10 MANUAL OF ANIMAL BIOLOGY 



ble for the green plant cell to utilize the energy of sunlight for 

 the manufacture, or synthesis, of the essential foodstuffs from the 

 simple inorganic materials which are abundantly present in the 

 immediate environment of the plant. This process of photo- 

 synthetic food manufacture is fundamental for all life because, in 

 the final analysis, all plant and all animal food is formed in that 

 way. Animals eat plants directly or they eat other animals which 

 have eaten plants. 



The substances taken into the plant cell consist of carbon 

 dioxide which is a gas composed of one part of carbon and two 

 parts of oxygen, and water which is composed of two parts 

 of hydrogen and one part of oxygen. Dissolved in the water 

 are various inorganic salts, particularly nitrates, which, as will be 

 seen, are necessary constituents of the more complex foods. 



Chlorophyll uses the energy of sunlight to separate the carbon 

 from the oxygen in the carbon dioxide and to combine the carbon 

 thus secured with the hydrogen and oxygen of the water to form a 

 type of sugar, or carbohydrate, which is the first food product of 

 photosynthesis. The oxygen which was separated from the carbon 

 is not utilized in photosynthesis but passes off as free oxygen, and 

 it may be later used in respiration by plant and animal cells as a 

 basis of their katabolic activities. These latter, it will be remem- 

 bered, disintegrate the complex compounds present in the cells in 

 order to secure energy for the essential life processes ; carbon and 

 oxygen again uniting to form carbon dioxide which passes off as 

 an excretory product. 



Sugar having been formed, it may be utilized at once by the 

 plant protoplasm (a) to secure energy through oxidation, or (6) it 

 may be changed to another carbohydrate, starch, and stored 

 for later use, or (c) the proportion of oxygen present in either the 

 sugar or starch may be decreased, and thereby the food material 

 changed from a carbohydrate into a fat which may also be stored, 

 or, finally, (d) the carbohydrate may be built up into the more 

 complex food material, protein, by the addition of nitrogen and 

 other inorganic salts dissolved in the water taken into the plant 

 cells. There is a question as to whether the protein formation 

 occurs as a part of the photosynthetic action or whether it is a later 

 process brought about in some other way. 



Plants. Since, then, plants are of basic importance in supply- 

 ing the nutritive requirements of animals, it will be well to present 



