142 



All Food 



Fig. 168 Plants ?nake protein as well as carbo- 

 hydrates and fats. How do they obtain the 

 necessary nitrogen, sulfur, and phosphorus? 



A busy factory. In a factory electrical 

 energy may be changed into mechanical 

 energy, or perhaps chemical energy may 

 be changed into mechanical energy. 

 Work is done. In the living leaf the hum 

 of machinery cannot be heard, the work 

 cannot be watched by the human eye. 

 Yet sugar is being made and energy 

 changes are taking place. It has been es- 

 timated that the leaves of a single corn 

 plant within a season make about two 

 pounds of sugar; the leaves of a medium- 

 sized apple tree may make 44 pounds 

 of sugar. 



This work goes on in the daytime, 

 while light strikes the plant. Light en- 

 ergy from the sun, or radiant energy as 



Is Made by Green Plants unit hi 



it may be called, is absorbed by the 

 chlorophyll. In the manufacture of sugar 

 this light energy is changed into chem- 

 ical energy, which remains locked up in 

 the food. All day long while the sun is 

 beating on the broad surface of the leaf 

 blade the chloroplasts within absorb the 

 rays of light. Even on gray days when 

 there is no direct sunlight, chloroplasts 

 absorb light energy and continue the 

 synthesis (making) of sugar, though 

 more slowly than in bright light. At night 

 the process stops. Devise and perform 

 an experiment to convince yourself that 

 light is necessary for making carbohy- 

 drates. Electric lamps may be used as 

 the source of light. 



Because light energy is used in the 

 synthesis of sugar the process is called 

 photosynthesis. Photo is the Greek word 

 for light. When carbon dioxide and wa- 

 ter are combined during photosynthesis, 

 free oxygen is left over. You can demon- 

 strate this by doing Exercise 5. 



Protein synthesis. Some of the sugar 

 made by the plant is built up into pro- 

 teins. You will remember that protein 

 contains more elements than sugar. Be- 

 sides carbon, hydrogen, and oxygen, it 

 contains nitrogen, sulfur, sometimes phos- 

 phorus, and others. These elements enter 

 the plant from the soil in the form of 

 mineral compounds. It is interesting to 

 note that a plant uses only simple com- 

 pounds, never elements, in making its 

 food. Small amounts of these minerals 

 combine with the sugar. The union does 

 not take place all in one step; compounds 

 simpler than proteins are made first. 

 Among these compounds are amino 

 acids. You will read more about amino 

 acids when you study digestion. 



