32 Practical Plant Biology. 



The sugar disappears and the production of alcohol and 

 bubbles of carbon dioxide may be .observed. This change makes 

 energy available, though how at first sight may not appear 

 evident. 



It is well known that when coal is burnt, heat is given off, 

 which may be utilised in a steam engine to do work. Evidently 

 the combustion of the coal sets free energy which is utilised by 

 the engine. The amount of energy available may be estimated 

 by the amount of heat produced. Using the usual standard this 

 heat may be measured by the number of kilograms (litres) of 

 water it can raise through i C. Thus one gram of coal, on 

 burning, can raise 7 to 8 kg. of water through one degree. In 

 other words, it can generate 7 to 8 Calories. In the same way 

 i gm. of glucose on complete combustion can generate about 

 376 Calories. On complete combustion the glucose is entirely con- 

 verted into carbon dioxide and water. In the process, however, 

 which we are considering combustion is not complete. One of 

 the products is ethyl alcohol which in itself, as is well known, may 

 be further oxidised, and may, in the oxidation, set free a consider- 

 able amount of heat. From the equation quoted above it appears 

 that one gram-molecule of hexose (180 gm.) produces two gram- 

 molecules (92 gm.) of alcohol. The fact that both bodies are 

 in solution must also be allowed for, so that it is found that, 

 during this incomplete combustion of hexose to alcohol, for each 

 1 80 gm. of hexose 92 gm. of alcohol is formed, and that while 

 the original hexose in solution had a heat of combustion equal to 

 about 300 Calories, the heat of combustion of the resulting alcohol 

 in solution amounts to only about 145 Calories. The remaining 

 155 Calories have been set free and are available to do the work 

 of the yeast cell. 



Reactions occurring within the cell, which like the foregoing 

 are a source of energy, are described as respiration. Generally 

 these reactions are of the nature of oxidations, i.e. are com- 

 bustions, and most usually they are complete and utilise free 

 oxygen ; but often they are incomplete and utilise combined 

 oxygen as in the yeast cell. Respiration utilising combined 

 oxygen in this way is called anaerobic respiration, inasmuch as 

 the oxygen used is not drawn from the atmosphere. Very usually 

 the substances oxidised are sugars as in this case. 



Inasmuch as assimilation, and other processes which require 

 an expenditure of energy, are characteristics of protoplasm it is 

 natural to find that all active protoplasm exhibits respiration in 

 one form or another. It is a general characteristic of protoplasm, 



