Chemical composition of protoplasm
When protoplasm is studied chemically, it is found to be built up of the same elements that occur in nonliving materials.
The 20 elements listed below appear to be essential to protoplasm
ESSENTIAL / SYMBOLS / PERCENT IN ELEMENTS PROTOPLASM
These elements, with the exception of oxygen, are generally combined to form compounds. Compounds can be divided into inorganic and organic. Organic compounds occur in nature only in living plants and animals, or in their products and remains. Inorganic compounds are principally water and salts, and organic compounds are principally proteins, fats, and carbohydrates. The percentages of the different compounds in protoplasm are on average as follows:
Compounds are made up of one or more molecules of the same kind; for example, water, sugar, and carbon dioxide are compounds. Molecules are so small that one computation shows that there are about 1,000,000 molecules in a single bacterium. A molecule is the smallest particle of a substance that possesses the chemical nature of that substance.
For example, a molecule of water can be subdivided, but it ceases to be water when it is broken down into the 2 elements, hydrogen, and oxygen, of which it is composed. Elements, such as hydrogen and oxygen, are known as atoms. More than 100 different elements or kinds of atoms are known. Many of these atoms can combine in various ways to form molecules; for example, 2 atoms of hydrogen, combined with 1 atom of oxygen, produce 1 molecule of water; 1 atom of carbon and 2 atoms of oxygen combine to form 1 molecule of carbon dioxide.
Evidently, there are vastly greater numbers of different kinds of molecules than of different kinds of atoms. Likewise, molecules of different kinds may be mixed together in various combinations so as to produce many more different kinds of substances than there are different kinds of molecules. Atoms and molecules are ordinarily indicated by means of symbols which provide a sort of chemical shorthand. Thus hydrogen is indicated by the letter H, and oxygen by the letter O. The molecular formula of water is written as H20, since each molecule of water is made up of 2 atoms of hydrogen and 1 of oxygen. Carbon is indicated by the letter C, and the molecular form of carbon dioxide is CO2.
Water is the most common compound in protoplasm, making up from about 60 to 96 percent of it. Water is ingested in greater amounts than all other substances combined, and it is the chief excretion. It is the vehicle of the principal foods and excretion products, for most of these are dissolved as they enter or leave the body. Actually, there is hardly a physiologic process in which water is not of fundamental importance.
Inorganic salts are essential for life processes. They are present in solution in the protoplasm and in body fluids. In body fluids, they are very similar in concentration to the salts in seawater. Although small in amount, they are important since certain salts in certain proportions are necessary for normal life activities. For example, if the calcium content of the blood is lowered sufficiently, convulsions and death ensue; and if sodium, calcium, and potassium are not properly balanced, the muscles of the heart do not function normally. The presence of certain salts is quite obvious to us since calcium phosphate and calcium carbonate make up about 65 percent of bone.
The three principal classes of organic compounds in protoplasm are known as carbohydrates, fats, and proteins. Carbohydrates and fats are composed entirely of carbon, hydrogen, and oxygen; protein has in addition nitrogen, sulfur, and phosphorus. Common carbohydrates are starch and sugar. The word carbohydrate is derived from the Latin term carbo, meaning coal, and the Greek term hydro, meaning water. Coal is a form of carbon. Carbohydrates are compounds of carbon, hydrogen, and oxygen in which the ratio of hydrogen and oxygen atoms is the same as that in water, that is, 2 of hydrogen to 1 of oxygen (H20). Carbohydrates are stored in the body in d form called glycogen, especially in the liver and the muscle cells. They are particularly valuable as fuel for the body but are also used in the structure of protoplasm. One of the simple carbohydrates, a sugar called glucose, seems to be of particular importance, probably as a fuel. If there is too little glucose present, nerves and muscles 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 condition 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 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, Since protoplasm is composed largely of proteins, we need plenty of protein in our food; and since different parts of the body, such as the liver and muscles, contain different kinds of proteins, we require food containing 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, carbohydrates into fats, and fats into carbohydrates.