BIOLOGICAL SIGNIFICANCE OF HEMOGLOBIN 193 



ficance of these differences. It is not suggested for instance that it 

 is a matter of any moment what crystaUine form a particular haemo- 

 globin assumes. That is not to say that the factor in the molecule 

 which decides the crystaUine form may not be very important for 

 other reasons. I only wish to warn the reader lest he should think 

 that because he discovers a fact, that fact may have a biological 

 significance, and I take crystaUine form as an example, because so 

 far as I know haemoglobin in a functional state never occurs in 

 crystals, and therefore the form in which it crystalhses is of no im- 

 portance. It is when we come to the relationship of haemoglobin to 

 gases that we have to consider rather closely whether such facts as 

 we observe are biologicaUy significant. It seems quite clear that 

 different forms of fife are possessed of haemoglobins with funda- 

 mentally different affinities for oxygen. For instance, man, Plan- 

 orbis and the frog at 15° C. have very different dissociation curves 

 — that is an understatement, man and the frog have affinities 

 of quite a different order at 15° C. Has this fact any biological 

 significance ? The first point that occurs to me is that human haemo- 

 globin does not operate at 15° C. but at 37° C. or thereabouts. But at 

 what temperature does frog's haemoglobin function? It may operate 

 at 37° C. in the swamps of tropical Florida, or at 7° C. in the ponds 

 during the British frosty spring time, and therefore not only must 

 its properties be appropriate to some one temperature, but the 

 temperature coefficient of its affinity to oxygen must be such as 

 to adapt it to the uses of the body over a wide range, and therefore 

 cannot be quite a haphazard affair — ^it must be related to the 

 temperature coefficients of other chemical processes in the frog. In 

 the case of man one may speculate too upon whether the temperature 

 coefficient has any particular meaning. It appears to be more than 

 twice that of the frog, and instead of being about 2 it is somewhere 

 in the region of 5. 



Now it may be that in an organism so nearly homoiothermic 

 as man it is a matter of no importance whether haemoglobin has 

 any temperature coefficient at all, but if the temperature coefficient 

 is to be of importance, clearly it must be very high. If, for instance, 

 it is important that during exercise or fever haemoglobin should 

 dissociate from oxygen more rapidly than at normal body tempera- 

 ture, a smaU temperature coefficient would effect nothing. On the 

 other hand, the high temperature coefficient of human haemoglobin 

 may be just an accident, it may be abnormal just because it does 



BII 13 



