132 - The Cell 



In the amoeba, absorbed foods can diffuse 

 quite rapidly throughout the cytoplasm and 

 nucleus. Moreover, the protoplasm constantly 

 flows, as the amoeba moves in search of 

 food, and the protoplasmic streaming tends 

 to accelerate distribution. 



Metabolism. Metabolism — the sum total of 

 all eh cm i nil processes occurring in the pro- 

 toplasm of an organism — represents the most 

 essential phase of nutrition. All other nutri- 

 tional processes are quite accessory to me- 

 tabolism, since they merely provide the 

 materials for metabolic activity, or remove 

 the metabolic wastes. 



Metabolism accomplishes two main objec- 

 tives in every organism: (1) it liberates en- 

 ergy, which finds tangible expression in the 

 movements and other activities of the organ- 

 ism; and (2) it achieves the synthesis of new 

 organic compounds, which are necessary for 

 the growth and maintenance of protoplasmic 

 structure. On this basis, in fact, the metab- 

 olism of each organism is subdivided into 

 two major parts: (1) catabolism, which in- 

 cludes all decomposition (exothermic) reac- 

 tions and provides energy for the organism; 

 and (2) anabolism, which embraces all syn- 

 thetic (endothermic) reactions and creates 

 new organic components in the protoplasm. 



The metabolism of even the simplest cell 

 involves a wide variety of chemical changes, 

 cataly/ed by a correspondingly wide variety 

 of intracellular enzymes. And although quite 

 a few matabolic reactions have been dupli- 

 cated outside the cell, many details of inter- 

 mediary metabolism, especially in lower ani- 

 mals, are still unknown. 



Regardless of details, we do know that food 

 substances in the protoplasm of an animal 

 cell, such as the amoeba, are launched upon 

 a series of chemical changes. In an anabolic 

 direction, these reactions give rise to new 

 proteins, lipids, carbohydrates, etc.; and 

 these substances, together with absorbed 

 water and salts, become organized into new 

 protoplasm as the organism grows. Simul- 

 taneously, many organic foods are oxidized 

 and decomposed in other ways. The energy 



from catabolism not only goes to sustain the 

 mechanical and other work of the cell, but 

 also activates anabolism, which as a whole is 

 endothermic. Moreover, the metabolism of 

 the amoeba produces a number of waste 

 products, which are referred to collectively 

 as the metabolic (or excretory) wastes. 



The metabolic wastes of the amoeba and 

 other animal cells include water, carbon di- 

 oxide, simple nitrogenous compounds such 

 as ammonium salts, and a variety of other 

 inorganic salts. Water and carbon dioxide 

 are produced from the oxidation of all or- 

 ganic foods, since hydrogen and carbon are 

 always present in these compounds. Nitro- 

 genous wastes, however, are derived entirely 

 from the decomposition of nitrogen-contain- 

 ing compounds, chiefly the amino acids. 

 Other inorganic salts — such as sulfates and 

 phosphates — result from the decomposition of 

 compounds containing sulfur (for example, 

 certain amino acids) and phosphorous (for 

 example, phospholipids), etc. 



Respiration. Without oxygen the metab- 

 olism of the amoeba and other animal cells 

 may continue for a time, but without oxida- 

 tive metabolism the intracellular reserves 

 of ATP and other immediately available 

 sources of energy (p. 143) are soon exhausted, 

 and metabolism as a whole begins to falter. 

 In addition, carbon dioxide must be dis- 

 posed of before it accumulates to toxic levels. 

 Tins continuous exchange of gases, in ivhich 

 oxygen enters and carbon dioxide leaves the 

 cell, constitutes the process of respiration. 1 

 Unless respiration continues, aside from rela- 

 tively short interruptions, most cells begin 

 to die of asphyxiation. 



Unicellular animals do not expend energy 

 in obtaining oxygen or in disposing of carbon 

 dioxide. In fact, among animals generally, 

 respiration proceeds on a diffusional basis, 

 although higher animals have circulatory 

 fluids, such as blood, which carry 2 and CO« 

 between the cells and environment. In the 



1 Many biologists prefer to use the term respiration 

 more broadly, to include what here is designated as 

 oxidative metabolism. 



