. The biology of marine animals. Marine animals; Physiology, Comparative. DIGESTION 267 the ends until dipeptides are formed; and the latter are split into their amino-acid components by dipeptidases. In some instances the secreted enzymes carry hydrolysis only as far as polypeptides, and further degrada- tion takes place intracellularly (7). Carbohydrases. Amylases capable of hydrolysing starch and glycogen probably occur in most animals, either extracellularly or intracellularly. Starches are broken down to dextrins and finally to maltose. Disaccharides,. 50 60 20 30 40 Digestion Time (minut
. The biology of marine animals. Marine animals; Physiology, Comparative. DIGESTION 267 the ends until dipeptides are formed; and the latter are split into their amino-acid components by dipeptidases. In some instances the secreted enzymes carry hydrolysis only as far as polypeptides, and further degrada- tion takes place intracellularly (7). Carbohydrases. Amylases capable of hydrolysing starch and glycogen probably occur in most animals, either extracellularly or intracellularly. Starches are broken down to dextrins and finally to maltose. Disaccharides,. 50 60 20 30 40 Digestion Time (minutes') Fig. Digestion of 2% Hb at pH 2-0 by Crystalline Salmon pepsin at various temperatures Ordinates represent the tyrosine equivalent liberated in 6 ml of digest. (From Norris and Elam (55).) especially maltose and sucrose, are broken down by sucrases (maltase and sucrase or invertase), which have been identified in many species. Of special interest are those enzymes attacking other complex carbo- hydrates, notably cellulose and chitin. These are both chain compounds, cellulose consisting of /S-glucoside (cellobiose) units, and chitin of acetyl- glucosamine units. Cellulases capable of digesting cellulose have a very restricted distribution among animals, some notable examples being herbivorous pulmonates and wood-boring insects. Among marine animals again we find a similar distribution, cellulase occurring in herbivorous molluscs (lamellibranchs and gastropods) and wood-boring species {Teredo, Limnoria). In Teredo and its relative Bankia, wood particles are passed to a specialized region of the gut diverticula, where they are attacked by an intracellular cellulase. Similarly, the digestive diverticula of Limnoria produce a cellulase capable of hydrolysing cellulose in the wood eaten by this animal. Two prosobranch gastropods Pterocera and Strombus. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability
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