38 GENERAL BIOCHEMISTRY 



decrease the concentration of hydroxyl ion. Bases have the opposite 

 effect. In addition, acids and bases function in the absence of water 

 and consequently without involving hydrogen or hydroxyl ions. 

 Hence some more general definition than production of aqueous 

 acidity or alkalinity is required. 



The most common definition in wide usage describes an acid as a 

 proton donor and a base as a proton acceptor. Thus water itself serves 

 in both capacities, for the dissociation 



shows that water supplies a proton and another molecule then takes 

 it up. In this case the reaction contains both acid and base, and both 

 are always found paired in any chemical process involving proton 

 transfer. 



Typical acids and bases are illustrated in the following reactions: 



Acid Base Acid Base 



In each case the acid transfers a proton to the base to form another 

 pair doing the same for the reverse reaction, so that acid plus base 

 yields another acid plus another base. All the illustrative reactions 

 save one involve water. And although there are many non-aqueous 

 acid-base reactions, they are seldom encountered in biology because 

 of the universal distribution of water. Water reacts first with both 

 acid and base before they react Aviih each other. Thus in aqueous 

 soliuion 



HCl -f HoO ±=. H3O+ + Cl- 

 H.O -f NH3 ^ NH4+ + OH- 

 and fmally 



H3O+ + OH- ^ 2HoO 



Acids differ in their tendencies to donate protons. Those with rela- 



