9 ELEMENTARY COURSE LESSON I THE CARBOHYDRATES AND FATS 1. Note the general appearance of the specimens of grape sugar or dextrose, dextrin, and starch which are given round. 2. Put some of each into cold water. Starch is insoluble; dextrose and dextrin dissolve after a time, but more readily in hot water. 3. Trommer's test for dextrose.-Put a few drops of copper sulphate solution into a test-tube, then solution of dextrose, and then strong caustic potash. On adding the caustic potash a precipitate is first formed, which, on adding more potash, redissolves, forming a blue solution. On boiling this a yellow or red precipitate (cuprous hydrate or oxide) forms. 4. Dextrin.—Add iodine solution to solution of dextrin, and a reddishbrown solution is produced. The colour disappears on heating. 5. Starch. (a) Examine microscopically the scrapings from the surface of a freshly cut potato. Note the appearance of the starch grains with their concentric markings. (b) On boiling starch with water an opalescent solution is formed, which, if strong, gelatinises on cooling. (c) Add iodine solution. An intense blue colour is produced, which disappears on heating, and if not heated too long reappears on cooling. N.B.-Prolonged heating drives off the iodine, and consequently no blue colour returns after cooling. (d) Conversion into dextrin and dextrose. To some starch solution in a flask add a few drops of 25-per-cent. sulphuric acid, and boil for 15 minutes. Take some of the liquid, which is now clear, and test for dextrin and dextrose. 6. Glycogen. Solution of glycogen is given round: (a) it is opalescent like that of starch. (b) With iodine it gives a brown colour like dextrin. (c) By boiling with 25-per-cent. sulphuric acid it is converted into grape sugar. 7. Lard is given round as an example of a fat. It is insoluble in water. By boiling with potash it yields a solution of soap. 8. Add to this solution a few drops of 25-per-cent. sulphuric acid. On heating a layer of fatty acid collects on the surface. CARBOHYDRATES The carbohydrates are found chiefly in vegetable tissues, and many of them form important foods. Some carbohydrates are, how ever, found in or formed by the animal organism. The most important of these are glycogen, or animal starch; dextrose; and lactose, or milk sugar. The carbohydrates may be conveniently defined as compounds of carbon, hydrogen, and oxygen, the two last named elements being in the proportion in which they occur in water. They may be for the greater part arranged into three groups according to their empirical formulæ. The names and formulæ of these groups, and the most important members of each, are as follows: The + and + Glycogen signs in the above list indicate that the substances to which they are The formulæ given above are merely empirical; and there is no doubt that the quantity n in the starch group is variable and often large; hence the name polysaccharides that Tollens gives to the group. Research has, moreover, shown that the glucoses are either aldehydes (dextrose) or ketones (levulose) of the hexatomic alcohol mannite. The amyloses may be regarded as the anhydrides of the glucoses [nC6H12O6-nH2O=(C6H10O5)n]. The sucroses are condensed glucoses-i.e. they are formed by the combination of two molecules of glucose with the loss of one molecule of water (C6H12O6+C6H12O6 FIG. 1.-Dextrose crystals. -H2O=C12H22O11). The following are the chief facts in relation to each of the principal carbohydrates. Dextrose or Grape Sugar.-This carbohydrate is found in fruits, honey, and in minute quantities in the blood and numerous tissues, organs, and fluids of the body. It is the form of sugar found in large quantities in the blood and urine in the disease known as diabetes. Dextrose is soluble in hot and cold water and in alcohol. It is crystalline (see fig. 1), but not so sweet as cane sugar. When heated with strong potash certain complex acids are formed which have a yellow or brown colour. This constitutes Moore's test for sugar. In alkaline solutions dextrose reduces salts of silver, bismuth, mercury, and copper. The reduction of cupric to cuprous salts constitutes Trommer's test, which has been already described at the head of the lesson. Another important property of grape sugar is that under the influence of yeast it is converted into alcohol and carbonic acid (C6H12O6=2C2H6O+2C02). Dextrose may be estimated by the fermentation test, by the polarimeter, and by the use of Fehling's solution. The last method is the most important: it rests on the same principles as Trommer's test, and we shall study it in connection with diabetic urine (see Lesson XII.) Levulose. When cane sugar is treated with dilute mineral acids it undergoes a process known as inversion-i.e. it takes up water and is converted with equal parts of dextrose and levulose. The previously dextro-rotatory solution of cane sugar then becomes levo-rotatory, the levo-rotatory power of the levulose being greater than the dextrorotatory power of the dextrose formed. Hence the term inversion. Similar hydrolytic changes are produced by certain ferments, such as the invert ferment of the intestinal juice. Pure levulose can be crystallised, but so great is the difficulty of obtaining crystals of it that one of its names was uncrystallisable sugar. Small quantities of levulose have been found in blood, urine, and muscle. It has been recommended as an article of diet in diabetes in place of ordinary sugar; in this disease it does not appear to have the harmful effect that other sugars produce. Levulose gives the same general reactions as dextrose. Galactose is formed by the action of dilute mineral acids or inverting ferments on lactose or milk sugar. It resembles dextrose in being dextro-rotatory, in reducing cupric salts in Trommer's test, and in being directly fermentable with yeast. When oxidised by means of nitric acid it, however, yields an acid called mucic acid (C6H1008), which is only sparingly soluble in water. Dextrose when treated this way yields an isomeric acid-i.e. an acid with the same empirical formula, called saccharic acid, which is readily soluble in water. Inosite, or muscle sugar, is found in muscle, kidney, liver, and other parts of the body in small quantities. It is also largely found in the vegetable kingdom. It is a crystallisable substance (see fig. 2) and has the same formula (C6H12O6) as the glucoses. It is, however, not a sugar. It gives none of the sugar tests, and careful analysis has shown it has quite a different chemical constitution from the true sugars. Cane Sugar. This sugar is generally distributed throughout the vegetable kingdom in the juices of plants and fruits, especially the sugar cane, beetroot, mallow, and sugar maple. It is a substance of FIG. 2.-Inosite crystals. * great importance as a food. After abundant ingestion of cane sugar traces may appear in the urine, but the greater part undergoes inversion in the alimentary canal. Pure cane sugar is crystalline and dextro-rotatory. It holds cupric hydrate in solution in an alkaline liquid--that is, with Trommer's test it gives a blue solution. But only a slight reduction occurs on boiling. After inversion it is strongly reducing. By boiling with water, or, more readily, by boiling with dilute mineral acids, or by means of inverting ferments, such as that occurring in the succus entericus, it undergoes inversion—that is, it takes up water and is split into equal parts of dextrose and levulose (see p. 11). C12H22O11+ H2O=C6H12O6+C6H12O6 With yeast, cane sugar is first inverted by means of a special soluble ferment produced by the yeast cells, and then there is an alcoholic fermentation of the glucoses so formed. Lactose, or milk sugar, occurs in milk. It has also been described as occurring in the urine of women in the early days of lactation or after weaning. It crystallises in rhombic prisms (see fig. 3). It is much less soluble in water than cane sugar or dextrose, and has only a slightly sweet taste. It is insoluble in alcohol and ether; aqueous solutions are dextro-rotatory. Solutions of lactose give Trommer's test, but when the reducing power is tested quantitatively by Fehling's solution it is found to be a less powerful reducing agent than dextrose. If it required seven parts of a solution of dextrose to reduce a given quantity of Fehling's solution, it would require ten parts of a solution of lactose of the same strength to reduce the same quantity of Fehling's solution. FIG. 3.-Milk-sugar crystals. Lactose, like cane sugar, can be hydrolysed by the same agencies as those already enumerated in connection with cane sugar. The glucoses formed are dextrose and galactose. C12H22O11+H2O=C6H12O6+C6H12O6 With yeast it is first inverted, and then alcohol is formed. This, however, occurs slowly. With the lactic-acid organisms which bring about the souring of milk the lactic-acid fermentation is produced. This may also occur as the result of the action of putrefactive bacteria in the alimentary canal. The two stages of the lactic-acid fermentation are represented by the following equations: Maltose is the end product of the action of malt diastase on starch, and is also formed as an intermediate product in the action of dilute sulphuric acid on the same substance. It is also the chief sugar formed from starch by the diastatic ferments contained in the saliva (ptyalin) and pancreatic juice (amylopsin). It can be obtained in the form of acicular crystals; it is strongly dextro-rotatory. It gives Trommer's test; but its reducing power, as measured by Fehling's solution, is one-third less than that of dextrose. By prolonged boiling with water, or, more readily, by boiling with a dilute mineral acid, or by means of an inverting ferment, such as occurs in the intestinal juice, it is converted into dextrose. C12H22O11+H2O=2C6H12O6 [maltose] [dextrose] It undergoes readily the alcoholic fermentation. It Starch is widely diffused through the vegetable kingdom. occurs in nature in the form of microscopic grains, varying in size and appearance, according to their source. Each consists of a central spot, round which more or less concentric envelopes of starch proper or granulose alternate with layers of cellulose. Cellulose has very little digestive value, but starch is a most important food. Starch is insoluble in cold water: it forms an opalescent solution in boiling water, which if concentrated gelatinises on cooling. Its most characteristic reaction is the blue colour it gives with iodine. On heating starch with mineral acids, dextrose is formed. By the |