Growth and Form 93 



material, which we find by determining the weight or volume changes in 

 an organism. Or we can pick out some constituent of protoplasm that is 

 always a constant proportion of the whole, such as the nitrogen content 

 or protein content. If either were to increase, we would know to what 

 extent the total protoplasmic mass had increased. 



It should be noted that both cell number and protoplasmic content 

 do not always increase together. For instance, cell division can occur 

 without any increase in protoplasm; the result is a greater number of 

 smaller cells. Alternatively, protoplasm can be synthesized in the absence 

 of cell division, in which case the cells grow larger but not more numerous. 

 However, isolated growth of this kind can go on only under exceptional 

 conditions and for a relatively short time. Ultimately cell division must 

 cease without protoplasmic increase and vice versa. 



Nutrition 



The act of growth, when it involves an increase in protoplasm, re- 

 quires the synthesis of a wide variety of cell constituents— nuclei including 

 chromosomal material, centrioles, nucleoli, etc., mitochondria and other 

 cytoplasmic organelles, thousands of enzyme molecules, cell membranes 

 and other structural material. These require the synthesis of macromole- 

 cules such as proteins, nucleic acids, and polysaccharides in which many 

 subunits are linked together. The subunits themselves, amino acids, 

 sugars, fatty acids, vitamins, etc., must be synthesized from still simpler 

 compounds or must be assimilated from the environment. 



Two basic needs must be met if these activities are to be accom- 

 plished—the need for energy and the need for raw materials. Cells obtain 

 energy by the oxidation of materials obtained from the environment. 

 Sugars are the prime fuels for this purpose, but many other oxidizable 

 compounds can be employed by one or another kind of cell. The energy 

 thus made available is employed for the svTithetic reactions. At their 

 simplest levels, the raw materials for the synthesis of protoplasm are the 

 elements of which cells are composed. The major components are carbon, 

 hydrogen, oxygen, nitrogen, sulfur, phosphorus, magnesium, manganese, 

 iron, and traces of many other elements as well. The elements must, how- 

 ever, be supplied in usable form. For example, the amino acid, alanine, 

 CH3CH2-NH0 COOH, is essential for the synthesis of proteins. No known 

 organism can synthesize alanine if it is merely supplied with elemental 

 carbon, hydrogen, oxygen, and nitrogen. But some cells can do so by 

 converting nitrogen from the air to ammonia and combining the ammonia 

 with pyruvic acid as follows : 



NH3 + 2H + CH3CO COOH > CHgCHNHs-COOH + HoO 



