6 HUMAN PHYSIOLOGY 



1. If the yeast be mixed with the solid constituents of the 

 solution in a dry state no growth or reproduction occurs. 

 Water is essential. 



2. 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 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 under- 

 going any change, as in the seeds of plants and in dried 

 bacteria. 



These conditions being present, 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. 

 (6) A SUPPLY OF ENERGY to bring about the construction. 



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. 



The energy is got by the breaking down of the sugar, 

 C 6 H 12 6 , into alcohol, C 2 H 6 O, and carbon dioxide, CO 2 . Such 

 a breaking down of a complex into simpler molecules liberates 

 energy, as is well seen when nitro-glycerine explodes, breaking 

 into carbon dioxide, water, oxygen, and nitrogen 



2C 3 H 6 (N0 3 ) 3 = 6C0 2 + 5H 2 + + ON. 



The energy can be used for the performance of work of 

 any kind, as, for example, the work of building up a fresh 

 quantity of the yeast plant out of the substances contained 

 in the solution. The history of the yeast plant shows that 

 protoplasm, when placed in suitable conditions, has the 

 power of breaking down certain complex substances, and 

 of utilising the energy liberated for building itself up. It 

 is this power which has enabled living matter to exist and 

 to extend over the earth. 



How does protoplasm liberate the potential energy of such 

 substances ? At present the answer to this question cannot 



