fied as organic and inorganic. The organic compounds, are further classified as nitrogenous (those containing nitrogen) and non-nitrogenous (those without nitrogen). The majority of nitrogenous substances are known as proteins and the non-nitrogenous organic substances are classed as carbohydrates and fats. The inorganic constituents of plant and animal tissues are salts and water.

Table showing classification:

The substances in egg

Examples of proteins are: which solidify when heated; the substance in milk that forms as a scum when the latter is heated and that which clots or curds when rennin or acid is added to milk; the white substance which appears as a coating on meat when it is exposed to heat.

Familiar examples of carbohydrates are starches and sugars.

Proteins

The proteins are indispensable constituents of animal and plant cells; without them all life would

cease.

Composition of proteins.-Protein molecules have a very complex structure, knowledge of which is still

very limited, though it has been greatly added to lately. Proteins consist of the elements carbon, hydrogen, nitrogen, oxygen, sulphur, and sometimes phosphorus and iron. The more complex proteins

The

contain thousands of atoms of these elements. non-mineral elements of protein molecules are so combined that they form what are known as anhydrids of amino acids. These anhydrids are formed by reaction between the OH groups of two molecules of organic acid with a loss of water.

Amino acids.-These differ from other organic acids chemically in that their hydrogen is replaced by what is known as amine or the amino group or radical, which is NH2. For example, acetic acid is represented by the formula CH, CO2. H and amino acetic acid by the formula CH ̧CO2. NH2. Amine is somewhat similar to ammonia-NH, and ammonia is formed from it during the decay of protein-containing compounds.

Classification of proteins.-A very large number of different amino acids have been obtained both by the decomposition of organic compounds and by synthesis, and as different kinds of proteins contain different numbers and kinds of these acids it can be appreciated that there is considerable variation in the nature of the protein of different foods.

Certain proteins, however, have similar characteristics and according to these they have been classified as follows:

Simple proteins are those which, when digested or otherwise hydrolyzed,1 yield only amino acids or their derivatives.

Albumins and globulins.-Proteins belonging to these two classes are found in many of the same substances. As a rule, however, there are more albumins than globulins in animal fluids, as blood, and more globulins in animal tissues and plants.

The albumins and globulins have many characteristics in common. They are both colloidal and will not diffuse through animal membranes and, under the influence of heat, certain salts (as bichlorid of mercury, silver nitrate, etc.), and various other substances,

By hydrolysis is meant, the splitting of complex molecules due to their absorption of water.

their molecules tend to aggregate together to form a coagulum. They are both soluble in dilute salt solutions, dilute acids and alkalies, but albumins are also soluble in distilled water and concentrated sodium chlorid solutions and globulins are not. The classification of the simple proteins placed under these two headings was based largely upon these differences in their solubilities.

The several different kinds of albumins and globulins are given different names. Thus, albumin in serous fluids and in blood is called serum-albumin; that in eggs, ova-albumin; that in milk, lact-albumin; that in muscle tissue, myogen. The globulins of the blood are called fibrinogen and serum-globulin or paraglobulin; that in muscle tissue, myosin1; also there are a few albumins and various globulins in plants, but as their special names are not often used, they need not be given here.

The glutenins and alcohol-soluble proteins.The glutenins and the most common alcohol-soluble protein, gliadin, form the gluten of wheat flour, which is its principal nitrogenous constituent. Gluten is found also in other cereals, but, as will be seen later, that in the other cereals is not quite the same as that of wheat flour.

Albuminoids.-These are protein substances extracted from such matter as bone, cartilage, and similar substances. They form the basis of gelatin.

I

Myosin, it is thought, does not exist as such in the living muscle, but is formed there by coagulation after death. The change is thought to be similar to the transformation that occurs in fibrinogen when blood clots. Rigor mortis-the stiffening of the body after death-is due to the changes that occur in the proteins after death.

The composition of the albuminoids differs from that of the other simple proteins, and gelatin, though it contains nitrogen, cannot be used in the diet as a substitute for them, since it cannot be utilized in the body, as other proteins are, for the building of muscle tissue.

Histones and protamines.-These two simple proteins are not common, they have been obtained from the blood corpuscles of some animals and from the spermatozoa of fish.

Conjugated Proteins

Conjugated proteins are those which have a molecule or molecules of matter other than mineral united to the protein molecule.

Nucleoproteins.-These are compounds of simple proteins and nuclein or nucleic acid. They are contained in the nuclei of cells and are relatively abundant in such glandular tissues as the spleen, pancreas, and liver, the cells of such organs containing many nuclei.

Nucleic acid, on oxidation in the body, yields substances known as the purin bodies, e. g., uric acid. For this reason, foods containing a high per cent. of nucleoprotein are eliminated from the diet in diseases, such as gout, which are characterized by the presence in the system of a comparatively large amount of uric acid.

Glycoproteins.-These are proteins which contain a carbohydrate molecule attached to the protein. Mucin, which is a constituent of the secretions of certain glands and of the mucous membranes of the respiratory and alimentary tracts, is an example.