Animal Cells and Their Nufrition - 1 33 



amoeba, however, 2 and C0 2 are exchanged 

 directly between the protoplasm and the sur- 

 rounding pond water (see Fig. 6-4). 



Excretion. Excretion is the process by 

 which metabolic wastes (excluding carbon 

 dioxide) are eliminated from the organism. 

 The excretory wastes (p. 138) are not very 

 toxic, unless they accumulate in the proto- 

 plasm, but waste products are produced so 

 constantly during metabolism that most ani- 

 mals can only survive a few hours if excre- 

 tion fails to occur. 



Except for water, the excretory wastes of 

 amoeba and other one-celled animals are 

 eliminated by diffusion. Owing to metab- 

 olism, such wastes as ammonium salts and 

 other salts reach a higher concentration in- 

 side the cell than outside. Accordingly, these 

 waste products pass out to the environment 

 spontaneously. But the amoeba is forced to 

 expend energy to eliminate water, since the 

 surrounding pond water is distinctly hypo- 

 tonic to the protoplasm. The total quantity 

 of water eliminated by the contractile vacu- 

 ole represents the sum of two parts: (1) the 

 larger part, which constantly enters the cell 

 by osmosis from the hypotonic outside me- 

 dium, and (2) the smaller part, which is pro- 

 duced metabolically from the oxidation of 

 hydrogen compounds in the protoplasm. 



In higher animals the blood stream serves 

 as an intermediary in excretion as well as in 

 respiration. The metabolic wastes pass into 

 the blood from the cells all over the body, 

 and are carried to the kidneys, or other ex- 

 cretory organs, where excretion finally occurs. 



METABOLISM, A DYNAMIC EQUILIBRIUM 



Recent studies, stemming from the bril- 

 liant work of the late Rudolph Schoen- 

 heimer of Columbia University, have forced 

 biochemists to the conclusion that the or- 

 ganic components of an animal's body are in 

 a dynamic state of flux — to a most astonish- 

 ing degree. Schoenheimer, in 1938, was 

 among the first to use food substances labeled 

 with tagged atoms (p. 142) as a means of 



following their metabolic fate. He fed his 

 animals (rats) with amino acids labeled with 

 heavy nitrogen (N 15 ) and fatty acids labeled 

 with heavy hydrogen (H 2 ) and traced the rate 

 at which these organic compounds became 

 incorporated into the various tissues of the 

 body. The amazing conclusion that must be 

 drawn from this and many similar experi- 

 ments is that the amino acids are rapidly 

 incorporated into the proteins all over the 

 body, even when no increase in total pro- 

 tein is occurring; and a similar incorporation 

 of the fatty acids into the fat deposits also 

 occurs. In fact, the calculations show that 

 the whole protein content of the body, strik- 

 ing a general average, is broken down and 

 rebuilt again about four times a year, al- 

 though the proteins of some structures (for 

 example, the liver) show a much more rapid 

 turnover than those of others (for example, 

 bone). In other words, each animal is not 

 exactly the same individual from day to day, 

 at least in a chemical sense; and what has 

 been found true for man and other higher 

 animals must certainly be even more appli- 

 cable to one-celled animals, such as the 

 amoeba and paramecium. 



Metabolic Reactions of Animal Cells. 

 Typical animals, regardless of their size and 

 complexity, exhibit the same eight funda- 

 mental nutritive processes. In fact, the pat- 

 tern of holozoic nutrition is coextensive with 

 the animal kingdom, since it depends upon 

 the kind of metabolism that characterizes 

 animals. The metabolic enzymes of animals 

 are generally similar, although many specific 

 differences have arisen in the course of evo- 

 lution. In short, similar enzymes and similar 

 metabolic processes have been inherited by 

 practically all members of the animal king- 

 dom. 



Constructive Metabolism: Protein Synthesis. 

 A most vital problem faced by every cell is the 

 necessity to synthesize its own unique pro- 

 tein structures — the protein components of 

 its genes, ribosomes, enzymes, membrane sys- 

 tems, and so forth. In fact, the distinctive 

 characteristics of each kind of cell are pre- 



