96 - The Cell 



centrated limit, control, and regulate the 

 metabolic reactions. Despite the size, each 

 cell may be considered as a delicately organ- 

 ized "chemical engine." In the cell a wide 

 variety of reactions go on simultaneously and 

 more or less independently. When this deli- 

 cate organization breaks down, the chemical 

 activity does not cease immediately. Rather 

 the metabolism becomes disorganized, un- 

 regulated, and purely destructive, leading to 

 a decomposition of many of the essential 

 protoplasmic compounds. 



Because protoplasmic structure and metab- 

 olism are mutually interdependent, a cell 

 once dead cannot be "started up" again: the 

 metabolism cannot be resumed in the absence 

 of the living structure; and this structure 

 cannot be restored without the metabolism. 

 But the "death" of a cell is not usually a very 

 abrupt and tangible change. In fact the 

 intergradations between disease, injury, and 

 death are very subtle; but when the proto- 

 plasmic structure has degenerated beyond 

 restoration, the cell is dead, however difficult 

 it may be to determine exactly when this 

 point of degradation is reached. From a sci- 



entific standpoint, the death of a multicellu- 

 lar individual, such as man, is still more 

 indefinite. The many cells of the body do 

 not all die at once. Some of the tissues may 

 continue to live many hours after the man 

 is "officially" dead. The usual verdict of 

 death is given when the heartbeat and breath- 

 ing stop, but these movements can sometimes 

 be restored many minutes later. But without 

 respiration and circulation, no oxygen is car- 

 ried to the body cells; and in the absence of 

 oxidative metabolism, the protoplasmic struc- 

 ture gradually crumbles. Then sooner or 

 later in each of the tissues the degradation 

 becomes irreversible. 



Thus all living organisms originate from 

 the protoplasm of other living organisms; 

 and the "endless" chain of specific structure 

 and metabolism goes on unbroken, except 

 when a species becomes extinct. Of the ulti- 

 mate origin of the protoplasmic system, we 

 have very little direct knowledge. Presum- 

 ably protoplasm was evolved very gradually, 

 through the millions of years of early geo- 

 logical time, from much simpler structures 

 and processes (Chap. 10). 



TEST QUESTIONS 



1. Among the elements represented in cells gen- 

 eral]) 1 , which of them are: 



a. never present in uncombined form 



b. present as the free element, as well as 

 in combined lorni 



c. present in the merest traces 



d. exceptionally abundant 



e. univalent 



f. divalent 



g. tetravalent 



2. In what way might you be able to convince 

 your younger brother that water (and all 

 other forms of matter) is composed of sub- 

 visibly small particles, or molecules? 



3. Specify at least six kinds of energy; what are 

 the primary distinctions between matter and 

 energy? 



4. Carefully define and exemplify each of the 

 following terms: (a) molecules and atoms; 

 (b) a substance and a mixture: (c) electrons 

 and protons; (d) kinetic and potential energy. 



5. Explain how the valence of an element is 

 related to its atomic structure. 



6. Distinguish between electropositive, electro- 

 negative, and inert atoms— giving an example 

 in each case. 



7. Discuss the physical properties of water in 

 relation to the structural and functional char- 

 acteristics of protoplasm. 



8. Distinguish between electrolytes (strong and 

 weak) and nonelec trolytes: what is the gen- 

 eral importance of electrolytes in the life of 

 a cell? 



9. Distinguish between the members of each 

 group of terms: 



a. acid, basic, and neutral solutions 



b. an acid substance and an acid solution 

 10. Explain a method by which the reaction 



(acidity or alkalinity) of the protoplasm of 

 the nucleus and cytoplasm of certain cells 

 has been measured. State the results of such 

 experiments. 



