Biochemistry: Water, pH, and Buffers

• Biochemistry Lecture 2 covers water, pH, and buffers.

Body Content of Water

• Approximately 60% of human body weight is water.
• 2/3 of body water is intracellular fluid.
• 1/3 of body water is extracellular fluid.
• The percentage of water in the body varies with age (decreases), fat content (decreases), and gender (males have more body water than females).

Functions of Water in the Human Body

• Water is an ideal biological solvent.
• Water solubilizes proteins, nucleic acids, and carbohydrates.
• Interaction with water influences the structure of biomolecules.
• Water modifies the properties of molecules in biological systems.
• 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 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 is a slightly skewed tetrahedron with oxygen at its center.

Polarity of Water

• The water molecule is polar.
• The oxygen atom is more electronegative than the hydrogen atoms, resulting in an asymmetry in the distribution of charge.
• Water solvates polar molecules due to H-bond interactions.
• Water weakens H-bonds and electrostatic bonds between polar molecules.

Acids and Bases

• According to Brønsted-Lowry, an acid is a proton donor, and a base is a proton acceptor.
• HA + H₂O ⇌ H₃O⁺ + A⁻ represents a conjugate acid-base pair, where HA is the acid and A⁻ is its conjugate base.
• HA + H₃N: ⇌ NH₄⁺+ A⁻ represents a conjugate acid-base pair.

Strong and Weak Electrolytes

• Strong electrolytes ionize completely in solution, like HCl → H⁺ + Cl⁻.
• Weak electrolytes ionize only partially, like CH₃COOH + H₂O ⇌ CH₃COO⁻ + H⁺.

Ion Product for Water (Kw)

• Water is an extremely weak electrolyte that undergoes autoionization.
• Autoionization forms very small, but equal, amounts of hydronium and hydroxide ions.
• The equation for autoionization is: H₂O(L) + H₂O(L) ⇌ H₃O⁺(aq) + OH⁻(aq).
• K = [H+][OH-]/[H₂O]² or K[H₂O]² = [H+][OH⁻].
• 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 term (K) is referred to as the ion product of water.

pH

• To express very low values of [H⁺] in biological systems, the concept of pH was introduced by Sorensen.
• pH is the negative log₁₀ of hydrogen ion concentration: pH = -log₁₀[H⁺].
• Example: The pH of 0.001 M HCl is 3.
• The pH scale ranges from very acidic to very basic.
• The physiological pH must remain nearly constant in the human body.
• pH inside cells is approximately 7.
• pH in blood is approximately 7.4 ± 0.05.

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 for it (Kₐ) can be calculated.
• In the reaction of a weak acid: HA ⇌ A⁻ + H⁺.
• Kₐ= [H+][A⁻]/[HA].
• For the weak acid HA and its salt A: HA ⇌ H⁺ + A⁻.
• The equilibrium dissociation constant for a weak acid is: Kₐ = [H+][A⁻]/[HA].
• The Henderson Hasselbalch equation is 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 and its conjugate base (e.g., CH₃COOH/CH₃COO⁻) or a weak base and its conjugate acid (e.g., NH₃/NH₄⁺).
• Buffers accept hydrogen ions from the solution when they are in excess and donate hydrogen ions when they have been depleted.
• H₂CO₃ ⇌ H⁺ + HCO₃⁻ is an example of a buffering system.
• The effectiveness of a buffer, called buffer capacity, depends on two factors: the concentration of buffering agents and the ratio of the weak acid to its conjugate base.
• The maximum buffering capacity at a ratio of one: pH = pKₐ.

Physiological Buffers

• Acid/base balance is one of the major problems in clinical practice.
• H⁺ concentration in plasma and extracellular fluid is 40 nmol/L, i.e., pH 7.4.
• The normal pH range is 7.35-7.45.
• A pH below 7.0 or above 7.8 can have serious consequences.