The Chemistry of Plant Life

by Roscoe Wilfred Thatcher

HETEROGENEOUS STRUCTURE OF THE CELL

Examination of cell protoplasm under the microscope reveals that it is not a simple homogeneous mass. In the first place, it has a definite structure, composed of (a) a nucleus; (b) numerous granular bodies of different sizes and kinds; and (c) a clear mass of colloidal material, which (if observed under the ultramicroscope, or photographed by ultra-violet light) is apparently made up of very minute particles of many different types of materials; the whole mass, in the case of plant protoplasm, being generally surrounded by (d) a differentiated layer known as the cell-wall. The actual internal structural arrangement of the clear colloidal mass is uncertain; but its properties indicate that it may be considered to be like a mass of foam (resembling a compact mass of soap-bubbles) the compartments of the foam being, of course, very minute and the films themselves almost infinitely thin, the contents of each compartment being probably liquid, and the whole composing a typical colloidal gel of complex composition.

This conception may not be accurate in every detail, but it seems to fit very closely the conditions and reactions of cell protoplasm. Furthermore, it is obvious that the definite structure, or form, of the cell is essential to its life; since, if the structure be destroyed by any kind of mechanical injury (freezing of the cell contents, resulting in the puncturing of the membranes by ice crystals; rupturing of the films, or cell-walls, by grinding with sharp sand, etc.) so as to bring about an intermingling of the parts which are segregated from each other in the organized structure, there results an immediate exhibition of abnormal chemical actions, accompanied by the liberation of carbon dioxide, and the death of the cell.

A proper mental picture of the organization of the cell structure and of the interrelation of all its working parts is suggested by the figure of a well-organized chemical factory, with the different chemical transformations which are involved in the whole process being carried on in different portions, or rooms, of the factory, with the various intermediate and final products regularly and systematically transported from one room to another as they are needed to keep each individual step in the whole process going at the proper rate, and with the different parts of the whole factory working in smooth coordination with each other. Any disturbance of the mechanism in any particular room, or any abnormal condition which breaks down the coordination or results in the mixing of the reagents or processes of adjoining rooms in improper order or proportions, produces instant destruction of the normal process, abnormal reactions take place, and the factory output is interrupted.

No other conception than this one of a definite structure and coordination of the different working parts of a cell can adequately account for the great variety of chemical changes which are constantly going on in any given cell. It is wholly inconceivable that a homogeneous mass of all the varying chemical compounds which are contained in any given quantity of protoplasm could either exist or produce any regular sequence of chemical reactions. Structure, or organization of the cell-contents into separate colloidal compartments, and the segregation of cell-contents into masses having different functions, is essential to any reasonable conception of how the cell performs its various activities.

The best understanding of the structural arrangement is afforded by the conception that protoplasm consists of a colloidal gel, or sometimes a very viscid sol, containing water, salts, carbohydrates, fats, proteins, and enzymes. Evidence in favor of this conception is afforded by the appearance of protoplasm under a high-power microscope, and by the close resemblance of the processes which go on in it, and its responses to external stimuli, to those of an artificial gel of similar chemical composition.

Two different conceptions of the form in which the chemical components exist in this mass have been advanced. One is that they are in true molecular unions, known as "biogens," and that the reactions which take place in the mass may, therefore, be studied from the same basis as are reactions between similar substances when they take place in a beaker or test tube in the laboratory. It would seem, however, that the constantly varying proportions of the materials themselves, and the lack of homogeneity of cell contents, afford insurmountable difficulties to this conception as a basis for the study of cell activities. The other, and seemingly more reasonable, conception is that these bodies exist in the form of colloidal complexes, whose composition may vary within wide limits and whose reactions are responsive to the usual phenomena incident to the colloidal condition of matter.

According to the latter conception, vital activities of cell protoplasm may be due to changes in water content, to electrical disturbances, to the phenomena resulting from the conditions brought about by surface boundaries between the different phases of the gel, to varying osmotic pressure, to changes in chemical reaction, etc., and may be controlled by various stimuli of chemical, physical, or mechanical nature. This conception seems, therefore, to fit most closely the actual conditions under which the protoplasm exists and carries on its vital functions.

With this conception in mind, we may now proceed to a consideration of how the various components of the complex organic colloidal system, and their specific properties, can affect its chemical activities.

The components of the system are, of course, water, salts, and the various organic compounds (fats, proteins, carbohydrates, and enzymes in all cells; and other groups, such as essential oils, tannins, pigments, etc., in cells which have certain special functions to perform) which constitute the solid phase of the colloidal mixture. In addition to the definite chemical properties of each of these component groups, which have been studied in detail in preceding chapters, there are many physical, or physical-chemical, properties of the system as a whole, and of its component parts, which are of the utmost importance in the physiological activities of the protoplasm. These we may now proceed to consider in some detail.