3IO FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



properties of chitin. Young Lepidopferan scales yield a curve of rodlet 

 birefringence with a minimum at ng = 1.57 instead of 1.61 (Fig. 151). 

 From this behaviour Picken (1949) concluded that growing scales are 

 not yet chitinous, and he proved that they consist at this stage of 

 protein similar to muscle protein (n = 1.57) or keratin (n = 1.55)- 

 No intrinsic double refraction is then visible ; it appears only in fully 

 grown scales after the formation of chitin with its typical negative 

 character. 



d. Starch Grains (Amylose and Amylopectin) 



Molecular structure of starch. The reserve carbohydrates sucrose, 

 maltose and starch are a-glucosides (see page 60), in contradis- 

 tinction to the skeletal carbohydrates cellulose, xylan, etc. which are 

 jS-glucosidic. Compared to the straight cellulose chains, the glucosan 

 chains with a- 1-4 bonds are rather kinked (Meyer and Mark, 1930). 

 The result is that a spatial lattice of such chains must be less compact 

 and, therefore, is more soluble, as indeed its physiological function 

 as a reserve material requires it to be. Evidently the voids formed by 

 this particular molecular configuration are partly filled with water 

 molecules. Even the simplest a-glucoside, maltose, crystallizes with 

 water of crystallization, and loosely bound water molecules also play 

 an important part in the crystal lattice of starch. They do not, ad- 

 mittedly, escape from the lattice as easily as from protein crystals, but 

 when grains of starch are crushed, their lattice structure is likewise 

 wrecked as the result of loss of water; they become amorphous, the 

 birefringence and their X-ray diagram (Sponsler, 1922) vanishing. 

 Hence additive water molecules apparently stabilize the lattice order 

 of starch, as is the case in the reserve proteins. 



The chemistry of starch is complicated by the presence in the starch 

 grains of two chemically distinct substances, viz., amylose and amylo- 

 pectin. Amylose is soluble in hot water and is stained blue by iodine, 

 whereas amylopectin swells in boiling water and gives a violet iodine 

 coloration. Thus, when the starch grains 'form into a paste, amylose 

 goes into solution, while the amylopectin becomes a swollen, in- 

 soluble jelly. Neither component exhibits any reducing power upon 

 Fehling's solution, which signifies that neither contains free aldehyde 

 groups. K. H. Meyer (1940b) has discovered the difference in con- 

 stitution between amyloses and amylopectin. He states that amyloses 



