442 - Multicellular Animals, Especially Man 



merely by discharging a volley of action po- 

 tentials, but the cells are connected in series, 

 like a string of serially wired batteries. Con- 

 sequently the action potentials of the indi- 

 vidual cells are summated, forming a total 

 discharge of considerable magnitude. In some 

 cases the total potential of the electric organ 

 may exceed 400 volts, which is sufficient to 

 stun or even kill a small fish if it receives the 

 full charge. The amperage from an electric 

 organ is not very great, because the effector 

 cells have a rather limited electrical capacity, 

 and each discharge endures for only about 

 0.005 second. However, enough current is 

 obtained from the electric organ of a sting- 

 ing ray to produce a series of brief flashes 

 when a lamp bulb is brought into the circuit. 



LUMINESCENT ORGANS 



Light is emitted by a wide variety of or- 

 ganisms. These include a number of bacteria 

 and fungi, as well as certain protozoans, 

 sponges, coelenterates, mollusks, crustaceans, 

 centipedes, millepedes, insects, and verte- 

 brates. Among vertebrates, however, light 

 production by the organism itself occurs only 

 in certain fish, especially deep-sea species. 

 Luminescence has not been observed in any 

 of the amphibians, reptiles, birds, or mam- 

 mals. 



Certain bacteria and other fungi luminesce 

 more or less continuously, when oxygen is 

 available, and in this case the light appears 

 to be an incidental by-product of oxidative 

 metabolism. But most luminescent animals, 

 like the firefly, give out flashes of light only 

 when the specialized luminescent organs are 

 stimulated. In some cases, the luminescent 

 organs are glands, which give forth luminous 

 secretions; but more specialized organs, such 

 as are found among insects, crustaceans, mol- 

 lusks, and fishes, produce their light intra- 

 cellularly. 



In the case of fireflies, at least, luminescent 

 flashing provides a system of signals, based 

 on timing, that guide the male flies to the 

 females of the same species. 



Chemistry of Bioluminescence. As was 



shown many years ago by Raphael Dubois in 

 France and by E. Newton Harvey in the 

 United States, it is possible to extract an or- 

 ganic substrate, called luciferin, and an en- 

 zyme, luciferase, from the luminescent organs. 

 These extracts give a flash of light when they 

 are mixed together in the presence of free 

 oxygen (0 2 ). However, different organisms 

 seem to have developed different varieties of 

 luciferin and different luciferases. Bacterial 

 luciferin, for example, appears to be a phos- 

 phorylated riboflavin-nucleic acid complex; 

 whereas the luciferin from one of the Crus- 

 tacea seems to be a yellow chromopeptid 

 compound. Moreover, ATP seems to play an 

 essential role in some cases, as in the flash 

 emissions of the firefly. The color of the 

 light, on the other hand, may vary according 

 to what luciferase is being used. 



The evidence indicates that the actual 

 emission of light is from the molecules of an 

 enzyme-substrate complex that becomes ener- 

 gized during oxidation. However, the ener- 

 gized state is short-lived. Free enzyme quickly 

 is liberated and catalysis continues so long 

 as free oxygen and other necessary substances 

 are available. 



Considerable progress has been made re- 

 cently toward the purification and identifi- 

 cation of the luciferin and luciferase com- 

 ponents of the various luminescent systems. 

 The bacterial system has been studied inten- 

 sively by F. H. Johnson and co-workers at 

 Princeton; the crustacean (cypridina) system 

 also has been studied by Johnson in collabo- 

 ration with an active group of investigators 

 in Japan; and the insect (firefly) system has 

 been investigated by W. D. McElrov and co- 

 workers 1 at Johns Hopkins University. In 

 the case of the firefly, it is now possible to 

 specify the precise molecular structure of 

 luciferin (Fig. 24-12); and it is well estab- 

 lished that the firefly luciferase is a long- 



1 Grateful acknowledgment should also be given to 

 the school children of Baltimore, who collected a vast 

 number of fireflies and delivered them to the bio- 

 logical laboratory. 



