Acids and bases play several important roles in biology. The pH of a biological system can significantly impact the function of biological molecules, including enzymes, proteins, and nucleic acids. For example, enzymes have optimal pH ranges for their activity, and changes in pH can denature or alter their structure, affecting their function. Acids and bases also play a crucial role in cellular signaling and communication. The pH of the extracellular fluid around cells can influence the activity of ion channels, which are essential for cellular communication and signaling.

Acids

An acid is a substance that releases hydrogen ions (H⁺) in solution. Because an atom of hydrogen has just one proton and one electron, a positively charged hydrogen ion is simply a proton. This solitary proton is highly likely to participate in chemical reactions. Strong acids are compounds that release all of their H⁺ in solution; that is, they ionize completely. Hydrochloric acid (HCl), which is released from cells in the lining of the stomach, is a strong acid because it releases all of its H⁺ in the stomach's watery environment. This strong acid helps in digestion and kills ingested microbes. Weak acids do not ionize completely; that is, some of their hydrogen ions remain bonded within a compound in solution. An example of a weak acid is vinegar or acetic acid.

Bases

A base is a substance that releases hydroxyl ions (OH^-) in solution or one that accepts H⁺ already present in solution. The hydroxyl ions (also known as hydroxide ions) or other basic substances combine with H⁺ present to form a water molecule, thereby removing H⁺ and reducing the solution's acidity. Food mixed with hydrochloric acid from the stomach would burn the small intestine, the next portion of the digestive tract after the stomach, if it were not for the release of bicarbonate (HCO₃^-), a weak base that attracts H⁺. Bicarbonate accepts some of the H⁺ protons, thereby reducing the acidity of the solution.

Neutralization reactions

A neutralization reaction is a specific type of acid-base reaction in which the reactants are an acid and a base (but not water), and the products are often salt and water.

Consider what happens when a typical antacid such as milk of magnesia (an aqueous suspension of solid Mg(OH)₂) is ingested to ease symptoms associated with excess stomach acid (HCl). In addition to water, this reaction produces a salt, magnesium chloride. Such reactions are of central importance to numerous natural and technological processes, ranging from the chemical transformations that take place within cells and the lakes and oceans to the industrial-scale production of fertilizers, pharmaceuticals, and other substances essential to society.

This text is adapted from Opentsax, Anatomy, and Physiology 2e, Section 2.4 Inorganic Compounds Essential to Human Functioning and Opentsax, Chemistry Atoms First 2e, 7.2 Classifying Chemical Reactions