520 



COLLEGE ZOOLOGY 



temperature; (3) each requires a definite 

 type of medium— neutral, acid, or alkaline; 

 (4) their action is specific, each acting on 

 one type of food substance; ( 5 ) their action 

 may be reversible, that is, they may aid 

 in building up as well as breaking down 

 substances. Enzymes are secreted into var- 

 ious parts of the digestive tract, bringing 

 about hydrolysis (p. 514) of different sub- 

 stances in different organs as noted in the 

 next paragraph. 



Carbohydrate digestion 



Two enzymes are present in saliva, ptyalin 

 and maltase. Ptyalin (salivary amylase) 

 hydrolyzes starch to dextrin and sugar (mal- 

 tose). Maltase converts maltose to glucose. 

 In the small intestine, the enzyme amylase 

 from the pancreas hydrolyzes starch that was 

 not acted upon in the mouth to maltose. 

 Enzymes secreted by the small intestine con- 

 tinue the process: maltase hydrolyzes mal- 

 tose to glucose; invertase (sucrase) acts 

 upon sucrose and hydrolyzes it to glucose 

 and fructose; and lactase hydrolyzes lactose 

 to glucose and galactose. Glucose, fructose, 

 and galactose are all simple sugars that can 

 be absorbed into the blood. 



Fats 



Included among the fats are true fats or 

 lipids and compound fats or lipoids. True 

 fats are composed of carbon, hydrogen, and 

 oxygen, as are carbohydrates, but less oxygen 

 is present in proportion to the carbon and 

 hydrogen; consequently this makes a more 

 concentrated form of fuel. A molecule of 

 fat can be split into two kinds of smaller 

 molecules, one of glycerol (glycerin) and 

 three of a fatty acid. For example, the fat 

 known as stearin hydrolyzes as follows: 



Stearin Water 1 Glycerol 



C3Hb03(Ci7H35CO)3 + 3H2O -> C3H5(OH)8 -|- 



3 Stearic acid 

 BHCisHsbO. 



The best-known lipoid is lecitliin, a sub- 

 stance that is abundant in the yolk of the 



hen's egg; it contains nitrogen and phos- 

 phorus in addition to carbon, hydrogen, and 

 oxygen. Another t)pe of lipoid is the sterols, 

 which are solid waxy substances. The best- 

 known sterol is cholesterol, which is excreted 

 in bile and is widely distributed in the body. 

 The fats we use are derived from plants and 

 animals; some of the most common are fats 

 ingested with meat, lard, butter, olive oil, 

 cod-liver oil, etc. 



Digestion of fat 



No digestion of fat occurs in the mouth. 

 In the stomach very little occurs, although a 

 gastric lipase may begin the digestion of 

 emulsified fats such as cream. Most fat is 

 digested in the small intestine. An enzyme, 

 lipase, from the pancreas splits the fats into 

 glycerol and fatty acids that can be ab- 

 sorbed. Bile from the liver breaks up the fat 

 droplets into small ones and thus hastens 

 the action of the lipase. 



Proteins 



The proteins are always present in proto- 

 plasm and are, as the name means, of the 

 first importance in the life of the cell. They 

 are extremely complex; hundreds or thou- 

 sands of atoms are required to form a single 

 protein molecule. Proteins always contain 

 nitrogen in addition to carbon, hydrogen, 

 and oxygen, and are on this account often 

 called nitrogen compounds. They sometimes 

 contain sulphur, phosphorus, and iron also. 

 A protein in our red blood corpuscles, hemo- 

 globin, has been given the approximate 

 formula C3032H48i60872N78oSsFe4. Protein 

 molecules can be broken down into simpler 

 molecules called amino acids; about 40 dif- 

 ferent kinds have been described as occur- 

 ring in nature. Different combinations of 

 these amino acids account for the differences 

 between the proteins of different animals 

 and of different tissues in the same animal. 

 Our food contains proteins from animal 

 sources such as meat, milk, fish, and al- 

 bumin, and from plant sources such as glu- 

 tenin from wheat, and legumin from peas, 

 beans, peanuts, etc. 



