Page 39 - Color Atlas of Biochemistry
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30 Basics
Acids and bases
B. pH values in the organism
pH values in the cell and in the extracellular
A. Acids and bases
fluid are kept constant within narrow limits.
In general, acids are defined as substances In the blood, the pH value normally ranges
that can donate hydrogen ions (protons), only between 7.35 and 7.45 (see p. 288). This
while bases are compounds that accept pro- corresponds to a maximum change in the H +
tons. concentration of ca. 30%. The pH value of
Water enhances the acidic or basic proper- cytoplasm is slightly lower than that of blood,
ties of dissolved substances, as water itself at 7.0–7.3. In lysosomes (see p. 234; pH
+
can act as either an acid or a base. For exam- 4.5–5.5), the H concentration is several hun-
ple, when hydrogen chloride (HCl) is in aque- dred times higher than in the cytoplasm. In
ous solution, it donates protons to the solvent the lumen of the gastrointestinal tract, which
(1). This results in the formation of chloride forms part of the outside world relative to the
–
ions (Cl ) and protonated water molecules organism, and in the body’s excretion prod-
(hydronium ions, H 3 O+, usually simply re- ucts, the pH values are more variable. Ex-
+
ferred to as H ). The proton exchange be- treme values are found in the stomach (ca.
tween HCl and water is virtually quantitative: 2) and in the small bowel (> 8). Since the
in water, HCl behaves as a very strong acid kidney can excrete either acids or bases, de-
with a negative pK a value(seep.18). pending on the state of the metabolism, the
Bases such as ammonia (NH 3 )takeover pH of urine has a particularly wide range of
protons from water molecules. As a result of variation (4.8–7.5).
–
this, hydroxyl ions (OH )and positively
+
charged ammonium ions (NH 4 , 3)form. Hy-
dronium and hydroxyl ions, like other ions, C. Buffers
exist in water in hydrated rather than free Short–term pH changes in the organism are
form (see p. 26). cushioned by buffer systems.These are mix-
Acid–base reactionsalwaysinvolve pairs of tures of a weak acid, HB, with its conjugate
–
acids and the associated conjugated bases base, B , or of a weak base with its conjugate
(see p.18). The stronger the acid or base, the acid. This type of system can neutralize both
weaker the conjugate base or acid, respec- hydronium ions and hydroxyl ions.
–
tively. For example, the very strongly acidic In the first case (left), the base (B )binds a
+
hydrogen chloride belongs to the very weakly large proportion of the added protons (H )
basic chloride ion (1). The weakly acidic am- and HB and water are formed. If hydroxyl
–
monium ion is conjugated with the moder- ions (OH ) are added, they react with HB to
–
ately strong base ammonia (3). give B and water (right). In both cases, it is
–
The equilibrium constant K for the acid— primarily the [HB]/[B ] ratio that shifts, while
base reaction between H 2 Omolecules (2)is the pH value only changes slightly. The titra-
very small. At 25 °C, tion curve (top) shows that buffer systems are
most effective at the pH values that corre-
–
+
K= [H ] [OH ]/ [H 2 O] = 2 10 –16 mol L –1 spond to the pK a value of the acid. This is
where the curve is at its steepest, so that the
In pure water, the concentration [H 2 O] is pH change, ∆pH, is at its smallest with a given
–1
–
+
practically constant at 55 mol L .Substitut- increase ∆cin [H ]or[OH ]. In other words,
ing this value into the equation, it gives: the buffer capacity ∆c/ ∆pH is highest at the
pK a value.
+
–
K w =[H ] [OH ]= 1 10 –14 mol L –1
–
+
The product [H ] [OH ]—the ion product of
water—is constant even when additional
acid–base pairs are dissolved in the water.
+
–
At 25 °C, pure water contains H and OH at
concentrations of 1 10 –7 mol L –1 each; it is
neutral and has a pH value of exactly 7.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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