38 FUNCTIONS OP A GREEN CELL 



synthesis — the plant absorbs minerals, in the form of nitrateS;, 

 sulphates, and phosphates, which are dissolved in soil water. These 

 substances furnish the plants with nitrogen, sulphur, and phos- 

 phorus which are thought to combine with some of the sugar to 

 form protein, a substance containing carbon, hydrogen, oxygen, 

 nitrogen, sulphur, phosphorus, and perhaps other minerals. 

 Protein in protoplasm may be represented by the symbols 

 C-H-0-X-S-P. INIost protein is used for growth (protoplasm 

 making), repair, and storage. Some is decomposed and the sugar 

 (one of the products of decomposition) is burned as fuel, while 

 the other products are thrown out as nitrogenous wastes or stored 

 for further protein-synthesis. 



Digestion. When the food substances that have been stored 

 in the cells of the plant as fats, starches, and proteins are to be used, 

 they must first be changed to a soluble form. The process of 

 changing an insoluble nutrient into a soluble form that can be 

 used by the organism is called digestion. Digestion is brought 

 about by the presence and action of chemical substances called 

 digestive enzymes. These act as catalytic agents, that is, they 

 bring about a chemical change, but are themselves not affected 

 by nor changed during the process. In the cell where digestion 

 takes place, there is an amylase for the breaking down of starch, 

 a protease for the digestion of proteins, a lipase for the digestion 

 of fats. (The names of most of the enzymes end in ase.) After 

 being digested, the liquid nutrients may be carried by means of 

 the streaming, flowing motion of the protoplasm to that part of 

 the cell where they are to be utilized. If there is an excess of 

 food nutrients, they may again be combined into an insoluble 

 form and stored in the cells until needed. 



Respiration. Each living plant cell takes in oxygen through the 

 cell membrane and gives off carbon dioxide by means of diffusion. 

 The oxygen unites with the compounds of hydrogen, oxygen, and 

 carbon and releases the energy, in the form of heat, stored in them. 



