Fortunately, the very complicated compounds which the plants provide and which both plants and animals use for food and growth, can be grouped into three great classes called: (1) Proteids, (2) Carbohydrates, (3) Fats. These are sometimes taken all together and called organic nutrients.
Proteids. These are very numerous and are found in all living substances; the following are some that are common and found in large amounts. You can find protein in gluten in grains, legumin in peas and beans, myosin in lean meat, albumen in the white of egg, and casein in milk and cheese
It is not necessary to learn these names but the list is put in to show that proteids are of many kinds and, though first provided by plants, are needed in animal tissue as well. To test proteids is quiet easy. Proteids differ in many ways but there is one point in which they all behave alike and which is different from any other substance hence we can use it as a test. If a substance supposed to contain any proteid is put into nitric acid and heated gently, it will turn bright yellow. Then if the acid be washed off and ammonia added the proteid, if present, will become orange color. This is the test for any proteid for no other substance will act in the same way.
The proteids are the most useful of the nutrients for they make up most of the active living substance of plant and animal; they are called tissue builders on this account. Proteids are composed of the elements carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, with sometimes mineral salts as well, so we see they are very complex organic compounds.
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