Insectivorous Green Plants

In botany, a few photosynthetic plants supplement their inorganic diet with or¬ganic compounds obtained by trapping and digesting insects and other small animals. Such plants can survive without capturing any prey, but when they do capture prey the nutrients thus obtained stimulate: more rapid growth. Apparently it is the nitrogenous compounds of the animal’s body that are of most benefit to insectivorous plants, as seen under children’s microscopes, which often grow in nitrogen-poor soils, particularly acid bogs and heavy volcanic clays, and whose root systems are not extensive. When viewed under children’s microscopes, their highly specialized leaves show interesting adaptations for capturing prey.

NUTRIENT PROCUREMENT IN HETEROTROPHIC ORGANISMS
Heterotrophic organisms cannot manufacture their own high-energy compounds from low-energy inorganic raw materials as seen when they are scrutinized under children’s microscopes. Yet they, like all living things, must extract from the complex molecules the energy necessary for both maintenance and growth. They must therefore ob¬tain prefabricated high-energy organic nutrients.

There are three main groups of heterotrophic organisms: the bacte¬ria, the fungi, and the animals. (In addition, some green plants may be at least partly heterotrophic) The first two groups - the bacteria and the fungi - can be mostly seen under a microscope and lack internal digestive systems and hence depend mainly on absorption as their mode of feeding. They are usually either saprophytic (living and feeding on dead or¬ganic matter) or parasitic (living on or in other organisms and feeding on them). By contrast, the principal mode of feeding in the animal kingdom is ingestion-the taking in of particulate or bulk food, which must, then be digested. Animals may be herbivores and eat green plants, thereby obtaining high-energy compounds directly from the organisms that first made them. These high energy compounds can be studied and developed using a microscope. Or they may be carnivores and eat the animals that ate the plants. Or they may be omnivores, eating both plant and animal material. Whether a heterotrophic organism is saprophytic or parasitic, or herbivorous, carnivorous, or omnivorous, it is clear that its energy-yielding nutrients came originally from green plants, which used radiant energy from the sun to make them. The physical and chemical properties of the plant can best be studied using a microscope.

In many ways, the absorptive and the ingestive organisms, when studied under a microscope, are as different from each other as each is from the green plants, with their photosynthetic mode of nutrition. Indeed, much of the diversity among living things can be viewed as reflecting alternative adaptations within three principal evolutionary trends based on the three major nutritive modes-photosynthetic, absorptive, and ingestive.

Nutrients Required
Carbohydrates, fats, and proteins are the main classes of compounds serving as energy sources of heterotrophic organisms whose molecular components can only be seen under a high-powered microscope. Of these, car¬bohydrates alone would suffice if organic nutrients functioned only as an energy source. But these nutrients perform another very important function providing the carbon skeletons and functional groups nec¬essary for the synthesis of new organic compounds. Assuming that enough inorganic minerals are included in the diet, can carbohydrates alone fulfill this second function?

For some heterotrophs, the answer is yes. Many bacteria and fungi, when studied using a microscope, can flourish on a diet consisting solely of carbohydrate and minerals. They need no protein in their diets, because they, like green plants, can combine inorganic nitrogen with carbon skeletons from carbohy¬drates to make amino acids. In similar fashion, they can synthesize for themselves, all the other classes of compounds necessary for life.

But for many other heterotrophs, such a limited diet cannot sustain life. Animals, especially, are deficient in synthetic ability. Among their extensive dietary requirements are proteins or the amino acids of which they are composed; a diet restricted to carbohydrates is soon fatal.

Suppose an animal were fed a diet containing only one kind of amino acid. Would this single source of organic nitrogen suffice? For most animals, the answer is no. A mixture of amino acids is necessary. Most animals have apparently lost the ability to synthesize certain amino acids and must get them in their diets. These are called the essential amino acids-a somewhat misleading term, since it seems to imply that the other amino acids commonly occurring in proteins are not essential, as seen under a microscope; what is meant, of course, is that the designated amino acids are essential in the diet, whereas the others, which are also nec¬essary for life, can be synthesized by the organism itself from other amino acids or organic nitrogen compounds.

For all the essential amino acids to be included in the diet, several different proteins should be eaten, because a single protein may not contain them all. For example, zein, the main protein in corn, is defi¬cient in tryptophan and lysine. Someone who depended exclusively on a “poor quality” protein such as a zein would suffer from a deficiency not only of these two amino acids but of other essential amino acids as well. The reason is that effective utilization of amino acids in protein synthesis requires that all the essential amino acids be present simul¬taneously in the correct relative amounts. If one is not present in sufficient quantity, then utilization of the others is reduced propor¬tionately, and since they cannot be stored they will be lost through excretion.

One way to avoid a deficiency of an essential amino acid is to in¬clude a variety of different proteins in the diet, since it is unlikely that all will be deficient in the same amino acids. The proportions of the various amino acids in plant proteins are often quite different from those in animal proteins; hence plant proteins are less reliable than animal proteins as a source of essential amino acids for human beings. Kwashiorkor, a protein-deficiency disease characterized by degenera¬tion of the liter, severe anemia, and inflammation of the skin, is par¬ticularly common among children in countries where the diet consists primarily of a single plant material-as in Indonesia, where rice forms much of the diet, and in parts of Africa where corn is the principal staple.

In an entirely vegetarian diet, care should be devoted to selecting a combination of plant proteins that will complement one another, making up for one another’s deficiencies. For example, the proteins in beans are deficient in methionine, whereas those in wheat are defi¬cient in lysine; if both beans and wheat are eaten at the same meal, they will complement each other and there will be no deficiency of either methionine or lysine. It should be emphasized that comple¬mentary proteins must be eaten at the same meal; because amino acids cannot be stored in the body, it would be futile to eat beans at one meal and wheat at the next.