102 BIOLOGY AND HUMAN LIFE 



77. Nutrients. It is convenient to distinguish those incomes 

 of an organism that serve as protoplasm-building material and 

 those that serve as sources of energy, or fuel, as foods proper or, 

 better, nutrients. These materials occur naturally only in the 

 bodies of living things, and so are organic (see section 12). 



Protoplasm-builders all contain the element nitrogen ; they 

 are called proteins. The proteins are represented in our com- 

 mon food by albumen, or white of egg ; casein, or the curd 

 formed when milk sours ; gluten, or the pasty substance in 

 wheat flour or bread. Similar nitrogen-containing substances 

 are found in the muscle (flesh) cells of many animals and are 

 called myosin. Others, found in the seeds of plants belonging 

 to the bean family (the Leguminosae) , are called legumin. 



The non-nitrogenous nutrients are of two main classes, fats 

 and carbohydrates. We are familiar with such fats as butter, 

 suet. lard, tallow, olive oil, peanut oil, and others. The car- 

 bohydrates include all the sugars and starches. These substances 

 serve as fuel, combining with oxygen and so yielding energy. 



Protoplasm may be said to consist fundamentally of proteins sus- 

 pended in it'ater containing various salts in solution and various other or- 

 ganic substances dissolved or floating in it. The proteins have in recent years 

 been further broken up into simpler nitrogen-containing substances called 

 amino acids, which combine in various ways to make up the proteins. 



Since protoplasm consists fundamentally of proteins, we shall expect 

 to find protein in almost every part of every animal or plant. That does 

 not mean that all animal or plant materials are suitable for food. In 

 some miaterials the proportion of protein is very low; other materials 

 contain additional substances which render them unsuitable as food, or 

 at least as human food. All seeds contain some proteins ; some in rather 

 large proportions (for example, beans, peas, lentils). In addition to this 

 all seeds contain either fat (as the castor bean, peanut, cotton seed, flax 

 seed) or some carbohydrate (as the bean, cereals, the date). 



78. Vitamins. During the past dozen years or so experiments 

 made upon mice, guinea pigs, pigeons, and human beings have 

 shown that, in addition to the protoplasm-builders (proteins) 

 and fuels (fats and carbohydrates), certain substances must be 

 present in our food to insure growth. Very little is known of 



