CHAPTER III 



BIOCHEMISTRY AND TAXONOMY 



I. BIOCHEMICAL DIVERSITY 



"Each species consists of groups of individuals with more or less similar 

 gene combinations, optimally adapted for a given environment" (Mayr, 

 1949). This definition of species implies the existence of biochemical 

 characters typical of the species and that these specific characters are 

 adapted to the ecological niche in which the species prospers and dominates 

 its competitors. 



To illustrate this, let us consider the special properties of the haemo- 

 globin in animals. 



Typically, the respiratory function of the internal environment operates 

 by means of a cycle in which the internal environment circulates around the 

 organism between the various tissues and the point where it is equilibrated 

 with the external environment or a continuation of the latter. 



At the point of contact with the external environment the blood becomes 

 charged with oxygen and gives up carbon dioxide. On reaching the tissues 

 it loses oxygen and takes up carbon dioxide. This typical respiratory cycle 

 requires the mediation of a physico-chemical system to bring about the 

 transport of oxygen in one direction and of carbon dioxide in the other. 



There are a number of properties of haemoglobin which are important 

 when it functions as a carrier. It can be oxygenated and the degree of 

 oxygenation is a function of the partial pressure of oxygen. Being a hetero- 

 protein, it bears acid groups which can combine with bases, the dissociation 

 of these groups varies with pH and cations are lost when the pH approaches 

 the isoelectric point. The dissociation does not only vary with the pH but 

 also according to the degree of oxygenation, and inversely, the degree of 

 ox}^'genation at a given partial pressure of oxygen will vary with the disso- 

 ciation of the acid groups in the neighbourhood of the oxygen-bearing 

 groups. On the other hand, the carrier molecule bears free — NHg groups 

 which are able to form compounds of the carbamate type with carbon 

 dioxide. 



We may plot the amount of absorbed oxygen as a function of its partial 

 pressure for different bloods and coelomic fluids, the temperatures and 

 carbon dioxide partial pressures being those existing in the arterial blood. 



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