Nutrient Procurement by Protozoans and Animals

Nutrient procurement by animals, when examined under microscopes such as teaching microscopes, usually involves much more activity than it does in plants and fungi. Animals must often resort to elabo¬rate methods of locating and trapping their food. Like the fungi al¬ready examined under a teaching microscope, most animals must digest their food before it can cross the membranes of their cells. Usually their food material is in the form of polysaccharides, fats, proteins, and other large molecules, and must be hydrolyzed. But animals rarely simply secrete digestive enzymes directly onto their food, in the manner of fungi. The vast majority ingest particles or lumps of food into some sort of digestive structure, in which enzymatic digestion takes place.

Though both the nutritional requirements and the basic processes of digestion are essentially alike in protozoans and all types of animals, from worms to human beings, as seen under teaching microscopes, the body plans of these organisms vary so greatly that the structures involved in food processing and the de¬tails of that processing are often very different. We shall briefly examine the digestive mechanisms of protozoans and a variety of animals.

NUTRIENT PROCUREMENT BY PROTOZOANS

Since Protozoa, as single-celled organisms (these can be viewed using a light microscope such as a teaching microscope), have a body plan obviously very different from that of multicellular animals, we would expect their adaptations for food procurement, to be likewise markedly dif¬ferent. And the differences are, in fact, considerable when they are examined under a microscope. But a more in¬teresting point, one with important biological implications, is that the similarities are often more striking than the differences.

Let us look first at an amoeboid protozoan, which constantly changes shape as its protoplasm flows along, pushing out new arm like pseudopodia and withdrawing other. When an amoeba is stimulated by nearby food, some of the pseudopodia may flow around the food until they have completely surrounded it. This is the process known as phagocytosis. The food is completely engulfed by the cytoplasm and is enclosed in a food vacuole, digestive enzymes are secreted into the vacuole, and digestion takes place. Amoebas exemplify protozoans that lack specialized permanent digestive structures, though their food vacuoles correspond in func¬tion to the digestive systems of higher animals.

The ciliates, another important group of protozoans, of which Para¬mecium is an example, are characterized by innumerable cilia cover¬ing the surface of their bodies. Like all Protozoa, they are commonly regarded as unicellular. Though they lack actual subdivision into recognizable cellular units, the more complex ciliates show much of the internal specialization, as seen under a microscope, usually associated with multicellularity. Unlike the amoeba, Paramecium has a permanent structure, an organelle that functions in feeding. When examined under a microscope, food particles are swept into an oral groove, a ciliated channel located on one side of the cell, by water currents produced by the beating of the cilia, and are carried down the groove into a cytopharynx. As food accumu¬lates at the lower end of the cytopharynx, a food vacuole forms around it. Eventually the vacuole breaks off and begins to move toward the anterior end of the cell. Digestive enzymes are secreted into the vacuole and digestion begins. As digestion proceeds, the products (simple sugars, amino acids, and many more) diffuse across the membrane of the vacuole into the cytoplasm, and the vacuole begins to move back toward the posterior end of the cell. When the vacuole reaches a tiny specialized region of the cell surface called the anal pore, it becomes attached there and ruptures, expelling by exocytosis any remaining bits of indigestible material. Not only does the food vacuole function as a digestive chamber, but also by its movement it helps distribute the products of digestion to all parts of the cell.

We have said that digestive enzymes are secreted into the food vac¬uoles of both the amoeba and Paramecium. But if these powerful en¬zymes, usually seen under a microscope, are capable of hydrolyzing such compounds as polysaccharides, fats, proteins, and nucleic acids, and if the cell itself is composed of these kinds of compounds, how can the cell contain the digestive en¬zymes without being destroyed by them? Digestive enzymes are packaged in lysosomes, vesicles whose membranes are apparently both impermeable to the enzymes and capable of resisting their hydrolytic action. The digestive enzymes are presumably synthesized on the ribosomes, move through the endoplasmic reticulum to the Golgi apparatus, and there become sur¬rounded by a membrane to form the lysosome. These cell structures can be further studied and analyzed using different types of microscopes such as teaching microscopes and other light microscopes. When a food vacuole is formed, a lysosome soon fuses with it. Food materials and the digestive enzymes are mixed in the resulting digestive vacuole. As already described, this vacuole circulates in the cytoplasm, the prod¬ucts of digestion are absorbed, and indigestible materials are eventually expelled from the cell by exocytosis. Although the process we have described here is classified as intracellular digestion, it should be noted that the food material is separated from the rest of the cellular material by a membrane that it cannot cross until after digestion has occurred. Thus intracellular digestion and extracellular digestion are alike in that the digestion always precedes the actual absorption of complex foods across a membrane, as evident by microscopic examination of the cell.

Although this description of lysosome activity pertains to digestion in Protozoa, it applies equally well to intracellular digestion in any animal cell when examined under a microscope. Lysosomes, you will recall, were, in fact, first discovered in rat liver cells.