Protoplasm exists only in discrete units almost always of mi croscopic dimensions. The living cell contains the protoplasmic unit which exhibits all the properties characteristic of living beings.
In many kinds of cells the protoplasm becomes highly specialized by the predominance of one or a few of the life functions, as explained in the article HISTOLOGY. In multicellular organisms there are well-established cases which indicate a continuity be tween the protoplasm of neighbouring cells. The extent to which this phenomenon occurs and its significance are still in question. There is no doubt, however, that the cells composing an organism exert mutual interactions which are of the greatest functional importance in the general economy of the whole.
The protoplasmic unit within each cell always is bounded by a sharply outlined surface and contains at least one differentiated structure, the nucleus, the removal of which ultimately leads to the death of the protoplasm. From what is known regarding the chemical constituents, especially the proteins and fatty substances, and from the optical behaviour of protoplasm, it has been con cluded that the components of protoplasm exist largely in the colloidal state. Beyond the visible structure of protoplasm as revealed by the microscope we must therefore look for the exis tence of an ultra-microscopic, colloidal structure. Whatever this structure may be there is no doubt that the extraordinary proper ties of surfaces especially characteristic of colloids play a large part in protoplasmic phenomena. (See COLLOIDS.) One of the many difficult problems in a consideration of the con stitution of protoplasm is the significance of the many and varied visible structures within the protoplasm. (See CYTOLOGY.) In addition there is reason to believe that the nucleus of the cell con tains a large number of discrete, self-propagative particles of col loidal dimensions—the genes. (See HEREDITY.) So far as the rest of the protoplasm is concerned investigations have pointed in a different direction. Nevertheless, experimental studies in embry ology have indicated that the cytoplasm of the unsegmented egg, for example, contains specifically different, but not necessarily visibly formed, materials which play an important part in develop ment. We must conclude that the concept of regional differentia tion within the cell must be taken into serious account in any attempt to interpret protoplasm in terms of physics and chemistry.
The common salts in protoplasm are the chlorides, carbonates, and phosphates of sodium, potassium, ammonium, calcium, mag nesium and iron. These substances are present in certain relative concentrations so that the specific properties of one balance those of others in the normal chemical equilibrium of the cell. Gases are also present in solution, viz., oxygen and carbon dioxide which play an important role in the oxidation processes.
(b) Organic Components.—The organic compounds obtained from protoplasm are the carbohydrates, lipins and proteins. The most stable of the carbohydrates probably serve as structural materials, others supply energy by undergoing oxidation. The lipins, or fatty substances, serve similar purposes. They have a much higher energy content. Their insolubility in water together with their ability to combine with electrolytes to form soaps with varying water-soluble properties are of considerable significance in the constitution of protoplasm.
The proteins are the most complex of the organic compounds. In common with the fats and carbohydrates they are composed chiefly of carbon and hydrogen, but they also possess certain radicals (NH, and COOH) which give them the peculiar property of behaving both as acids and as bases. The proteins readily com bine with inorganic ions to form salts; this is one of the factors which enables protoplasm to retain a high concentration of certain inorganic elements relatively independent of that of the environ ing medium. Because of their varying acid and basic groups the proteins can likewise maintain the protoplasm at its own peculiar degree of acidity and basicity. Proteins are mostly all colloidal, their solutions are often viscous and exhibit reversible states of solidity and fluidity. These remarkable properties of the proteins probably serve as a basis for the complex system of structure and chemical interactions in protoplasm.