82 IXTLMATE ASSOCIATIOXS OF IXOEGAXIC IONS 



holding iDOwer is now only 0.04, so that (0.18 — 0.04) 0.14^ 

 is the figure representing the acid fijxed by the proteins alone. 

 This represents 0.51;«^ of hydrochloric acid itseK. jSTow this is 

 rather a ponsiderahle amount ; and its physiological^ significance 

 is that, within pretty wide chemical limits, no free acid 

 can reach the living tissues, for the circulating proteins 

 can combine with them and so constitute a protective mechanism 

 against acidosis or an excessively acid condition of blood. These 

 native or serum-proteins, therefore, behave in an amphoteric 

 fashion, for they can fix alkalies like acids and acids like 

 alkalies. This explains how serum is acid to phenolphthalein, 

 and alkaline to methyl-orange, while it is physico-chemically 

 neutral. This double power proteins possess is now believed to 

 be due to their polypeptide composition. This means that after 

 any number of amino-acids have united together in chain 

 fashion, there will be left an amidogen group at one end and a 

 carboxyl group at the other, thus conferring a chemical polarity 

 or what is otherwise called ''residual afiinity." 



Thtis the dipeptide giycyl-giycin is formed, 

 XHoCHoCOOH + H XH CH, COOH,. 

 which gives us XH,CH,.C0-XH-CH2-C00H + H.O, a com- 

 pound is basic on account of XH, and acidic on account of the 

 COOH. 



Hence owing to its acidity, glyein can form the copper 



salt thus 



CH,XH.,CO O---^^ , . . , . . . 



■ ■ ^Ou: and owmo^ to its basicitv it can unite 



Lri., JsrljLO \j^,^^ 



with an acid like benzoic and form hippuric acid thus ; 



QH, CO 'OH + H' XH CH.-, COOH = C,H- CO XH CH., COOH 



+ HA 



The union of oxygen with haemoglobin is. however, not 

 merely an adsorption due to residual, affinities, for it is strictly 

 mono-molecular, and the reduced form of the pigTuent is differ- 

 ent from the oxidised in colour and therefore in spectrum. 



But not merely are acids and inoro-anic substances imited 



