18 



in addition nitrogen, sulphur, and phos- 

 phorus. Common carbohydrates are starch 

 and sugar. The word carbohydrate is de- 

 rived from the Latin term carbo, meaning 

 coal, and the Greek term hydor, meaning 

 water. Coal is a form of carbon. Carbo- 

 hydrates are compounds of carbon, hydro- 

 gen, and oxygen in which the ratio of hy- 

 drogen and oxygen atoms is the same as 

 that in water, that is, 2 of hydrogen to 1 of 

 oxygen (H2O). Carbohydrates are stored in 

 the body in a form called glycogen, espe- 

 cially in the liver and the muscle cells. They 

 are particularly valuable as fuel for the body, 

 but are also used in the structure of proto- 

 plasm. One of the simple carbohydrates, a 

 sugar called glucose, seems to be of particu- 

 lar importance, probably as a fuel. If there 

 is too little glucose present, nerves and mus- 

 cles become more irritable, and death may 

 follow convulsions, just as when the calcium 

 content of the blood becomes too low. If the 

 sugar content of the blood is too high, a 

 disease known as diabetes results; this condi- 

 tion can be corrected by injection of the 

 hormone insulin. 



Fats differ from carbohydrates in the 

 structure of their molecules. Less oxygen is 

 present in proportion to the carbon and 

 hydrogen. This is evident when the formulas 

 of a carbohydrate and a fat are contrasted. 



Carbohydrate 



Fat 



Fats, like carbohydrates, serve principally as 

 fuel, and much fat is stored in the body 

 where it can be used when needed. When 

 deposited just beneath the skin, it insulates 

 the body, since it is a poor conductor of 

 heat. 



Proteins are the primary constituents of 

 protoplasm. Their molecules are much larger 

 than those of fats and carbohydrates; a 

 common protein (hemoglobin) in our red 

 blood corpuscles, for example, has the ap- 

 proximate formula C3032H48160872N78oSsFe4, 



which means that each molecule is built up 

 of 6 different kinds of atoms, totaling about 

 10,000. Since protoplasm is composed largely 



COLLEGE ZOOLOGY 



of proteins, we need plenty of protein in our 

 food; and since different parts of the body, 

 such as the liver and muscles, contain differ- 

 ent kinds of proteins, we require food con- 

 taining various types of proteins. Common 

 animal proteins are present in meat, fish, 

 milk, and eggs, and common plant proteins 

 in peas, beans, and peanuts. 



Proteins play the leading role in the 

 chemical composition of protoplasm; fats 

 and carbohydrates serve principally as fuels. 

 Fats and carbohydrates cannot be converted 

 into proteins in the body, but proteins can 

 be converted into carbohydrates, carbo- 

 hydrates into fats, and fats into carbohy- 

 drates. 



Metabolism and growth 



The term metabolism is used to include 

 all chemical changes that take place in the 

 protoplasm. Growth in any living thing in- 

 volves a complex series of changes. The 

 chemical compounds which make up the 

 bodies of animals are extremely unstable; 

 they are constantly breaking down into sim- 

 pler substances or becoming more complex 

 by the addition of new materials. There is 

 no time during the life of any individual, 

 even after growth ceases, when elaborate 

 chemical reactions are not taking place. 

 Metabolism is the term used to include 

 this great complex of incessant chemical 

 changes. Those processes which use energy 

 to build up compounds are said to be ana- 

 bolic; those by which substances are broken 

 down, thereby releasing energy, are termed 

 catabolic. 



Animals are primarily catabolic organisms. 

 They cannot make organic compounds from 

 simple inorganic substances; in this respect 

 they differ from plants, which manufacture 

 sugar (glucose) from carbon dioxide and 

 water, in the presence of light energy and 

 chlorophyl. The green plants obtain carbon 

 dioxide (CO2) from the air, water (H2O) 

 from the soil, and energy from light. Chloro- 

 phyl, an additional substance, which is re- 

 sponsible for the green color of plants is also 



