Perhaps the most visible characteristic of living organisms is that they are complicated and highly organised. Cells, which form the basic biological unit, possess complex internal structures. This structural complexity is very much related to the cellular functions. The structural and functional properties of all cell components are determined by the molecular structure, arrangement and properties of their chemical constituents.
Chemically, the cell is composed of water, proteins, lipids, salts, nucleic acids, carbohydrates, and minute quantities of a variety of organic compounds, such as vitamins and growth-factors. Approximately 93 per cent of the cellular material is composed of carbon, hydrogen and oxygen. The elements such as nitrogen, phosphorus, chlorine and sulphur constitute about 2 per cent of the total material in the cell. Other elements such as iodine, fluorine and boron occur in minute quantities only. Elements such as calcium potassium, sodium and magnesium form less than 5 per cent of the cellular constituents, and cobalt, zinc etc., are found only in traces.
Salts, mineral ions and water are the important inorganic materials present in a cell.
Mineral salts are essential components of the cell. They are vital for the growth of cells. The need for these minerals varies from cell to cell and particularly from plant to animal cells. They are often classified into two broad categories: macronutrients, which are required in large quantities and micronutrients, which are required only in traces. For both plants and animals, the major macronutrients are sodium, chlorine, potassium, calcium, phosphorus, and magnesium, and the principal micronutrients are iron, copper, manganese and zinc. In addition to these, animals need cobalt, iodine and possibly vanadium and selenium, and plants require boron, molybdenum, and vanadium, although vanadium is essential for only certain forms of plants. Most of the elements of the periodic table have been found in living cells.
Minerals occur in living cells as components of organic and inorganic molecules and ions. In body fluids there is a balance between the amounts of a mineral present as ions and in complexes.
Minerals are important elements for the structure and function of cellular organelles as well as cellular macromolecules. They form part of the cellular structure (calcium and phosphorus in bones and teeth), in the exoskeleton of invertebrates (calcium pectate in cell wall, etc.) and biologically active substances (e.g. iron in haemoglobin, cytochromes, myoglobin, Mg2+ in chlorophyll pigment; iodine in thyroid hormones, etc.) and assist enzyme action as cofactors.
Sodium and potassium are responsible for the maintenance of fluid balance in the cell. These two ions are also responsible for the maintenance of membrane potential. Minerals also act as acid-base buffers to maintain the hydrogen ion concentration of cellular fluids.
Water is an essential component of protoplasm. It can occur in the cell bound to the molecules of proteins, or it may exist 'free'. Free water acts as a major reagent in photosynthetic cells (for oxygen evolution), a medium for various metabolic reactions, and as the constituent of the protoplasm to give it the colloidal state. Most of the cellular functions of the protoplasm thus seem to be dependent on the formation and breakdown of ordered states of water molecules. Water, therefore, is no longer an inert medium filling up the space between cellular components but it is an essential unit of the organic matter of protoplasm.