470 Physiology 



the stimulatory effects of fumarate, succinate, and a-ketoglutarate on 

 oxygen consumption of the former (513). 



Synthesis of carbohydrates and lipids 



Many Protozoa synthesize and store carbohydrates and lipids as 

 visible deposits. Little is known about the relations of particular sub- 

 strates and other factors to such syntheses. Photosynthesis^" makes an 

 important contribution in many phytoflagellates, but those without 

 chromatophores also store carbohydrates. In pure cultures, lipids may 

 accumulate as the cultures grow older, whereas carbohydrates may be 

 predominant in young cultures. 



The lipids synthesized by Tetrahymena pyriformis have been estimated 

 quantitatively (511); sterols make up about 0.05 per cent of the total 

 (514). A mixture of fatty acids extracted from T. pyriformis has shown 

 bacteriostatic activity against several Gram-positive bacteria in vitro but 

 not in vivo. Similar material from Chilomonas Paramecium showed activ- 

 ity against pneumococcus type III in vitro (370). Acetate is an effective 

 substrate for the synthesis of lipids and carbohydrates by T. pyriformis. 

 Although arsenite and malonate inhibit oxidation of acetate, they do not 

 influence synthesis of either carbohydrates or lipids (515). 



CONTRACTILE VACUOLES IN 

 HYDROSTATIC REGULATION 



The major function of contractile vacuoles seems to be that of 

 hydrostatic regulation. Although they probably do eliminate some soluble 

 wastes, their excretory function is of doubtful importance. The many 

 Protozoa which lack contractile vacuoles must carry on excretion through 

 the general body surface or some permeable portion, and the same mech- 

 anism probably is operative in species with contractile vacuoles. 



The nature of the excretory products is uncertain for most Protozoa. 

 So-called excretion-crystals have been described in various species, but the 

 chemical nature of these inclusions has been disputed and their excretory 

 significance has not been demonstrated satisfactorily. In attempts to iden- 

 tify less problematical Avaste products, Howland (211) was unable to 

 demonstrate uric acid in the vacuoles of Amoeba, Paramecium and Vor- 

 ticella, but did detect it in fluid from cultures of Amoeba and Para- 

 meciurji. Weatherby (565) found urea in culture fluid but not in the 

 contractile vacuole or cytoplasm of P. caudatum. However, urea has been 

 reported in the vacuolar fluid of Spirostommn, the low concentration 

 suggesting that only about 1.0 per cent of the theoretical urea produc- 



1" Major experimental investigations on photosynthesis in phytoflagellates are yet to 

 be completed. Reviews of photosynthesis in general have been published by Rabino- 

 witch (470) and by Franck and Looniis (132). 



