CELL METABOLISM 77 



The amount of light produced by certain himinescent animals is 

 amazing. Many fireflies produce as much light, in terms of lumens per 

 square centimeter, as do modern fluorescent lamps. Different kinds of 

 animals may emit lights of different colors, red, green, yellow or blue. 

 One of the more spectacular luminescent beasts is the "railroad worm" 

 of Uruguay, the larva of a beetle, which has a row of green lights along 

 each side of its body and a pair of red lights on its head. The light pro- 

 duced by luminescent organisms is entirely in the visible part of the 

 spectrum; no ultraviolet or infrared light is produced. Since very little 

 heat is given off in the process, bioluminescence has been called "cold 

 light." 



What advantage an animal derives from the emission of light can 

 only be guessed at. For deep sea animals, which live in perpetual dark- 

 ness, light organs might be useful to enable members of a species to 

 recognize one another, to serve as a lure for prey or as a warning for 

 woidd-be predators. Experiments have shown that the light emitted by 

 fireflies serves as a signal to bring the two sexes together for mating. The 

 light emitted by bacteria and fungi probably serves no useful purpose 

 to the organisms, but is simply a by-product of oxidative metabolism, 

 just as heat is a by-product of metabolism in other plants and animals. 



Questions 



1. How would you define the term "metabolism"? 



2. What factors affect the rate of a chemical reaction in the test tube? In a living cell? 



3. Define the following terms: enzyme, coenzyme, apoenzyme, substrate, turnover num- 

 ber, energy-rich phosphate, coupled reactions. 



4. What might be the advantage to a cell of having all the enzymes that act in sequence 

 on a given substance localized in a particular intracellular organelle such as a mito- 

 chondrion or microsome? 



5. Discuss the several meanings of the term "respiration." 



6. Indicate brieflv how the carbon chain of an amino acid might become part of (a) a 

 glycogen molecule and (b) a fatty acid molecule in an animal cell. 



7. What factors do you suppose have led to the evolution of luminescent organs in ani- 

 mals? 



8. Suppose you discovered a new species of bioluminescent worm. How could you prove 

 that it was the worm itself and not some contaminating bacterium that was producing 

 the light? 



Supplementary Reading 



A series of articles on the many different fields of biology in which enzymes play a 

 role is found in Enzymes: Units of Biological Structure and Function, edited by O. A. 

 Gaebler. Baldwin's Dynamic Aspects of Biochemistry gives a technical but extremely 

 interesting account of the details of cellular metabolism. Rudolf Schoenheimer presents a 

 summary of his classic experiments demonstrating the rapid renewal of the chemical 

 constituents of the body in The Dynamic State of the Body Constituents. The phenome- 

 non of bioluminescence is described by E. \. Harvey in Living Light and, in a more 

 detailed fashion, in Bioluminescence. L. J. Henderson, in his classic, The Fitness of the 

 Environment, advanced the thesis that the environment had to have certain chemical 

 and physical characteristics for life to develop. A number of eminent biochemists and 

 physiologists present their current theories and findings in Currents in Biochemical 

 Research, edited by D. E. Green. 



