Jethalal S. Zaveri
At this very second, thousands of chemical reactions are taking place in your body. Various constituents of the body are joining together or being split apart or undergoing intricate exchanges and rearrangements. The sum total of all the chemical reactions that go on in the living organism is called metabolism [1]. Some of these reactions result in a build up of body structure; they form the category of anabolic reactions. Catabolic reactions, on the other hand, are the breakdown reactions of the body. Both go on constantly in a bewildering interplay that is life.
Chemical Constituents of the Body
The substance of life, protoplasm is a complex mixture of inorganic and organic compounds. None of its elements is unique to living things. However, the distribution of the elements in the living and non-living worlds is quite different. And even more important is that the element in living organisms are confined into an assortment of complex compounds, many of which have no counterpart in the nonliving world. The main constituents of protoplasm are: water, inorganic salts, proteins, carbohydrates, lipids (fats) and nucleic acids.
Water: Water is the most abundant compound in the body, constituting more than 65 percent of the total body weight. It is one of the simplest compounds yet one of the most important. A human being can survive for only a few days without taking water.
Water serves a great variety of functions in the body. It provides the fluid medium in which the chemical reactions of the cells take place; it furnishes the major transport medium for distributing oxygen, nutrients and other substances as well as carrying away waste products. It is important in excretion of wastes through the kidney etc. and it plays a key role in the body's temperature-control mechanisms. The water content of the tissues is continuously monitored in the brain.
Inorganic Salts: 4½ percent of the weight of the body is comprised of inorganic salts—partly in dissolved form dissociated into ions [2] and partly in combination with organic compounds.
Sodium and chloride ions are the most abundant ions in the body fluids; within the cell, potassium and phosphate ions are the major inorganic constituents. Calcium phosphate is the major constituent of bones. Various other ions in smaller amounts, some in barely detectable traces are also present. Some of these trace elements are constituent of key enzyme systems, without which the normal reactions in the body could not take place. The acid-base balance, coagulation of blood, transport of oxygen and carbon dioxide, nerve conductivity, muscle contraction and coordination of metabolic activities—these all need inorganic salts.
Carbohydrates: Carbohydrates contain three elements: carbon, hydrogen and oxygen combined in the ratio CxH2yOy. They include sugars and starches and are the major sources of energy. Glucose is the form of simple sugar that is carried in the blood. It is an essential nutrient for all body-cells, particularly brain-cells. Starch consists of many glucose-units linked together. Whatever glucose is not immediately needed is synthesized into glycogen, which is stored within liver and muscle tissues.
Lipids (Fats): Although one does not like to be 'fatty', fats and other fatty substances also do play some valuable roles in the body. Fat-deposits serve as a source of reserve energy. Gram for gram, fats yield more than twice as many calories as carbohydrates and are thus a more economical means of energy-storage. They play an important role in maintaining normal cell permeability and thus regulate the flow of materials into and out of cells. They aid in transport of fat soluble vitamins.
Proteins: There are about 100,000 different kinds of proteins in the human body. Human proteins differ from those of other species and there are differences from one person to another. They are formed by linking together of smaller units called amino acids. Some are huge with molecular weight in the millions of moles.[3] There are about twenty naturally occurring amino acids.
Next to water, proteins are the most abundant substances in most cells, forming 10 to 20 percent of the cell-mass. Most of them can be grouped in one of the two main classes: (i) structural proteins and (ii) functional proteins.
Structural proteins are present in cell membranes and hold the cell-structure together. Most of them are in the form of long fibrous threads and provide tensile strength for the cell-structure.
Functional proteins include enzymes and hormones that regulate the body's activities. Enzymes are the organic catalysts, without which most of the chemical reactions in the body would not proceed. Many enzymes catalyze only a single chemical reaction with a single set of reactants and do not act even on somewhat similar compounds, thus forming a lock and key relationship. Usually they exist in complex with co-factors such as vitamins and metal ions. Hormones control and coordinate the activities of the cells, tissues and organs of the body. Nucleo-proteins are complexes of proteins and nucleic acids and carry the hereditary blueprints for all the body's structures and activities.
A continuous supply of protein is needed for growth of new and repair of old tissues. But the food proteins are not used as such. They are first digested and broken up into their constituent amino acids and then built up into the characteristic proteins of the body.
Nucleic Acids were first discovered in the cell nucleus (hence the name), but actually they are also found in the cytoplasm of the cell, outside the nucleus. The amount of information contained in the chromosomes (23 pairs) of a single human cell is estimated to "a thousand books of 500 pages" each. It is encoded in nucleic acid molecules. More than a dozen Nobel Prize winners have received their awards for work in the field of the chemicals of heredity.
There are several kinds of nucleic acids each with its own function in the cell. The master plans are carried in DNA (deoxyribonucleic acid) [4] molecules. Working parts of the plans are passed on through several types of RNA (Ribonucleic acid) and ultimately translated into proteins. DNA and RNA molecules are constructed from sugar, phosphate and nitrogen base components. Structurally they are similar in some respects and different in others.
Like proteins, the nucleic acids in foods are not utilized as such, but are digested into their constituents and resynthesized as needed.
The basic knowledge of the chemistry of life is essential to understand and appreciate the importance of "Nutrition" needed for growth, development and health, and is discussed in the next chapter.
Even the most complex molecules of life, containing millions of atoms, weigh only a small fracton of a gram. A gram molecular weight, called a mole, contains 6.20 x 1023 molecules of the compound.
