In considering plant metabolism, we shall not choose a specific example, inasmuch as our generalizations will be broadly applicable. Indeed, we could have taken this approach in our consideration of the metabolism of a complex animal body.
Virtually all that we said there applies to any mammal, and with some exceptions, to any vertebrate. However, in order that we might have a definite starting point, let us consider that we are talking about a seed plant, perhaps a bean or tomato plant of an oak tree.
Nutrition A seed plant has its beginning in the reproductive processes of a parent plant. Essentially, a seed is a reproductive unit which consists of an embryonic plant and certain associated tissues which are chiefly organic nutrient materials. Under suitable conditions of moisture, temperature, and oxygen availability, the embryonic plant within the seed begins to undergo cellular activity. However, it cannot grow without a source of oxidizable organic molecules, and as yet it does not possess the ability to manufacture any.
Under these conditions it produces enzymes which initiate hydrolytic reactions among the surrounding organic nutrients. As a result of this enzymatic activity, organic micro molecules are absorbed into the embryonic plant, where they are utilized in its metabolism. In essence, then, an embryonic seed plant starts its metabolic existence as a heterotrophy, not as an autotrophy.
With continued growth, a root system forms. Not only does this provide anchorage for the plant in the soil; water and dissolved mineral substances become available to it, inasmuch as they are transported by the root system to the remainder of the plant body.
Meanwhile, a stem with leaves grows upward, penetrates the soil surface, and becomes exposed to air and light. By this time, chlorophyll has developed, and the plant has become independent of the organic molecules originally stored in the seed from which it grew. From the point of chlorophyll development until its death, the plant is autotrphic in its nutrition.
The inorganic nutrient substances required by an autotrophic plant are carbon dioxide, oxygen, water, and a variety of mineral substances. Carbon dioxide and oxygen enter the plant through its leaves, and water and minerals enter through its roots.
The entrance of carbon dioxide and oxygen into leaves simply involves the diffusion of these substances through the stomata of the leaf. This arrangement makes it possible for atmospheric gases to circulate freely among the cells.
Even the more tightly packed cells of the upper layer have some space between them; thus, all cells of the leaf are in direct contact with oxygen and carbon dioxide. The passage of these gases into and out of cells is consistent with the gas laws which we mentioned in the previous topic.
The transport of water and minerals from the root system to the remainder of the plant is a much more complicated process, and some aspects of it are poorly understood. Probably the best explanation is the transpiration-cohesion-tension theory. According to this theory, the evaporation of water from cells within the leaf to the atmosphere through the stomata creates a water pressure deficit which eventually reaches to the xylem tissues.
A continuous column of water exists in these tissues, and as evaporation occurs from above, tension is placed on the column. Due to the cohesion of water molecules, the column is slowly lifted. Hence the force which causes the rise of water in the plant is held to be the evaporation of water from the leaf with its attending pressure deficit.
Although minerals are absorbed into root tissues by forces which are independent of water movement, as soon as they reach xylem tissues they are apparently swept along with the rising column of water.
Since we have already discussed photosynthesis as a cellular phenomenon, we shall simply describe some of its broader aspects at this point. The photosynthetic process yields PGAL and oxygen. Some of the oxygen may be utilized within the ell in respiration, but for the most part, it diffuses from the leaf by way of the stomata.
PGAL may be used by the cell in which it is produced, but during active photosynthesis, its production far exceeds the requirements of the cell for organic nutrients. Excess FGAL may then follow any one of three pathways: it may be stored within the cell as starch; it may be combined with minerals in the formation of complex compounds such as chlorophyll; it may be converted to transport carbohydrate and secreted to the outside of the cell.
In the case of this last alternative, the carbohydrate material moves into the conducting tubes of phloem, which convey it to various parts of the plant? Thus, although simple carbohydrates are the immediate products of photosynthesis, the green plant does not ordinarily build up great quantities of these substances. Rather, they serve as raw material for the further synthesis of organic compounds. They may be converted to more complex carbohydrates or to fats in the plant, or they may be combined with nitrogen and other elements available to the plant in its environment to form proteins.
Vitamins, enzymes, and various materials essential to the well-being of the plant may finally be formed by such modification of these carbohydrates, and even more chlorophyll can be synthesized from them. It may be said that the green plant is a very able chemist, producing a variety of substances from fundamental materials. The actual chemistry of reactions which occur in green plants is extremely complex and is still the subject of much intense research.
There is a common notion that green plants obtain their organic nutrients from the soil, or, in the case of aquatic species such as algae, from the aqueous medium. This is a mistaken idea; as we have seen, the green plant manufactures its organic nutrients from inorganic precursors.
It is true that these plants depend upon their environments for essential substances such as water, carbon dioxide, and inorganic salts, but these are not energy-yielding compounds. As for plants that grow in the soil, a simple experiment will show that it is not from the soil itself that the substance of a plant is chiefly derived.
A container may be filled with dirt, oven-dried, and weighed. If the seed of some plant is inserted into the soil and thoroughly watered, the seed will germinate into a plant. After the plant has grown to a considerable size, it may be pulled up and separated from the soil in which it was growing.
If great care has been taken to ensure that all the original soil is still present, and if all plant parts are removed from it, a second diying and weighing will indicate that the soil has lost only an extremely small percentage of its original weight. When the plant is weighed, it will be found that it is many times heavier than the soil which has been lost. Of course, much ofthe weight ofthe plant is accounted for by the water it has absorbed.
However even its dry weight will be found to equal many times that lost by the soil. It was experiments such as this that led early plant physiologists to the realization that the body of a land plant derives its mass from some source other than soil. We realize now that synthetic reactions account for this increase in mass and that the soil furnishes only an infinitesimal portion of the matter which eventually composes the plant body.
Those green plants which exhibit bodies of considerable size and complexity generally produce more organic materials in favourable seasons than they can use, and these may- be stored tn some form within one of the plant organs.
The most common sites of storage are roots and stems, but the leaves may also serve in this capacity. In seed plants the seeds are sites of considerable deposition of organic nutrients. Under certain circumstances the plant may utilize these resources or, as is more frequently the case, they may serve in the propagation of the species.
Respiration because photosynthesis occurs in green plants, there is a prevalent but erroneous belief that respiration occurs only in animals. Perhaps this idea results from attempts on the part of authors and teachers to simplify metabolism by saying that animals “breathe” oxygen and plants “breathe” carbon dioxide.
As is the case with many erroneous ideas, there is some truth to such a viewpoint, since animals do consume oxygen from their environments, and plants that are active in photosynthesis exhibit a net intake of carbon dioxide over that of oxygen. However, it should be obvious at this point that whenever a green plant derives energy from a foodstuff it is obliged to utilize oxygen exactly as an animal does.
This may easily be demonstrated by allowing a number of seeds to germinate within a closed container. It will be found that they deplete the oxygen in the atmosphere of their container within a short time and will stop growing until they are supplied with more.
Another way of showing this is to measure gaseous exchange in a green plant which is maintained for a time in darkness. Under these conditions, it -will be found that oxygen is utilized, and carbon dioxide is produced.
Nevertheless, in its overall metabolism, the green plant produces far more oxygen than it utilizes. In this respect, green plants serve the oxygen requirements of animals and heterotrophic plants, which in turn produce the carbon dioxide that is vital to the life of the green plant. Hence, the world of life is involved in a gaseous cycle in which oxygen and carbon dioxide play the predominant roles.
Returning to our hypothetical green plant, let us suppose that it is a perennial plant which has reached an age of fifty years. How much respiration occurs in such a plant, and in what areas? Actually, a great portion of such a plant is composed of dead tissues, including almost all of the cells of the main body from the bark inward, as well as much of the bark.
The same is generally true of the root system. In the main, living cells are restricted to the growing tips of stems and roots, a thin layer of the bark, and the current crop of leaves during a growing season. This knowledge can sometimes be put to practical use.
This apparently interrupts the flow of oxygen to root tissues which remain active in respiration. Again, pruning a tree very closely may kill it, especially during the growing season, as the living cells have been decreased below a critical point.