HLTH1004 W2 L2.4.4

Buffers and Their Function

• Definition of Buffer: A compound that stabilizes pH by adding or removing hydrogen ions (H+).

  • Weak acids donate hydrogen ions (proton donor)

  • Weak bases absorb hydrogen ions (proton acceptor)

Buffer Systems in the Body

• Components of a Buffer System: Combination of a weak acid and ions from its dissociation.

• Three Main Buffer Systems:

  • Phosphate buffer system.

  • Protein buffer system.

  • Carbonic acid-bicarbonate buffer system.

Phosphate Buffer System

• Role: Buffers pH of intracellular fluid and urine.

  • Composed of a weak acid, H₂PO₄^- and its corresponding anion HPO₄^2- (a weak base).

  • Cells contain a weak base (Na₂HPO₄)

    • Provides additional HPO₄^2– for this buffer system

Protein Buffer System

• Relies on amino acids as they respond to pH changes by accepting or releasing H⁺

• Presence: Functions in both intracellular and extracellular fluids.

  • Hemoglobin Buffer System: Found in red blood cells, hemoglobin can pick up carbon dioxide.

  • Amino Acid Buffers: Proteins that can accept or release hydrogen ions.

• Most buffering capacity of proteins is provided by ‘R-groups’ of the amino acids

• At normal pH (7.35-7.45)

  • Carboxyl groups of most amino acids have already given up their H⁺

• If pH decreases

  • Carboxylate ion and amino group act as weak bases - accepting H⁺ → forming carboxyl group and amino ion

• If pH increases

  • Carboxyl group of amino acids dissociates, releasing a hydrogen ion that acts as a weak acid → carboxyl group becomes carboxylate ion

Buffering Mechanisms

• Carboxylate Ion Formation: After donating H⁺ , the carboxyl group becomes a carboxylate ion; this occurs during increased pH levels.

• Amino Group Activity: In lower pH conditions, the amino group acts as a weak base, accepting hydrogen ions.

• R Groups Contribution: Most buffering capacity of proteins derives from R groups of amino acids, not from carboxyl and amino groups in peptide bonds.

zwitterions

• Definition: Slightly charged amino acids at neutral pH form zwitterions (charged at both ends).

• Responsiveness to pH Changes:

  • With rising pH (alkaline conditions), amino acids can release H⁺.

  • With falling pH (acidic conditions), amino acids can absorb excess H⁺.