48 



GENERAL CONCEPTS 



'^i'^'^S'-^ 



Figure 3.8. Histochemical demonstration of the location of the enzyme alkaline 

 phosphatase within the cells of the rat's kidney. The tissue is carefully fixed and sec- 

 tioned by methods which do not destroy the enzyme's activity. The tissue section is incu- 

 bated at the proper pH with a naphthyl phosphate. Some hydrolysis of the naphthyl 

 phosphate occurs wherever the phospliatase enzyme is located. The naphthol released 

 by the action of the enzyme couples with a diazonium salt to form an intensely blue, 

 insoluble azo dye which remains at the site of the enzymatic activity. The photomicro- 

 graph thus reveals the sites of phosphatase activity, i.e., the sites at which the azo dye 

 is deposited. The cells of the proximal convoluted tubules (left) have a lot of enzyme, 

 those of the loop of Henle (right) have little or no activity. (Courtesy of R. J. Barrnett.) 

 (Villee: Biology.) 



cules of foodstuffs as the chemical energy of the bonds connecting the 

 atoms in the food molecules. This chemical energy is a kind of potential 

 energy. When these food molecules are taken within a cell, chemical 

 reactions occur which change this potential energy into heat, motion, 

 or some other kind of kinetic energy. All forms of energy are at least 

 partially interconvertible, and living cells constantly transform potential 

 energy into kinetic energy or the reverse. If the conditions are suitably 

 controlled, the amount of energy entering and leaving any given system 

 can be measured and compared. Such experiments have shown that 

 energy is neither created nor destroyed, but simply transformed from 

 one form to another. This is an expression of one of the fundamental 

 laws of physics, the Law of the Conservation of Energy. Living things 

 as well as nonliving systeins obey this law. 



14. Molecular Motion 



The constituent molecules of all substances are constantly in mo- 

 tion. Despite the fact that wood, stone and steel seem very solid, their 

 component molecules vibrate continuously within a very restricted space. 

 The prime difference between solids, liquids and gases is the freedoin 

 of movement of the molecules present. The molecules of a solid are very 

 closely packed and the forces of attraction between the molecules permit 

 them to vibrate but not to move around. In the liquid state the molecules 

 are somewhat farther apart and the intermolecular forces are weaker, so 

 that the molecules can move about with considerable freedom. The 



