Chapter 3 Notes Part B (Acidic and Basic Conditions Affecting Living Organisms: Water Dissociation, Acids, Bases, pH, and Buffers)

Acidic and Basic Conditions Affect Living Organisms

Dissociation of Water Molecules

The dissociation of water molecules is a fundamental chemical process that influences the acidity and basicity of biological systems. This process involves the reversible transfer of a hydrogen atom between two water molecules, resulting in the formation of ions.

• Hydrogen Ion Formation: A hydrogen atom leaves its electron behind and is transferred as a proton, forming a hydrogen ion (H+).

• Hydronium Ion: The water molecule that gains the extra proton becomes a hydronium ion (H3O+). In biological contexts, H+ is often used as shorthand for H3O+.

• Hydroxide Ion: The molecule that loses the proton becomes a hydroxide ion (OH-).

• Equilibrium: In pure water, only a very small fraction of molecules dissociate, but this process is critical for cellular chemistry.

Equation:

$2H_2O \rightleftharpoons H_3O^+ + OH^-$

Example: Changes in the concentrations of H+ and OH- can drastically affect proteins and other complex molecules in cells.

Acids and Bases

Definitions and Properties

Acids and bases are substances that alter the concentration of hydrogen ions (H+) and hydroxide ions (OH-) in aqueous solutions. Their behavior is essential for understanding biological pH regulation.

• Acid: Any substance that increases the H+ concentration by donating H+ to the solution. Example: Hydrochloric acid (HCl) dissociates as follows: $\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$

• Base: Any substance that reduces the H+ concentration by accepting H+ or by adding OH- to the solution. Example: Ammonia (NH3) accepts H+: $\text{NH}_3 + \text{H}^+ \rightleftharpoons \text{NH}_4^+$ Sodium hydroxide (NaOH) dissociates: $\text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^-$

• Neutral Solution: Concentrations of H+ and OH- are equal.

pH Values and Biological Fluids

The pH scale measures the concentration of hydrogen ions in a solution, which determines whether it is acidic, basic, or neutral. Most biological fluids maintain a pH close to neutral, with some exceptions.

• Acidic Solutions: pH < 7 (high H+ concentration)

• Basic Solutions: pH > 7 (low H+ concentration)

• Neutral Solutions: pH = 7 (equal H+ and OH- concentrations)

• Biological Range: Most biological fluids have pH values between 6 and 8. Exception: Stomach fluid (gastric juice) has a pH of ~2.

Solution Type	pH Value	H+ Concentration	OH- Concentration
Acidic	< 7	High	Low
Neutral	7	Equal	Equal
Basic	> 7	Low	High

The pH (Power of Hydrogen) Scale

Definition and Calculation

The pH scale is logarithmic and quantifies the concentration of hydrogen ions in a solution. The product of H+ and OH- concentrations in any aqueous solution at 25°C is constant.

• Constant Product: $[H^+][OH^-] = 10^{-14}$

• pH Definition: pH is the negative logarithm of the H+ concentration: $\text{pH} = -\log [H^+]$

• Neutral Solution Example: For [H+] = $10^{-7}$ M, $\text{pH} = -\log(10^{-7}) = 7$

• Logarithmic Nature: Each unit change in pH represents a tenfold change in H+ concentration.

Example: A solution with pH 3 is 1,000 times more acidic than a solution with pH 6.

Buffers

Role in Biological Systems

Buffers are crucial for maintaining stable pH in living organisms, as cellular biochemistry is highly sensitive to pH changes. Most cells and organisms use buffers to minimize fluctuations in H+ and OH- concentrations.

• Function: Buffers accept H+ when they are in excess and donate H+ when they are depleted.

• Composition: Most buffers consist of a weak acid and its corresponding base, which can reversibly combine with H+.

• Example: The bicarbonate buffer system in human blood involves carbonic acid (H2CO3) and bicarbonate ion (HCO3-): $\text{H}_2\text{CO}_3 \rightleftharpoons \text{HCO}_3^- + \text{H}^+$

• Biological Importance: Human blood pH is maintained at ~7.4; deviations below 7.0 or above 7.8 are life-threatening.

Chapter 3 Key Concepts Summary

• Water is essential to life: It is a polar molecule capable of forming hydrogen bonds.

• Emergent properties of water: Cohesion, adhesion, temperature moderation, expansion upon freezing, and versatility as a solvent.

• Hydrophilic vs. Hydrophobic: Hydrophilic substances interact well with water; hydrophobic substances do not.

• Acids and Bases: Defined by their effect on H+ concentration.

• Buffers: Maintain stable pH in biological systems.