Biochemistry: Water, pH, and Buffers

• Biochemistry Lecture 2 covers water, pH and buffers.

Body Content of Water

• Water makes up approximately 60% of human body weight.
• Intracellular fluid comprises 2/3 of the body's water.
• Extracellular fluid makes up 1/3 of the body's water.
• The percentage of water varies with age, fat content, and gender.
  • Water content decreases with age and fat content.
  • Males generally have more body water than females.

Functions of Water

• Water is an ideal biologic solvent, solubilizing most biological molecules like proteins, nucleic acids, and carbohydrates.
• Water affects most molecular interactions in biological systems, modifying the properties of these molecules.
• Water is important for the transport of molecules and heat.
• Water is a product of oxidative metabolism of food.
• Water is a reactant in many metabolic reactions.
• The hydrogen ion content and the amount of body water are controlled to maintain a constant environment for the cells, which is called homeostasis.
• Significant deviations from a constant environment, such as acidosis or dehydration, may be life-threatening.

Structure of a Water Molecule

• A water molecule consists of two hydrogen atoms bonded to an oxygen atom by a covalent bond.
• The angle between the H atom and O atom is 105°.
• A water molecule has a slightly skewed tetrahedron shape with oxygen at its center.

Polarity of Water

• Water is a polar molecule.
• Oxygen is more electronegative than hydrogen, leading to asymmetry in charge distribution.
• Water solvates polar molecules due to H-bond interactions, weakening H-bonds and electrostatic bonds between polar molecules.
• Water is a dipolar molecule that forms H-bonds.

Acids and Bases

• An acid is a proton donor and a base is a proton acceptor according to Brønsted-Lowry.
• HA + H₂O ⇌ H₃O⁺ + A⁻ represents a conjugate acid-base pair.

Strong and Weak Electrolytes

• Strong electrolytes ionize completely in solution.
  • HCl → H⁺ + Cl⁻ (0.01M each).
• Weak electrolytes ionize only partially.
  • CH3COOH + H2O ⇌ CH3COO⁻ + H⁺.

Ion Product for Water (K)

• Water is an extremely weak electrolyte that undergoes autoionization.
• Autoionization forms very small, but equal, amounts of hydronium and hydroxide ions.
• The concentration of H₂O is very high (55.5M) relative to that of [H+] and [OH-].
• K = [H+][OH-] = 1.0 X 10⁻¹⁴ M² at 25°C.
• The ion product of water is K.
• In pure water, [H+] = [OH-] = 1 x 10⁻⁷ M.

pH

• pH expresses very low values of [H⁺] in biological systems.
• Sorensen introduced the concept of pH.
• pH is the negative log₁₀ of hydrogen ion concentration.
• pH = -log₁₀[H⁺].
• The pH of 0.001 M HCl has a pH of 3.0

Physiological pH

• pH remains nearly constant in the human body.
• pH is approximately 7 inside cells.
• pH is 7.4 ± 0.05 in blood.

Henderson-Hasselbalch Equation

• The Henderson-Hasselbalch equation describes the behavior of weak acids and buffers.
• Weak acids and bases in solution do not fully dissociate.
• There is an equilibrium between the acid and its conjugate base.
• The equilibrium constant (Kₐ) can be calculated.
• For the reaction HA ⇌ A⁻ + H⁺, the equilibrium constant is Kₐ = [H⁺][A⁻]/[HA].
• The equilibrium dissociation constant for a weak acid is Kₐ = [H⁺][A⁻] / [HA].
• pH = pKₐ + log [A⁻]/[HA]

Buffers

• A buffer solution resists changes in pH upon the addition of acid or base.
• A buffer consists of a mixture of a weak acid & its conjugate base (CH3COOH/CH3COO⁻).
• A buffer can consist of a weak base and its conjugate acid (NH₃/NH₄⁺).
• Buffers accept hydrogen ions when they are in excess.
• Buffers donate hydrogen ions when they have been depleted.
• H₂CO₃ ⇌ H⁺ + HCO₃⁻ represents buffering action.
• The effectiveness of a buffer called buffer capacity depends on:
  • The concentration of buffering agents.
  • The ratio of the weak acid to its conjugate base.
• The maximum buffering capacity occurs at a ratio of one, where pH = pKₐ.

Physiological Buffers

• Acid/base balance is a major problem in clinical practice.
• H⁺ concentration in plasma & extracellular fluid is 40 nmol/L, which is pH 7.4.
• The normal pH range is 7.35-7.45, and pH below 7.0 or above 7.8 can have serious consequences.