THE FUNCTIONS OF PROTOPLASM AND CELLS 



47 



dioxide and some of the water, however, are put to a totally different use. 

 Carbon dioxide is a by-product of the burning of coal or wood or the 

 decay of dead animals and plants or of anything else composed partly of 

 carbon. It is constantly' being thrown off as a waste product by animals 

 and by plants, except as they use it in the process about to be described. 

 Plants absorb the carbon dioxide into their leaves or other green parts 

 and there combine it with water to form one of the simple sugars, glucose. 

 The final results of this reaction are indicated by the equation 



6CO2 + 6H0O + energy^CeHijOe + 6O2 



Tn words this means that six molecules of carbon dioxide and six of 

 water are decomposed and their parts recombined to form one molecule 

 of glucose and six molecules of oxygen. The energy 

 expended in bringing about this change comes from 

 sunlight, hence the process is called photosynthesis, 

 literally construction by light. In most plants 

 production of glucose can occur only in the pres- 

 ence of chlorophyll, the green substance in their 

 plastids, and certain enzymes. The energy of the 

 sun in this reaction appears not to affect the car- 

 bon dioxide directly, but to decompose the water. 

 The hj'drogen set free from the water is picked up 

 by other substances which then, without any aid 

 from light, proceed to attack the carbon dioxide. 

 The oxygen that is liberated is not produced 

 directly by the decomposition of the original raw 

 materials; it comes from a peroxide which is an 

 intermediate product. That oxygen is liberated 

 may be demonstrated by an experiment with water 

 plants. In such an experiment the cut ends of a 

 Avater plant, as Elodea, are inserted in a test tube 

 filled with water, the plant and tube are immersed 

 in water, and the tube is inverted (Fig. 29). 

 When the plants are placed in sunlight, bubbles of gas escape from their 

 cut ends and collect in the tube. Suitable tests show the gas to be 

 oxygen. 



Photosynthesis is not absolutely limited to plants, for there are some 

 simple animals which contain chlorophyll, and in these glucose is pro- 

 duced in the same way as in plants. Nor are chlorophyll and light 

 always necessary for the production of glucose, since some colorless 

 organisms are capable of doing this in darkness. 



Plant Products as Food of Animal Cells. — Inasmuch as most animals 

 are incapable of producing carbohydrates directly from inorganic com- 



FiG. 29. — Method 

 of collecting oxygen 

 produced by the 

 aquatic plant Elodea 

 during photosynthesis. 

 The oxygen rises from 

 the plant into the 

 closed end of the test 

 tube. 



