Phylum Ciliophora

The nearly 8,000 species that make up the phylum Ciliophora (siL-ee-AHF-uh-ruh) share one key feature: they have cilia. Cilia are short, hairlike cytoplasmic projections that line the cell membrane. Cilia make it possible for these protists to move. Members of the genus Paramecium, shown in Figure 25-5, are among the most thoroughly studied ciliates. Paramecia are found in ponds and slow-moving streams that contain plants and decaying organic matter. A paramecium has cilia arranged in rows across its cell membrane. The cilia beat in waves, moving the cell through the water. Ciliates often feed on bacteria, algae, and other small organisms in their marine and freshwater habitats.

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figure 25-5

Like other ciliates, paramecia move by using hundreds of short projections called cilia. Paramecia have a large macronucleus, which controls many cell functions, and one or more micronuclei, which are involved in reproduction. The oral groove, mouth pore, and gullet collect food into vacuoles.

Cilia

Macronucleus

Micronucleus

Oral groove Mouth pore

Food vacuole (forming)

Anal pore

Macronucleus

Micronucleus

Mature food vacuole

Contractile vacuole

Pellicle

Oral groove Mouth pore

Food vacuole (forming)

Anal pore

Characteristics

Word Roots and Origins macronucleus from the Greek makros, meaning "long," and the Latin nucleus, meaning "nut" or "kernel"

figure 25-6

Flagella are a key characteristic of mastigophorans, such as these trypanosomes (Trypanosoma sp.), shown with red blood cells.Trypanosome flagella are structurally similar to the tails of human sperm.

Ciliates have the most elaborate organelles of any protist. A clear, elastic layer of protein, called a pellicle, surrounds the cell membrane. The pellicle has a funnel-like depression called an oral groove. Cilia lining the oral groove create currents that sweep food down the groove to the mouth pore. The mouth pore opens into a gullet, which forms food vacuoles that move throughout the cytoplasm. Enzymes in the vacuoles digest food into small organic molecules that enter the cytoplasm. Undigested materials move to the anal pore, which contracts and expels them. Ciliates also have contractile vacuoles, saclike organelles that expand to collect excess water and contract to squeeze the water out of the cell.

Ciliates have two types of nuclei. The large macronucleus contains multiple copies of DNA that direct the cell's metabolism and development. The smaller micronucleus participates in the exchange of genetic material during conjugation.

Reproduction

Asexual reproduction in ciliates occurs by binary fission. In this process, the micronucleus divides by mitosis. The macro-nucleus, which has up to 500 times more DNA than the micro-nucleus, simply elongates and splits in half. One half goes to each new cell.

Sexual reproduction in ciliates involves conjugation. During conjugation, two cells join, and their macronuclei disintegrate. Each diploid micronucleus then undergoes meiosis, producing four hap-loid micronuclei. In each cell, all but one micronucleus disintegrates. The remaining micronucleus divides by mitosis, producing two identical haploid micronuclei. The two cells then exchange one micronucleus. The two micronuclei in each cell then fuse to form one diploid micronucleus. The two cells separate, and a macronucleus forms in each cell from products of mitotic divisions of the micronucleus. Although the cells exchange genetic material during conjugation, they produce no new cells. After conjugation, the two cells divide, forming four offspring.

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