PROTOPLASM AND CELLULAR ORGANIZATION 



19 



necessary. We do not know how chlorophyl 

 is able to convert the hght energy into chem- 

 ical energy, nor how this chemical energy is 

 used to synthesize glucose from carbon 

 dioxide and water. 



Because this synthesis is dependent on 

 light, it is called photosynthesis. The photo- 

 synthetic equation is written as follows: 



CO2 4- H.O + Light 



-f Chlorophyl 



Glucose + Oxygen 



We know that the above equation is no 

 more than a statement of input and output. 

 Chemical studies involving the use of "la- 

 beled" carbon dioxide reveal that there are 

 probably dozens of intermediate chemical 

 processes. 



Since animals must have organic food, 

 plant products are necessary either directly 

 or in the form of protoplasm built up by 

 other animals out of plant food. Before 

 animal growth is possible, food must be con- 

 verted into living substance. 



Digestion is the process by which food 

 materials are broken down into simpler sub- 

 stances so they can be absorbed. This is a 

 nutritive process, and, while not a part of 

 metabolism as defined above, it is necessary 

 if metabolism is to continue. Material can- 

 not be absorbed unless it is in a liquid condi- 

 tion. Water may be absorbed without 

 change. Many mineral salts are easily ab- 

 sorbed, the process depending on their con- 

 centration. Carbohydrates must be broken 

 down into simple sugars, such as glucose, 

 before their absorption is possible. This is 

 accomplished with the help of complex 

 substances produced by the protoplasm, 

 which are known as enzymes.* Fats must be 

 broken down by enzymes into glycerin and 

 fatty acids before they can be absorbed. Pro- 

 teins are likewise acted upon by enzymes, 

 eventually becoming amino acids, which 



* The importance of enzymes in life processes 

 cannot be overemphasized. The modern biochemist 

 is inclined to believe that living things are chiefly a 

 matter of enzymatic reactions. It has been estimated 

 that there are 3000 to 5000 different enzymes in a 

 cell. 



are absorbable. In very small animals, di- 

 gested food does not need to be transported 

 very far in order to become distributed 

 throughout the body, but in larger animals 

 some sort of circulatory system is necessar}' 

 for this purpose. 



Assimilation, an important part of ana- 

 bolism, is the process of converting absorbed 

 material into protoplasm. During this proc- 

 ess comparatively simple materials are built 

 up into more complex compounds with the 

 aid of enzymes produced by the protoplasm; 

 that is, the protoplasm manufactures en- 

 zymes which convert digested and absorbed 

 materials into more protoplasm. The result 

 is replacement of the protoplasm that is 

 broken down; and after this has been re- 

 placed, growth takes place. 



Energy is defined as the ability to do 

 work, to produce a change in matter; it may 

 take the form of motion, heat, light, or elec- 

 tricity. Energy is derived ultimately from 

 sunlight and is stored in the molecules of 

 food as chemical energy. Chemical reactions 

 inside the body occur, changing the chem- 

 ical energy to heat, motion, or some other 

 kind of energy. Under experimentally con- 

 trolled conditions, the amount of energy en- 

 tering and leaving any given system may be 

 determined and compared. It is always 

 found that energy is neither created nor de- 

 stroyed, but only changed from one form to 

 another. This generalization is known as the 

 Law of the Conservation of Energy. This 

 law applies to living as well as nonliving sys- 

 tems. 



Energy is contained in the organic mole- 

 cules in protoplasm and in stored substances 

 in the body and is liberated when these 

 molecules are broken down by oxidation. A 

 simple example of the oxidative process is 

 as follows: 



Carbon 

 Sugar Oxygen dioxide Water 

 CflHiaOe + 6O2 -> 6CO2 -f 6H2O + Energy 



According to this equation, oxygen splits the 

 sugar molecule into carbon dioxide and 

 water, thereby liberating energy. Oxidation 



