36 PROTOPLASM AND THE CELL. 



There is a corresponding likeness in the general properties and reactions 

 of proteids. They are colloidal or non-diffusible, i.e., they will not pass 

 through the membrane of a dialyser, or only with great difficulty ; they 

 are rarely crystalline ; they rotate the plane of polarized light to the left. 

 Though not all soluble in water, they may be dissolved by the aid of heat 

 in strong acetic acid and in caustic alkalies, but are insoluble in cold ab- 

 solute alcohol and in ether. They may be precipitated from solution by 

 strong mineral acids, etc. Many proteids are precipitated by heat (a pro- 

 cess which is called coagulation) ; and it is worthy of note that tempera- 

 tures which produce coagulation of proteids (40 75 C.) produce also the 

 death of most organisms. 4k Amongst the organic proximate principles 

 which enter into the composition of the tissues and organs of living beings, 

 those belonging to the class of proteid or albuminous bodies occupy quite 

 a peculiar place and require an exceptional treatment, for they alone are 

 never absent from the active living cells which we recognize as the pri- 

 mordial structures of animal and vegetable organisms. In the plant, whilst 

 we recognize the wide distribution of such constituents as cellulose and 

 chlorophyl, and acknowledge their remarkable physiological importance, 

 we at the same time are forced to admit that they occupy altogether a 

 different position from that of the proteids of the protoplasm out of which 

 they were evolved. We may have a plant without chlorophyl, and a vege- 

 table cell without a cellulose wall, but our very conception of a living, 

 functionally active, cell, whether vegetable or animal, is necessarily asso- 

 ciated with the integrity of its protoplasm, of which the invariable organic 

 constituents are proteids. 



"In the animal, the proteids claim even more strikingly our attention 

 than in the vegetable, in that they form a very much larger proportion of 

 the whole organism, and of each of its tissues and organs. We may indeed 

 say that the material substratum of the animal organism is proteid, and 

 that it is through the agency of structures essentially proteid in nature 

 that the chemical and mechanical processes of the body are effected. It is 

 true that the proteids are not the only organic constituents of the tissues 

 and organs, and that there are others, present in minute quantities, which 

 probably are almost as widely distributed, such as for instance phosphorus- 

 containing fatty bodies, and glycogen, yet avowedly we can (at the most) 

 only say probably, and cannot, in reference to these, affirm that which we 

 may confidently affirm of the proteids that they are indispensable constit- 

 uents of every living, active, animal tissue, and indissolubly connected 

 with every manifestation of animal activity." (Gamgee, Physiological 

 Chemistry, Chap. I.) 



The molecular instability of proteids is proved by the ease 

 with which they may be decomposed into simpler compounds ; 

 their complex constitution by the numerous compounds, them- 

 selves often highly complex, which may thus be derived or 

 split off from them. 



