Respiration and Metabolism 



111 



claimed to have a very low R.Q. (0.5) at 4° and a high R.Q. (0.86) at 22° 

 C.^^^ A similar increase in R.Q. value with rise in temperature, from 0.70 at 

 15° C. to 0.99 at 35° C, prevails in Paramecium caiidatiim.-^'* 



The R.Q. is not necessarily constant throughout the life cycle of an individu- 

 al and may reflect changes in diet. In those invcrtehrates with rather special- 

 ized and changeable feeding habits, the R.Q. variations may be striking. The 

 R.Q. of the shore crab, Carcinus maenas, falls from a value of 1.0 in the early 

 stage of development to 0.7 a few hours later, to rise again in successive stages 

 of ontogeny--"'" (Fig. 65). The R.Q. of the frog egg increases during develop- 

 ment from cleavage to gastrulation,-^' -•'''■• perhaps indicatmg a change from 

 lipoid to carbohydrate food reserves. The R.Q. of the silkworm embryo, 

 Bomhyx viori, is initially 0.79, drops to 0.64 at about the eighth day of incuba- 

 tion, and then increases to 0.75 just before hatching.^""' 



R.Q. 



I la 23456 78 



DEVELOPMENTAL STAGES 



Fig. 65. Respiratory quotient change during embryonic development of the crab, 

 Carcinus, indicating different food sources at various stages of development. After 

 Needham.-"" 



Despite certain limitations and precautions which must be exercised in 

 interpreting R.Q. data, the respiratory quotient is of considerable value in 

 identifying metabolic adaptations in organisms. Further critical analyses of 

 the sources of variation in R.Q. determinations, of "true" and "false" respira- 

 tory quotients, of the fixation and release of COj, should extend our concepts 

 rapidly along the route of the actual metabolic processes involved in gas 

 exchange and energy conversion. 



CONCLUSION 



The respiratory mechanisms which have e\'ol\'ed to provide the necessary 

 gas exchange for animals are varied and highly adaptive, both morphologically 

 and physiologically. Diffusion of oxygen into the organism proves adequate 

 with very small organisms, and, in fact, cutaneous respiration necessarily occurs 

 if a sufficient oxygen gradient is established across the more or less permeable, 

 usually moist, integument. Gas exchange always occurs across a water film. 

 The increase in respiratory surface made possible by the development of gills, 

 lungs, and tracheae, as well as the added efficiency of ventilating mechanisms, 

 permit increase in body size and metabolic activity. All of these specialized 

 mechanisms were adaptable to life on land as well as in water, and aerial 



