Since changes in pH can dramatically influence the function of many biomolecules, unicellular and multicellular organisms have developed various means to protect themselves against changes in pH. One of these mechanisms is the use of small molecules that can, based on their chemical properties, be classified as buffers. Buffers are typically small molecules that can reversibly bind and unbind protons in solution. If the pH in an environment is lower than the pKa of a protonatable functional group on the buffer molecule, that group will tend to become protonated and therefore "remove" a proton from solution. Alternatively, if the pH in an environment is higher than the pKa of of the same protonatable functional group on the buffer molecule, that group will tend to become or stay deprotonated, lower the local pH.
of the body carefully maintained in the narrow range required for survival. Maintaining a constant blood pH is critical to a person’s well-being. The buffer maintaining the pH of human blood involves carbonic acid (H2CO3), bicarbonate ion (HCO3–), and carbon dioxide (CO2). When bicarbonate ions combine with free hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly excess carbonic acid can be converted to carbon dioxide gas and exhaled through the lungs. This prevents too many free hydrogen ions from building up in the blood and dangerously reducing the blood’s pH. Likewise, if too much OH– is introduced into the system, carbonic acid will react with it to create bicarbonate, lowering the pH. Without this buffer system, the body’s pH would fluctuate enough to put survival in jeopardy.
Figure 1: This diagram shows the body’s buffering of blood pH levels. The blue arrows show the process of raising pH as more CO2 is made.
Other examples of buffers are antacids used to combat excess stomach acid. Many of these over-the-counter medications work in the same way as blood buffers, usually with at least one ion capable of absorbing hydrogen and moderating pH, bringing relief to those that suffer “heartburn” after eating. The unique properties of water that contribute to this capacity to balance pH—as well as water’s other characteristics—are essential to sustaining life on Earth.