UNITS OF LIFE-CELLS 



61 



contains all of the food requirements of the 

 cell. Hence, the molecules need only trav- 

 erse the membrane to gain entrance into the 

 cell. Within the cell they are transported 

 throughout the protoplasm by diffusion. 

 Having reached this destination, the food 

 is available for metabolism by the cell. 



Metabolism of food is the most important 

 part of nutrition because it is the process 

 by which energy is released from the 

 energy-rich compounds and building ma- 

 terials are incorporated into the structure 

 of the cell. The business of providing the 

 cells with food and removing the waste 

 products is merely accessory to the real 

 job of extracting energy from the food and 

 building it into new protoplasm. During 

 constructive metabolism (anabolism) the 

 absorbed amino acids are built into the pro- 

 tein framework of the cell's protoplasm. 

 Those not needed for this purpose are 

 deaminized (amino groups removed) and 

 converted to glucose. Fats are stored and 

 some of the glucose is converted to glyco- 

 gen and stored. In the destructive metabolic 

 ( catabolic ) phase, the glucose is burned to 

 carbon dioxide and water through a long 

 series of steps producing many intenne- 

 diary products. Fats are also oxidized and 

 release large amounts of energy, leaving 

 end products of carbon dioxide and water. 

 Such energy is used to produce heat and 

 movement, and to bring about the anabolic 

 conversions just mentioned, for they are all 

 endothermic reactions requiring energy be- 

 fore they can take place. 



In order that oxidation can occur in the 

 cell, oxygen must be supplied to it. This 

 is abundantly furnished in the waters sur- 

 rounding the single cells and it must like- 

 wise be supplied in the fluids such as lymph 

 surrounding the metazoan cell. Oxygen 

 must be available at all times if the process 

 of metabolism is to go forward normally. 

 This gas, like food products, passes readily 

 through the membrane of the cell by diffu- 

 sion and mingles with the molecules in the 

 protoplasm, ready to combine with glucose 



or with fats in order to release energy. 



As a result of these metabolic activities 

 within the cell, certain waste products are 

 formed which must be removed before they 

 accumulate to toxic proportions. These 

 wastes consist of carbon dioxide and water 

 which result from the burning of glucose 

 and fats, and nitrogen wastes including 

 urea which results from the breakdown of 

 nitrogen-containing compounds, principally 

 amino acids. Some inorganic salts, such as 

 phosphates and sulfates, accumulate as a 

 result of the decomposition of phospho- 

 lipids and certain sulfur-containing amino 

 acids, and they too must be eliminated or 

 excreted. Wastes leave the cell through 

 the membrane by the same process that the 

 food entered, and are deposited into the 

 surrounding fluid in the case of metazoan 

 cells ( many-celled animals ) or the envelop- 

 ing water in the case of single cells. Any- 

 thing that interferes with the elimination of 

 these products affects the metabolism of the 

 cell itself; in fact, the cell can survive only 

 a short time if the waste products are forced 

 to accumulate in any quantity. 



Reproduction. Another fundamental 

 problem that the cell must solve is repro- 

 duction. All isolated cells must duplicate 

 themselves periodically if they are to in- 

 crease their numbers. The ability to do this 

 often determines whether or not a species 

 will succeed as a race. The individual cells 

 of a multicellular animal multiply rapidly 

 during embryonic life and some do through- 

 out life. Others are produced once and are 

 never duplicated again. Just how this is 

 accomplished at both the cellular and mul- 

 ticellular level will be discussed in subse- 

 quent chapters. 



We have seen then that cells, while 

 highly variable in size and morphology, all 

 have the same fundamental needs which 

 must be cared for whether they are a part 

 of a many-celled animal or live as isolated 

 individuals. Let us now consider how these 

 needs are satisfied when cells began to live 

 in groups, that is, in a metazoan. 



