CELL-CONTENTS AND CELL-WALLS. 3 



prolamins, are insoluble in alcohol; some are soluble in 

 water, others insoluble ; others, again, are soluble in saline 

 solutions. All are soluble in strong acids and alkalies, but 

 undergo decomposition in the process. 



The constitution of the proteins is very complex. When 

 decomposed in various ways, e.g. by acids, alkalis, or pro- 

 teolytic enzymes, they yield a great variety of substances. 

 When acted upon by Bacteria they undergo putrefaction, 

 offensive gases (ammonia, sulphuretted hydrogen, phos- 

 phoretted hydrogen, etc.) being given off. 



Proteins are composed largely of amino acids ( 60), which form 

 the chief units of the protein molecule. These include leucin, 

 tyrosin, aspartic acid, glutamic acid, arginin, tryptophane, etc. 

 By synthesis compounds have been prepared which contain from 

 two to about twenty amino acid units. The most complex of these 

 compounds, or polypeptides, would be regarded as true proteins if 

 they were found in nature. Proofs of the polypeptide constitution 

 of the proteins are (1) the isolation of polypeptides from the natural 

 proteins ; (2) the hydrolysis of polypeptides by trypsin into their 

 constituent units or amino acids, in the same way as the natural 

 proteins are hydrolysed. The various amino acids can be combined 

 together in many different ways, hence an enormous number of 

 isomers is possible among the polypeptides, while the proteins found 

 in nature show still greater variety. 



The known proteins are classified mainly according to their 

 origin, solubility in different reagents, coagulability on heating, 

 and other physical characters, without strict reference to their 

 chemical composition, though the classification is borne out by 

 their actual composition so far as this is known. For our purposes 

 we may divide the vegetable proteins into (1) primary proteins, 

 (2) conjugated proteins, (3) derived proteins. 



38. Primary proteins. These, including the chief pro- 

 teins found in seeds, are divided into albumins (soluble 

 in water and coagulated on boiling), globulins (insoluble 

 in water, but soluble in saline solutions), prolamins (in- 

 soluble in water or saline solutions, but soluble in alcohol), 

 and glutelius (insoluble in water or saline solutions or 

 alcohol, but soluble in alkalis). 



Of these the globulins include the majority of seed 

 proteins, e.g. the legumiu of Broad Bean and Pea, the 

 phaselin of Phaseolus, the conglutin of Lupin, and the 

 crystalline globulins found in many oily seeds. 



