METABOLISM AND TRANSPORT IN ANIMALS 



519 



is necessary for normal bone development. 

 Iron occurs principally in the red corpuscles 

 in the blood where it serves to carry oxygen; 

 it enters our bodies especially by eating beef 

 liver, egg yolk, whole grains, fruits, and green 

 vegetables. Iodine is used by the thyroid 

 gland in the production of the hormone 

 thyroxin, which consists, in weight, of about 

 65 per cent of iodine; deficiency in the quan- 

 tity of thyroxin may result in an enlargement 

 of the thyroid gland to form a goiter. Iodine 

 may be supplied by iodized salt, certain fish, 

 lobsters, oysters, milk, leafy vegetables, and 

 fruits from regions where the soil contains 

 plenty of iodine. Copper is an essential ele- 

 ment in nutrition although its role is not 

 known; it occurs in liver, nuts, legumes, 

 fruits, and leafy vegetables. Other salts in 

 minute quantities are necessary for normal 

 health. For example, a daily intake of 

 0.0003 gram of manganese is required. 



Carbohydrates 



The carbohydrates are the foods that pro- 

 vide energy most abundantly and economic- 

 ally. They contain carbon (C), hydrogen 

 (H), and oxygen (O), the latter two being 

 in the proportion of 2 atoms of hydrogen 

 to 1 of oxygen as in water (HoO). They oc- 

 cur as simple sugars, or as substances that 

 can be converted into simple sugars by hy- 

 drolysis. Three kinds are recognized on the 

 basis of the complexity of the molecule: 

 (1) simple sugars (monosaccharides), (2) 

 double sugars (disaccharides), and (3) com- 

 plex carbohydrates (polysaccharides). A 

 simple sugar contains 6 carbon atoms in its 

 molecule and has the formula CeHioOe- 

 Simple sugars are soluble in water, can be 

 absorbed in the intestine without further 

 change, and consequently are not digested. 

 Glucose is one which occurs in certain fruits, 

 especially grapes, and in the blood; fructose 

 (levulose) also is a simple sugar which oc- 

 curs in fruits. 



Double sugars (disaccharides) have the 

 formula C12H22O11 and are hydrolyzed dur- 

 ing digestion into two simple sugars. One 



molecule of a disaccharide when combined 

 with 1 molecule of water gives 2 mono- 

 saccharides as follows: 



Disaccharide Water Glucose Fructose 

 Ca^H^^Ou + H2O ^ CoHx^Oo -f C«Hi.O« 



Sucrose or cane sugar occurs in vegetables, 

 fruits, and in many plant juices. Lactose or 

 milk sugar occurs in the milk of all mam- 

 mals. Maltose is a product of starch diges- 

 tion and occurs in germinating cereals, 

 malts, and malt products. 



Polysaccharides when hydrolyzed break 

 up into many simple sugars. Their molecular 

 formula is written (CeHioOs)™, the n being 

 a number supposed to range from 7 to 200 

 for different substances. For example, n for 

 starch is large and for dextrin smaller, since 

 each molecule of starch is hydrolyzed into 

 several molecules of dextrin. Dextrin is then 

 hydrolyzed into the disaccharide, maltose, 

 and this into the monosaccharide, glucose. 

 We obtain starch from grains, tubers, roots, 

 etc. The solid matter of cereal grains con- 

 sists of from 50 to 75 per cent starch, and 

 of potatoes about 75 per cent starch. 

 Another polysaccharide of which we ingest 

 large quantities is cellulose. This is derived 

 from the covering of starch grains and from 

 plant cells; it resists digestion but by its 

 bulk aids in peristalsis in the digestive tract. 

 Glycogen is a polysaccharide which is stored 

 in the liver and muscles. It is hydrolyzed to 

 glucose in these organs, when needed by 

 the body, and liberated into the blood. It is 

 also hydrolyzed in the digestive tract when 

 liver or muscle is eaten. 



Hydrolysis by enzymes 



Enzymes are complex substances pro- 

 duced by living cells. Their characteristics 

 are as follows: (1) they are catalysts, that 

 is, they bring about or hasten chemical 

 changes in other substances without under- 

 going significant change in the process; (2) 

 they are most effective in the body at body 



