Unit 11. Protozoa.

WARM-UP: Spend a minute writing down all of the different words you associated with Protozoa. Share your ideas with the partner. Together put them into categories.

VOCABULARY: Learn the following words and word combinations.

Protozoa vestibule gullet to accumulate digestive juice to dissolve food vacuole contractile vacuoles to split into cyst encystment immobile to burst drought

Guess the meaning of the following words.

Microscope; one-celled organism; absorbing; organic matter; vacuole; condition; magnifying lenses; discovery; micronucleus; bacteria; hair-like; to accumulate; to assimilate; outer layer.

Practice the following for pronunciation.

protozoa [prəυtəυ´zəυə]

vestibule [´vestıbju:l]

gullet [´g۸lıt]

cyst [sıst]

vacuole [´vækjυəl]

burst [bз:st]

oar [o:]

bacterium [bæk´tıərıəm]

drought [draυt]

immobile [ı´məυbaıl]

READING: Read the text and do the tasks that follow.

More, than 300 years ago an event took place that was very important in the history of natural science. A Dutchman Anton van Leeuwenhoek, a skilful polisher of magnifying lenses, while examining, under a microscope he himself had built, a drop of rainwater, which had been kept in a barrel for some time, was astonished to find that the turbid liquid he had considered lifeless was very far from being so. It was such an unexpected discovery that he could not help calling out in surprise. Very soon he realized that what he had seen were tiny living animals no one had even heard of before. Leeuwenhoek called them “the smallest animals”. Later, having studied their structure, scientists named them Protozoa.

A Paramecium is easier to detect in a drop of fresh water viewed under a microscope than other Protozoa. It is shaped like a slipper, whence it derives its name.

The Paramecium is made up of protoplasm and surrounded by a somewhat tough outer layer which helps the Paramecium to maintain a constant shape. Inside is a large oval macronucleus seen only upon the introduction of certain stains. Next to it is the so called micronucleus.

The Paramecium is a one-celled living organism with two nuclei. Like all other animal cells it has no chlorophyll and is incapable of absorbing carbon dioxide from the air.

In its search for food the Paramecium swims rapidly about, propelled by the action of little hair-like cilia, which cover its body and act like tiny oars. The Paramecium feeds on organic matter called bacteria.

By beating its cilia, the Paramecium sets up a current which draws the bacteria into a depression called the vestibule. From there it is taken into the mouth which leads into the gullet. The food is accumulated in the gullet and surrounded by digestive juices. A food vacuole is formed, which breaks away changed by the digestive juices, dissolved and absorbed by the protoplasm. The Paramecium assimilates the digested food while the undigested remains are thrown out of the food vacuole through a hole at the broader end of the animal’s body.

Paramecia die when placed in cool, boiled, airless water. This means they can not exist without air, and are in constant need of oxygen. Like plants, they respire through their body surface.

As the Paramecium develops, part of it is constantly being destroyed and the waste matter harmful to the organism, is passed out together with surplus water through two contractile vacuoles.

In the warmer seasons when there is a plentiful food supply in the pond, the Paramecia grow fast and then divide. Each of the nuclei splits into two, and a groove appears across the middle of the body. It deepens and the body separates into two approximately equal parts. The “daughter” cells then grow to their maximum size and again divide about a day later.

If the water in the pond where the Paramecia live dries out suddenly, they die. If the drying out process is slow the body of the Paramecium takes a round shape and forms a hard resistant wall, a cyst. This condition is known as encystment. The Paramecium does not take in food and remains immobile. But the animal does not die. If a gust of wind carries the cysts into another reservoir they burst and the Paramecia become lively and active again. The cysts are capable of withstanding drought, low temperatures, and other unfavorable conditions.