LECTURE VII. 



WE have seen that Chlamydomonas obtains its carbon from the 

 carbon dioxide of the air dissolved in water. Complex carbon 

 compounds such as proteins and carbohydrates are unnecessary 

 for it. It builds up these compounds itself from inorganic sub- 

 stances. Similarly it does not require organic nitrogen-compounds 

 to enable it to form protoplasm, but strangely enough, like other 

 plants possessed of chlorophyll, it cannot directly utilise organic 

 nitrogen. It draws its supplies from the inorganic nitrates and 

 possibly, to a less extent, from the salts of ammonia dissolved in 

 the water where it lives. In these respects green plants occupy a 

 unique position : they alone can build up organic substances from 

 inorganic. All other living beings require a supply of organic 

 substances, and hence are dependent, directly or indirectly, on 

 green plants for the food which they require to build up their 

 bodies, as completely as they are dependent on green plants for 

 the energy which they require for their work. 



The nutrition of organisms which, like Chlamydomonas, draw 

 their supplies of carbon and nitrogen from inorganic sources is 

 termed holophytic in contrast to the others which depend on 

 organic substances for their food, whose nutrition has been called 

 holozoic. 



It may be noted here that, so far as is known, no organisms can 

 utilise elemental carbon or free hydrogen, and only few can make 

 use of free nitrogen. Free oxygen, on the other hand, as we will 

 presently see, enters very usually into metabolism. 



When light is cut off from Chlamydomonas, as you saw, photo- 

 synthesis ceases. Observation also shows that under this condi- 

 tion the starch in the pyrenoid slowly disappears. Comparison 

 with other similar plants also shows us that during this process 

 oxygen is absorbed and carbon dioxide is formed. Evidently a 

 carbohydrate is being oxidised and its products of combustion, 

 carbon dioxide and water, are being eliminated. The latter is lost 

 in the already large quantity of water in the cell and is not notice- 

 able. You will observe that this process, which is called respira- 

 tion, is the reversal of photosynthesis, and so, when photosynthesis 



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