps maintain pH by responding to changes in H⁺ or OH⁻ levels via the following equilibrium:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
• This system neutralizes excess H⁺ or OH⁻ to keep cellular environments stable.
In a Nutshell
Water’s polarity and hydrogen bonding give it unique properties essential for life, including cohesion, high specific heat, expansion upon freezing, and solvent ability. The concept of pH, rooted in hydrogen ion concentration, is vital in biological systems and is regulated by buffers such as the carbonic acid–bicarbonate system in blood.