16 PLANTS AND MAN 



usually elliptical in shape, are found in the cells which make up 

 the green parts of plants, notably the leaves. Since solar radia- 

 tion affords tremendous sources of easily available energy, the 

 synthesis of carbon compounds by autotrophic green plants far 

 surpasses in amount and efficiency that of the colorless auto- 

 trophic ones such as the bacteria. Green plants have multiplied 

 and increased in diversity until today they comprise the most 

 conspicuous part of the earth's vegetation — evidence of the 

 success of this pattern of living. 



This constructive phase of metabolism in green plants is 

 known as photosynthesis, and is a relatively simple process 

 when one considers the far-reaching effects of its activities. It 

 involves the utilization of solar energy by means of chlorophyll 

 associated with protoplasm, so that carbon dioxide and water, 

 absorbed into the plant cell, can be transformed into the complex 

 organic molecule of glucose. During the process oxygen is re- 

 leased as can be seen from the following chemical equation for 

 photosynthesis (compare with that of the autotrophic bacteria 

 previously given on p. 14). 



6CO2 + 6H2O + solar energy = GsHiaOe + 6O2 



Expressed otherwise, six molecules of carbon dioxide and six 

 molecules of water are decomposed and rebuilt by the chloro- 

 phyll-protoplasm combination, with the aid of sunlight, into a 

 single molecule of sugar (glucose) and six molecules of oxygen. 

 Thus during the photosynthetic process, in addition to manu- 

 facturing food, green plants give off oxygen as a by-product, in 

 this way purifying the air for all organisms depending upon 

 oxygen for respiration. All plants which lack chlorophyll are 

 heterotrophic,* and all animals are dependent upon photosyn- 

 thesis for their very existence. The importance of photosynthesis 

 to man in the production of food crops is obvious in the develop- 

 ment of agriculture. 



Secondary Differences between Animals and Plants 



The animal type of metabolism, lacking the ability to synthe- 

 size food, has become specialized in its own way for the securing 



* With the exception of the autotrophic bacteria already described on page 14. 



