Giáo trình Tiếng Anh sinh học - English in biology: Phần 2

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Giáo trình Tiếng Anh sinh học - English in biology: Phần 2 CHEMICAL ASPECTS OF DIGESTION I. T EX T Polysaccharides such as starch and glycogen form an important part of the food ingested by man and most animals. T\\e glucose units of these large molecules are joined by glycoskllc bonds liiiking carbon 4 (or carbon 6) of one glucose molecule with carbon 1 of tl٠ie adjacent molecule. These bonds are hydrolyzed by amylases . These enzymes will digest polysaccharides to tl٦e disaccharide, maltose , but will not split the bond between the two glucose units of maltose. Tlie amylases will split tlie a-glycosidic bonds present in starch and glycogen but not the B-glycosidic bonds present m cellulose. The garden snail's digestive juice contains ii-glycosidases that can hydrolyze cellulose. In most vertebrates, amylase is secreted only by the pancreas ; in man and certain other mammals, amylase is secreted by the salivary gland as well. Different individuals have salivas containing different amounts of amylase ; thus the time required for the salivary digestion of a given amoui'it of starch will vary. You can test the activity of your own saliva by this simple experiment. An estimate of the amount of starch in solution can be obtained by adding a standard amount of iodine solution, which yields a blue- coloured complex with starch but not with sugar. Place 10 ml. of a dilute boiled starch solution in a test tube and add 1 ml of saliva. Mix thoroughly and at tlie end of each minute after the addition of the saliva, remove a drop and add it to a drop of iodine solution. At first the resulting solution will be blue ; subsequently, samples will turn violet or red and eventually remain yellow, indicating that all the starch has been digested. The red colour results from the reaction of iodine with certain mtermediate substances formed in the breakdo ١vn of starch to sugar. All enzymes are proteins and are denatured, i.e., rendered inactive, by heating. You can prove that the digestion of starch by saliva is mediated by enzymes by repeating the test using saliva that has been boiled. 285 286 Unit Eleven Disaccharides are cleaved to monosaccharides by enzymes tl٦at are specific for the particular disaccharide. Maltose is split by maltases present in saliva and in the intestinal juice secreted by intestinal glands. The intestinal juice also contains sucrase , which splits tlie disaccharide sucrose to its constituents, glucose and fructose, and lactase, which split lactose (milk sugar) to glucose and galactose. The ultimate products of the digestion of carbohydrates - tlie hexoses, glucose, fructose and galactose - are absorbed into the blood stream through the intestinal wall. There are several kinds of hydrolases that attack tlie peptide bonds of proteins ; each is specific for peptide bonds in a specific location in a polypeptide chain. ExopeptiJases cleave the peptide bond joining the terminal amirio acids to tlie peptide chain. Carboxypeptidase splits the peptide bond joining the amino acid witli tl٦e free terminal carboxyl group to the chain, and aniinopeptidase removes the amino acid with a free terminal a-amino group. Other hydrolases, tl٠ie endopeptidases, will cleave only peptide bonds within a peptide chain. Pepsin , secreted by tlie chief cells in the gastric mucosa, and trypsin and chymotrypsin , secreted by the pancreas, are endopeptidases but differ in their requirements for specific amino acids adjacent to the peptide bond to be cleaved. These endopeptidases split peptide chains in smaller fragments which are then cleaved further by exopeptidases. The combined action of tlie endopeptidases and exopeptidases results in splitting the protein molecules to free amino acids, which are then absorbed through the intestinal wall into the blood stream by active transport. These powerful proteolytic enzymes would constitute a serious threat to the tissues secreting them. However, pepsin, tiypsin and chymotrypsin are not secreted as such, but are secreted in the form of inactive precursors - pepsinogen, trypsinogen and chymotrypsinogen. This prevents tlieir digesting the proteins of tlie cells that produce them. In the gut each is activated by tlie removal of part of the precursor molecule to yield the active enzyme and an inactive fragment. The digestion of fats is catalyzed by esterases that split the ester bond between glycerol and fatty acid. The principal mammalian esterase is lipase , secreted by the pancreas. Like other proteins, lipase is water-soluble, but its substrates are not. Thus the enzyme can attack only those molecules of fat at the surface of a fat droplet. The bile salts are detergents that reduce the surface tension of fats, breaking the large droplets of fat into very fine ones ; tliis greatly increases tlie surface area of fat exposed to tlie action of lipase and increases tlie rate of digestion of lipids. Conditions in the intestine are usually not optimal for the complete hydrolysis of lipids to glycerol and fatty acids. The products of digestion include glycerol and free fatty acids plus monoacylglycerols, diacylglycerols and some ti٠iacylglycerols, imdigested fats. The pancreas also secretes ribonuclease, an esterase which splits the phosphate ester bonds linking adjacent nucleotides in ribonucleic acids, and deoxyribonuclease, which splits the phosphate ester bonds linking adjacent nucleotides in deoxyribonucleic acids. Enzymes that complete the cleavage of nucleic acids are secreted by the intestinal mucosa. Phosphodiesterase removes nucleotides one at a time from the end of a polynucleotide chain. Tlie nucleotides in turn are attacked by phosphatases which remove the phosphate grou ...