542 INTERRELATIONSHIPS OF ORGANISMS 



radiation, and this condition has existed since the origin of the earth. 

 Of this energy, some 0.3 to 3 per cent is conserved and utilized in the 

 building up of protoplasm and the maintenance of life. The ability to 

 capture and transform this energy is confined to the green plants and 

 is due to the peculiar properties of chlorophyll, which is able to transform 

 light energy into chemical energy and store it in the form of chemical 

 compounds. 



The raw materials involved. Protoplasm and cell products are com- 

 posed chiefly of carbon, oxygen, hydrogen, and nitrogen, together with 

 smaller quantities of calcium, phosphorus, sodium, potassium, sulfur, 

 magnesium, iodine, copper, iron, zinc, and a few other elements. These 

 are distributed more or less generally over the earth, either as elements or 

 as simple compounds, and, as such, contain no energy that is available 

 to living organisms. 



The storage of energy; food synthesis. The building of organic 

 molecules out of the simpler inorganic molecules and elements requires 

 an expenditure of energy. Once synthesized, the complex organic mole- 

 cules contain not only the original raw materials (atoms and simple 

 combinations of atoms) but also the energy that was necessary for their 

 construction. We have seen earlier that the complex molecules from which 

 protoplasm is built include proteins or their component amino acids, 

 carbohydrates, and fats, and that these same substances are the foods on 

 which organisms depend for growth and all catabolic processes. Only 

 plants have the ability to synthesize these substances from simple inor- 

 ganic material, and only the green plants can perform the basic photo- 

 synthesis in which light is captured and stored as chemical energy. Once 

 this synthesis has taken place and sugar is available, other plant tissues 

 can utilize the energy in the sugar (or its derivatives) by oxidation, to 

 build simple nitrogen compounds into amino acids and proteins, and 

 simple carbohydrates into fats. 



The green plant takes in carbon in the form of carbon dioxide from the 

 air or in solution in water and takes in water from the soil. Carbon dioxide 

 and water are combined by photosynthesis to form sugar, with the giving 

 off of oxygen as a by-product. Nitrogen is obtained in the form of am- 

 monia or nitrate salts in solution in the soil water, as are the necessary 

 supplies of calcium, phosphorus, potassium, sulfur, etc. From these raw 

 materials are synthesized all the food substances needed for building more 

 protoplasm (growth and reproduction), for expenditure in maintaining 

 life, and for storage against future needs. The energy of the sun is thus 

 stored in the form of the complex molecules of protoplasm and plant 

 products. Altogether this amounts to billions of tons of synthesized 

 carbon compounds in existence on the earth. 



The release of stored energy. Free oxygen is not a necessity in any 



