412 LIFE : OUTLINES OF GENERAL BIOLOGY 



haemoglobin, formed at the place of oxygen-capture, in gills or 

 lungs, and haemoglobin which has parted with oxygen at the seat 

 of combustion, such as the muscles. 



There seems to be no more difference between chlorophyll-a 

 and chlorophyll-6 than there is between haemoglobin and oxyhaemo- 

 globin; in both cases the latter contains more oxygen. Both the 

 chlorophylls absorb red and orange light, and therefore appear 

 green. Very different from the chlorophylls, but similarly related 

 to one another, are the two yellowish pigments, carotin and xantho- 

 phyll, to which we shall return. How chlorophyll and its associated 

 pigments join in the process of reducing carbonic acid and building 

 up carbon compounds, from formaldehyde and sugar upwards, is 

 still uncertain; and the problem is discussed under the heading 

 photosynthesis. It is enough here to mention the certainty that 

 chlorophyll combines with carbonic acid. 



Blood-pigments. — As a convenient second group we may rank 

 the blood-pigments, notably the red pigment haemoglobin, which 

 is characteristic of Vertebrate animals and occurs in some Inverte- 

 brates as well. Earthworms and many seaworms have, in a general 

 way, the same blood-pigment (haemoglobin) as man, and so have 

 the larval stages of the Harlequin fly, popularly known as "blood- 

 worms". The last instance is interesting since these larvae live in 

 stagnant water that has less oxygen-content than usual, and 

 haemoglobin excels all other blood-pigments, such as the haemo- 

 cyanin of most crustaceans, in its capacity for entering into a 

 loose union with oxygen. The "blood- worm" illustrates physiological 

 adaptation. 



As we have mentioned, haemoglobin is remarkably like chloro- 

 phyll in several ways. Its molecule is even larger, and again it 

 may be split into two parts. But the colourless portion in this 

 case is not an alcohol, but a protein called globin. The coloured 

 portion, now called hcsfn, again consists of four pyrrol rings linked 

 together with an atom of metal, which in this case is not magnesium 

 but iron. These resemblances are striking. I'nlike chlorophyll, 

 which is the same in all green plants, hc-emoglobin varies from one 

 animal to another, but these differences concern not the essential 

 nucleus (hiem), but the attached protein (globin). It is a fine illus- 

 tration of specificity that when the haemoglobin of different kinds 

 of animals is made to crystallise, the blood-crystals show differences. 

 Even in nearly related species, such as horse and ass, or dog and 

 fox, there is a difference in the details of the hemoglobin crystals. 

 In fact, there is an e.xample of a chemical basis of species of which 

 we shall afterwards see more. 



Although haemoglobin shows an alternation of gaining and 

 parting with oxygen, its functions are very different from those 

 of chlorophyll, and they cannot be related to the jxiwer of ab.«;orbing 



