pH

pH is a measure of the concentration of hydrogens ions (= H⁺) (= protons) in a solution.

Numerically it is the negative logarithm of that concentration expressed in moles per liter (M).

Pure water spontaneously dissociates into ions, forming a 10^-7 M solution of H⁺ (and OH^-). The negative of this logarithm is 7, so the pH of pure water is 7.

Solutions with a higher concentration of H⁺ than occurs in pure water have pH values below 7 and are acidic.

Solutions containing molecules or ions that reduce the concentration of H⁺ below that of pure water have pH values above 7 and are basic or alkaline.

Is pH important? Yes!

The properties of most proteins, enzymes for example, are sensitive to pH.

As the pH drops,

H⁺ bind to the carboxyl groups (COO^-) of aspartic acid (Asp) and glutamic acid (Glu), neutralizing their negative charge, and
H⁺ bind to the unoccupied pair of electrons on the N atom of the amino (NH₂ ) groups of lysine (Lys) and arginine (Arg) giving them a positive charge.

The result: Not only does the net charge on the molecule change (it becomes more positive) but many of the opportunities that its R groups have for ionic (electrostatic) interactions with other molecules and ions are altered.

As the pH rises,

H⁺ are removed from the COOH groups of Asp and Glu, giving them a negative charge (COO^-), and
H⁺ are removed from the NH₃⁺ groups of Lys and Arg removing their positive charge.

The result: Again the net charge on the molecule changes (it becomes more negative) and, again, many of the opportunities its R groups have for electrostatic interactions with other molecules or ions are altered.

Link to an example of the importance of pH in separating serum proteins by electrophoresis.

The pH of the cytosol within a human cell is about 7.4. BUT, this value masks the pH differences that are found in various compartments within the cell. For example,

The interior of lysosomes is much more acidic (as low as pH 4) than the cytosol and the enzymes within work best at these low pH values.
The pH differential created within chloroplasts by the energy of the sun is harnessed to synthesize ATP which, in turn, powers the synthesis of food. (Discussion)
The pH differential created within mitochondria during the respiration of food is harnessed to the synthesis of ATP which, in turn, powers most of the energy-consuming activities of the cell such as locomotion and biosynthesis of cell components. (Discussion)

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14 June 1999