SYNTHESIS, STORAGE AND BREAKDOWN 13; 



of oxygen, probably because the alcohol produced is poisonous to 

 their tissues, or because the energy liberated by anaerobic respiration 

 is insufficient to maintain life. 



There is no doubt that the respiratory processes are very complex 

 and involve a chain of intermediate reactions, many of which are 

 those of catalysis activated by enzymes or other protoplasmic agents 

 present in the cell. This conclusion is forced upon us by consideration 

 of the fact that in the laboratory carbohydrates are relatively difficult 

 of oxidation. It is widely believed that in normal aerobic respiration 

 there are two chief phases, only the second of which actually requires 

 atmospheric oxygen. In the first phase the sugars are thought to 

 be broken down to simpler compounds, possibly through the agency 

 of the enzyme zymase which is present in higher plants, though 

 known best in yeast. This first phase is considered to be common 

 to both normal aerobic respiration and to that of the anaerobic type. 

 If oxygen is present the products of the first phase are believed to 

 be oxidised with the final production of carbon dioxide and water. 

 In this oxidative phase enzymes known as oxidases and dehydrases 

 are thought to play a part. If on the other hand oxygen is absent, 

 the degradation of the sugar is less complete, and the products of 

 the first phase are converted to alcohol and carbon dioxide. By 

 this scheme normal and anaerobic respiration are linked up and 

 regarded as alternative developments of a common initial phase. 

 Yeast differs from higher plants in that it continues to exhibit the 

 anaerobic form of respiration even when oxygen is present. 



Some of the respiratory energy of the plant is evolved as heat, 

 as may be shown if steps are taken to prevent loss of the heat by 

 radiation. Thus germinating seeds if enclosed in a thermos flask, 

 with precautions to prevent bacterial growths, will show an appre- 

 ciable rise in temperature. The amount of heat so produced in 

 plant tissues is small, and under normal conditions is quickly lost 

 by radiation, so that body heat is not a characteristic of plants. 

 The temperature of plant-organs, at least in darkness, is much the 

 same as that of the surrounding atmosphere. 



Concluding Remarks. 



In this study of metabolism we have seen how the green plant 

 utilises simple materials from its environment and from them ela- 

 borates carbohydrates, proteins, and other complex substances. 

 This ability to make use of entirely inorganic materials for purposes 



