H2O H2O molecule to H3O+ OH– the other hydronium hydroxyl(B) ion ion other words, a proton (H+). When the polar molecule becomes surrounded by water molecules, the proton will be attracted to the partial negative charge on the O atom of an adjacent water molecule. This proton can easily dissociate from its original partner and associate instead with the oxygen atom of the water molecule, generating a hydronium ion (H3O+) (Figure 2–5A). The reverse reaction also takes place very readily, so in the aqueous solution protons are constantly flitting to and fro between one molecule and another.
Substances that release protons when they dissolve in water, thus forming H3O+, are termed acids. The higher the concentration of H3O+, the more acidic the solution. H3O+ is present even in pure water, at a concentration of 10–7 M, as a result of the movement of protons from one water molecule to another (Figure 2–5B). By convention, the H3O+ concentration is usually referred to as the H+ concentration, even though most protons in an aqueous solution are present as H3O+. To avoid the use of unwieldy numbers, the concentration of H3O+ is expressed using a logarithmic scale called the pH scale. Pure water has a pH of 7.0 and is said to be neutral—that is, neither acidic (pH <7) nor basic (pH >7).
Acids are characterized as being strong or weak, depending on how readily they give up their protons to water. Strong acids, such as hydrochloric acid (HCl), lose their protons quickly. Acetic acid, on the other hand, is a weak acid because it holds on to its proton more tightly when dissolved in water. Many of the acids important in the cell—such as molecules containing a carboxyl (COOH) group— are weak acids (see Panel 2–2, pp. 92–93).
Because the proton of a hydronium ion can be passed readily to many types of molecules in cells, altering their character, the concentration of H3O+ inside a cell (the acidity) must be closely regulated. Acids—especially weak acids—will give up their protons more readily if the concentration of H3O+ in solution is low and will tend to receive them back if the concentration in solution is high.
The opposite of an acid is a base. Any molecule capable of accepting a proton from a water molecule is called a base. Sodium hydroxide (NaOH) is basic (the term alkaline is also used) because it dissociates readily in aqueous solution to form Na+ ions and OH– ions. Because of this property, NaOH is called a strong base. More important in living cells, however, are the weak bases—those that have a weak tendency to reversibly accept a proton from water. Many biologically important molecules contain an amino (NH2) group. This group is a weak base that can generate OH– by taking a proton from water: –NH2 + H2O →–NH3+ + OH– (see Panel 2–2, pp. 92–93).
Because an OH– ion combines with a H3O+ ion to form two water molecules, an increase in the OH– concentration forces a decrease in the concentration of H3O+, and vice versa. A pure solution of water contains an equal concentration (10–7 M) of both ions, rendering it neutral. The interior of a cell is also kept close to neutrality by the presence of buffers: weak acids and bases that can release or take up protons near pH 7, keeping the environment of the cell relatively constant under a variety of conditions.
Figure 2–5 Protons readily move in aqueous solutions. (a) The reaction that takes place when a molecule of acetic acid dissolves in water. at ph 7, nearly all of the acetic acid is present as acetate ion. (B) Water molecules are continuously exchanging protons with each other to form hydronium and hydroxyl ions. These ions in turn rapidly recombine to form water molecules.
a Cell is formed from Carbon Compounds