Protozoa

What Are Protozoa?

Protozoa are microscopic, unicellular, animallike organisms. Some are barely visible, and appear as tiny specks to the naked eye. Protozoa are quite different from more familiar plants and animals. Many scientists have placed them in a separate kingdom, the Protista.

Although protozoa are considered among the simplest organisms, they are far from simple. Protozoa perform all of the many functions of life that multicellular organisms perform, such as digestion, excretion, and respiration. However, in protozoa all of these functions take place within a single cell.

Some protozoa form a group, or cluster, of cells called a colony. Each cell within a colony maintains its indepen­dence as a separate organism because each cell performs all life functions. Only the function of reproduction may be shared between individual cells of a colony.

There are about 30,000 species of protozoa. Each has its own life style and habitat. Protozoan habitats include ponds, oceans, mud, soil, and even on or in certain plants and animals. Some species live only in the body cavity.

Not all protozoa are harmful to the animals with which they are associated. Some are beneficial. For example, cer­tain flagellated protozoa live in the intestines of termites, where they secrete digestive enzymes. The enzymes break down the wood that the termites eat. The usable nutrients that result are absorbed by both the protozoa and the ter­mites. The termites benefit by having their food digested; the protozoa benefit by having a source of food and a place to live.

Protozoa are classified in four phyla based on their method of movement. An example of a typical protozoan from each phylum will show you the characteristics com­mon to each group.

Phylum Sarcodina: Ameba

The genus Amoeba includes several interesting protozoan species. At first glance, you might mistake an ameba for something nonliving—a microscopic bit of jelly, for exam­ple. Yet this tiny blob of grayish "jelly" is a complete living organism. It moves, reproduces, and performs all other life functions.

Through the microscope an ameba appears as a shape­less mass of cytoplasm surrounded by a thin plasma mem­brane. In an active ameba you will see that the cytoplasm has a constant flowing motion. This streaming cytoplasm presses against the plasma membrane and pushes out pro- _pseudo = false jections called pseudopodia (soo-doe-POE-dee-uh).

Pseudopodia (singular, pseudopod) means "false feet." The name is fitting, since amebas move by means of their pseudopodia. The motion is sort of flowing, with new pseu­dopodia reaching out and old ones disappearing back into the cytoplasm. On the basis of this ameboid movement, amebas are placed in the protist phylum Sarcodina.

A closer look at an ameba such as Amoeba proteus shows that amebas have two types of cytoplasm. A clear, watery ectoplasm is found just inside the cell membrane. Ecto­plasm is outside the denser, grainier endoplasm, which looks like gray jelly with pepper sprinkled through it. The inner part of the endoplasm is more fluid than the outer part and flows more rapidly when the ameba moves. A disk-shaped nucleus, which changes position as the cyto­plasm flows, is present.

Amebas live in water. They are found in slime at the bottom of streams and ponds, and on the leaves of water plants. The oxygen needed for life enters through the cell membrane by diffusion from the surrounding water. Car­bon dioxide and soluble wastes such as ammonia pass out through the membrane in the same way.

Much useless water also enters the ameba cell, mainly by osmosis. If the ameba could not rid itself of this water, it would swell and burst like a balloon. This does not happen, of course. Instead, the extra water gathers in a contractile vacuole. When this vacuole reaches a certain size, it con­tracts sharply, and the water is squeezed out. A temporary break in the cell membrane occurs when this happens.

How Amebas Get Food

The food supply for amebas includes cells of algae and of certain other protists. When an ameba contacts such a cell, it simply engulfs the cell by surrounding it with pseudopo­dia. The food is taken into the ameba cell as a food vacuole. Part of the cell membrane of the ameba surrounds this vac­uole. A new membrane quickly forms at the point where the food entered the ameba cell.

Digestion is carried on by enzymes formed in the cyto­plasm. These enzymes pass into the food vacuole and act on the food. Digested food is absorbed by the cytoplasm. Then it is ready for use as an energy source or as raw material for building more protoplasm. Particles that are not digested remain in the vacuole and may pass out at any point on the cell membrane.

Response in the Ameba

Amebas respond to the conditions around them. Though they have no eyes, they are sensitive to light and seek dim or dark areas. They do not have nerve endings for a sense of touch. Yet they react to movement around them and move away from objects they touch.

Some species of ameba respond to conditions such as dryness, cold, or lack of food. Such amebas become inactive and withdraw into a round mass called a cyst. When conditions improve, the amebas become active again.

Reproduction of Amebas

Under good conditions, an ameba may double its volume in a day or two. When this occurs, it reproduces by simple cell division. The nucleus divides and the two daughter nuclei move to opposite ends of the cell. The cell then pinches in at the center, and the two halves pull apart. Each daughter cell has a nucleus and is capable of independent life and growth. The division itself takes about an hour.

Phylum Ciliophora: Paramecium

Amebas are fairly simple cells with few specialized struc­tures. Members of the genus Paramecium, however, are a much more complex form of one-celled life. Species of this genus live mainly in quiet or stagnant ponds. Great num­bers of paramecia are found in the scums that form on such ponds.

Oxygen gas (O2) dissolved in the water diffuses through the cell membrane and enters the cytoplasm. It is used in the process of respiration. Carbon dioxide (CO2), a waste product of respiration, diffuses out of the cell into the water.

The Paramecium cell is shaped like a slipper. Although they are flexible enough to bend, paramecia do not change shape like amebas. Their definite shape results from a thick outer membrane, the pellicle, which surrounds the cell membrane.

Paramecia move by means of hairlike threads called cilia. These cilia are arranged in rows and beat back and forth like tiny oars. They cover the whole cell but are most easily seen at the edges. Cilia can beat either forward or backward, moving the cell in either direction and allowing it to turn. This kind of movement places paramecia in the protist phylum Ciliophora.