Biochemical and Biophysical Phenomena 757 



source of the energy of our foods produced by green chlorophyll-bearing 

 plants is the sun (see Radiant Energy). The energy value of a food is 

 measured by a unit called a calorie, which is the amount of heat required 

 to raise the temperature of 1 Gm. of water 1° G. One gram of fat 

 produces about 9 calories of heat; 1 Gm. of carbohydrate, about 4 calo- 

 ries; 1 Gm. of protein, about 4 calories. 



Radiant Energy 



Radiant energy is the energy possessed by the sun's rays. When the 

 electromagnetic waves of sunlight are passed through a prism, there is 

 produced a spectrum (L. spectrum, vision) of various wave lengths and 

 colors. These waves of different lengths (Fig. 368) are capable of dif- 

 ferent types of work and of producing a variety of phenomena. The 

 longer, visible waves at the red end of the visible spectrum grade through 

 the orange, yellow, green, and blue to the shorter, visible violet rays at 



500^0 



3600 AZftO A920 5860 6470 



4550 5350 



800Q<? 

 —J 



VISIBLE 



SPECTRUM 



Fig. 368. — Diagram of a spectrum showing the divisions of the electromagnetic 

 scale with the wave lengths of each band shown (approximately) in Angstrom 

 units (A). One Angstrom unit (A) is one ten-billionth of a meter (0.000,000,- 

 000,1). Roentgen rays extend from 2 A to 12A; unexplored rays and x-rays from 

 12A to 500 A; infra-red rays extend from 8,000 A to 3, 100,000 A; unexplored rays, 

 31 X 105 to 160 X 105 A; radio rays, from 160 x 10^ to 120,000,000 x 10^ A. 



the opposite end of the spectrum. At each end of the visible spectrum 

 there are no visible wave lengths or colors, but rays continue indefinitely 

 to become longer and longer beyond the red end, and shorter and shorter 

 beyond the violet end. The waves at the red end are heat rays, as are 

 those, known as infrared rays, just beyond the red. Those waves just 

 beyond the violet end are the 'mvisihle,\ltraviolet waves. Certain waves 

 of the visible spectrum, as well as those of the violet end, show certain 

 chemical activities (Fig. 368). 



The chlorophyll of green plants in the presence of red, blue, and 

 ultraviolet waves photosynthesizes carbohydrates from carbon dioxide 

 and water. These waves of radiant energy split off the oxygen from the 

 carbon dioxide molecule, this permitting the free carbon to unite with 

 the water to form a carbohydrate. The free oxygen passes off into the 

 atmosphere. The carbohydrate can be ( 1 ) oxidized for metabolic pur- 

 poses, (2) stored for future use, and (3) combined with salts, especially 



