700 TEXTBOOK OF ZOOLOGY 



are large. The sun is continually transforming mass into radiation 

 consisting of cosmic rays visible, ultraviolet, and infrared radiations. 



Biological Effects of Sunlig-ht 



Organisms living on the surface of the earth are dependent on 

 solar radiation either directly or indirectly. It would be difficult 

 to imagine human life, for example, without the benefits of food, 

 heat, fresh water, and light. Most biochemical processes are ex- 

 othermic (give off heat) and lose energy. Photosynthesis constitutes 

 the chief means of counteracting the "running down" in the energy 

 of our planet. Green plants contain chlorophyll which absorbs en- 

 ergy from sunlight, decomposes the carbon dioxide of the air and 

 combines its carbon with the oxygen and hydrogen of water to make 

 carbohydrates (simple sugars). This process of photosynthesis is a 

 very complex one and some essential factors are chlorophyll, light, 

 carbon dioxide, oxygen, water, temperature, certain minerals, and 

 enzymes (organic catalysts which accelerate chemical reactions). 

 The energy absorbed from the sunlight is employed to activate 

 atoms which results in the synthesis of new compounds capable of 

 oxidation and release of energy. Plants also show other reactions 

 to light. The intensity and wave length of light affects size, time 

 of fruiting, amount of fruit produced, and transpiration (evapora- 

 tion of water). Animals, by eating the plants, liberate the energy 

 and utilize it in doing work, in producing heat, and in building 

 more living tissue. 



The eye is man's most important receptor of radiation. The light 

 coming into the eye is focused by the lens and the image (inverted) 

 is thrown upon the retina. Retinal cells contain photosensitive 

 substances which are affected by the quantity and quality of the 

 radiation. The photochemical reactions incite nerve impulses which 

 travel to the visual centers of the brain. Moderate amounts of 

 sunlight are not injurious to the eye, but extreme intensities are 

 harmful and may result in temporary or permanent blindness, eye 

 fatigue, and cataracts. The glare of carbon arcs and the reflection 

 of bright sunlight should be avoided. 



Infrared Radiation 



Both sunlight and artificial light sources have much of the radi- 

 ant energy output in the infrared. The wave lengths of the radia- 

 tion are too long for visibility and the energy is absorbed in tissues 



