GENERAL METABOLISM OF THE PLANT The term metabolism is applied to the whole complex of chemi cal changes going on in the plant, the building up of more com plex organic substances from simpler ones being spoken of as anabolism, and the breaking down of complex to simpler ones being termed katabolism. It has already been noted that very many of the chemical reactions in the plant are balanced or re versible ones, so that according to the conditions (e.g., the concen tration of the reacting substances) the process may at one time be anabolic, another time katabolic. A balanced reaction ultimately reaches an equilibrium point where the rate of the process in one direction is equal to the rate in the other; the equilibrium is thus a dynamic one. In those balanced reactions in which there is an energy exchange, i.e., heat is taken in or given out, the equilibrium point is altered by a change of temperature. Experiment in the laboratory shows that the rate of many chemical processes is markedly affected by a rise of temperature, the rate of many of them doubling for each rise of le C. The marked effect of temperature on the metabolism of plants is no doubt partly one of acceleration, and partly a shifting of the equilibrium points of reversible reactions. There is a further point in relation to balanced reactions; if such a reaction is proceeding in a particular direction and material A is being converted into material B, then the mere accumulation of B tends to slow down the reaction and ultimately brings it to a standstill ; should then the amount of B be increased further the reaction will proceed in the other direc tion, i.e., from B to A. A similar result is very common in reac tions caused by enzymes, the accumulation of the products of the reaction tending to bring the reaction to a standstill.
Stress must be laid on the fact that the chemical processes going on in the living plant take place not in a simple homogeneous medium like water but in the protoplasm, which is what is called a heterogeneous system, having many colloid phases. (See above.) The enzymes which seem to play so large a part in metabolism also appear to be colloidal at least in part; thus many of the reactions may take place on their surfaces and on the surfaces of the protoplasmic medium. The system in which the chemical changes occur being thus heterogeneous and of so complex a nature, we should naturally expect that a variation of conditions (such as of temperature, illumination, etc.) would affect differently the processes in the plant and similar processes occurring in the laboratory.
The metabolism of the green plant, to which our brief review must be confined, shows two main anabolic processes, (a) the manufacture of carbohydrates in the process of photosynthesis (carbon assimilation), (b) the building up of complex nitrogenous substances such as proteins from nitrates absorbed from the soil and from organic material such as carbohydrates. In addition there
is a main metabolic process, that of respiration, which is akin to the process of alcoholic fermentation by yeast.
The whole life of the globe is dependent on this photosynthetic power of green plants for they alone are able to manufacture food material for themselves. The green plant may be described as the great alchemist which alone of living things has mastered the secret of converting the sun's rays into food material. The pro cess of assimilation is associated with certain pigments which absorb the incident light ; the energy so obtained is employed in the building up of complex organic substances from carbon dioxide and water, oxygen being at the same time evolved. In the case of the higher plants the pigments of the green leaf, as we know mainly from the work of Willstatter and his collaborators, are four in number—Chlorophyll a, a blue-black crystalline substance, greenish blue in solution; Chlorophyll b, a green-black substance, green in solution; Caro tin, an orange-red substance found also in carrots, and Xanthophyll, C40115602, a yellow substance. The two first are the green pigments and are often termed "chlorophyll," the other two are the yellow pigments. The amounts of these do not vary greatly in different leaves. The average amounts are given below:— In green algae the same four pigments are present ; in the brown and red algae and blue-green algae chlorophyll is present in association with other pigments which give these plants their peculiar colour. The assimilating pigments are not dissolved generally in the cell but are associated with denser portions of the protoplasm of definite form, known as plastids; it is apparently in these plastids that the special physical and chemical processes characteristic of photosynthesis occur. It is generally accepted that the two green pigments are the most important in the process of photosynthesis. One of the functions of the green pigments is clearly that of absorbing the necessary energy for the decompo sition of carbon dioxide, and their solutions show very characteris tic absorption bands, particularly in the red-orange and the violet end of the spectrum. Whether in addition the chlorophyll reacts chemically with the carbon dioxide is still in doubt. Willstatter and Stoll hold that the carbon dioxide combines with the chloro phyll to form a definite compound on which light acts; it is thus converted into a substance of the nature of a peroxide which can be acted upon by an enzyme.