STRUCTURE AND ACTIVITIES OF CELLS 



anabolism and catabolism. Anabolism includes those reactions by which suit- 

 able substances are built up or synthesized into the constituent compounds of 

 cells. Catabolism includes the destructive reactions which occur in cells, 

 bringing about the release of energy, together with the production of heat and 

 certain waste products. Growth, or increase in cellular volume, occurs in 

 living organisms when anabolism occurs at a more rapid rate than catabolism. 

 Organisms grow from within; they difTer in this respect from inorganic 

 crystals, which grow by the process of accretion, the deposition of additional 

 material on the surface of that already present. Metabolic reactions- are 

 catalyzed by the enzymes (p. 28) typical of living cells. 



Responsiveness is the capacity as a result of which cells respond to stimuli, or 

 changes in their environment. It is on this characteristic that the interactions 

 not only among the various constituents of a cell but also among the differ- 

 ent cells and systems of an individual depend. It is well known that non- 

 living things react in certain definite ways to changes in their surroundings, 

 as when metals undergo particular amounts of contraction for particular 

 decreases in temperature. The reactions of living organisms, however, involve 

 both chemical and physical factors and are so much more complicated as to 

 be distinctive. 



Reproduction is the capacity as a result of which certain cells of an organism 

 can become detached and, either alone or after union with cells of another 

 organism of the same kind, can give rise to a new individual capable of be- 

 coming in all essential respects like the parent or parents. Nothing com- 

 parable with reproduction is known to occur among inanimate objects. The 

 capacities of metabolism and responsiveness enable the individual organism 

 to maintain itself as a living unit. Maintenance of the species is made 

 possible as a result of the capacity of reproduction. 



Methods of Studying Cells 



It has been stated that discovery of the fact that living animals and plants 

 are composed of cells containing nuclei was made possible by the invention 

 and improvement of what we now call the compound microscope or light 

 microscope. Increase in knowledge of the structure of the cell and its parts 

 has gone hand in hand with improvements in techniques for preparing the 

 cells for study and in instruments for magnifying small objects. 



From an early date, students of the cell found that rapid killing or so-called 

 fixation of tissues, followed by other special procedures, made it possible to 

 slice them in thin sections. This increased the amount of light which could 

 pass through them and facilitated microscopic examination. Staining of such 

 thin sections adherent to glass slides can be accomplished with dyes that have 

 selective affinities for diflferent parts of the cell. Skill in grinding and as- 

 sembling lenses from the finest glass greatly increased the magnification and 



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