6 VETERINARY PHYSIOLOGY 



This 'power of growth and reproduction under suitable 

 conditions is an essential characteristic of living matter. 



2. Conditions necessary for Manifestations of Life. — 

 (a) If the yeast be mixed with the solid constituents of 

 the solution in a dry state, no growth or reproduction 

 occurs. Water is essential. 



(b) If the yeast, mixed with the solution, be kept at the 

 freezing point no growth takes place, but this proceeds 

 actively at about 36° C. A certain range of temperature is 

 necessary for the vitality of protoplasm. 



In the absence of these conditions, protoplasm is only 

 potentially alive, and in this state it may remain for long 

 periods without undergoing any change, as in the seeds of 

 plants and in dried bacteria. 



3, Essentials for Growth. — In order that the growth of 

 the yeast may take place, there must be : — 



{a) A Supply of Material from which it can be 

 formed. The chemical elements in protoplasm are carbon, 

 hydrogen, oxygen, nitrogen, sulphur, and phosphorus. These 

 elements are contained in the ingredients of the solution 

 used. If yeast be sown in distilled water, even if it be kept 

 at a temperature of 36° C, it does not grow. 



(6) A Supply of Energy to bring about the construc- 

 tion. The source of the energy is indicated by an examina- 

 tion of the fluid in which the yeast has grown. The sugar, 

 CgHioOe' ^^^s decreased in amount, being converted into 

 alcohol, C2HgO, and carbon dioxide, COg — 



CeH,A=2COo+2C2HeO. 



It is this oxidation of the carbon to carbon dioxide which 

 yields the energy, just as the combustion of the coals in the 

 furnace yields the energy for an engine. But in this case 

 the Oo comes from the CgH^^Og, and not from outside, and 

 the energy evolved in the reaction is only about a tenth of 

 that liberated by the complete oxidation to CO, and H2O 

 which occurs in the animal body. The formation of alcohol 

 from sugar does not lead to the liberation of energy. So 

 far as this is concerned, the alcohol may be considered as a 

 by-product. 



