I. Read the text and answer the questions in written form

Bacteria: their Construction

1. Bacteria are very small single-celled organisms (micro-organisms) that exist in enormous numbers almost everywhere. They live in soil, water, air, and in living and dead animals and plants. A gram of soil can contain up to a thousand million bacteria, and there may be hundreds in a single drop of milk.

2. Bacteria differ from each other mainly in where and on what they live, and in the shape of their single cells. There are the spherical coccus types such as Staphylococcus and Streptococcus, which often occur in chains or masses, and the rod-shaped bacillus type such as Mycobacterium, which causes tuberculosis. Other disease-causing bacteria are Eberthella typhi (typhoid), and Vibrio oholerae (cholera). The type of bacterium which forms a coil on spiral is Spirillum.

3. Although bacteria cells are more complicated than viruses they are still very simple. Their structure has been worked out with optical microscopes which magnify by one hundred thousand times. All bacteria have a tough outer cell wall so their food must be soluble before it can be absorbed into the cell. In some bacteria there is a protecting layer of jelly enclosing the cell wall and also one or more minute fibres (flagella) used for swimming. Inside the cell there is a coil of DNA and other chemical substances, but there is no definite nucleus or any of the other structures found in plant and animal cells.

4. Bacteria usually reproduce by simply splitting in two. When temperature conditions are favourable, about 37⁰С for most bacteria, they can divide about once every 30 minutes. In theory, one bacterium could form about 140 000 000 000 000 bacteria at the end of 24 hours.

Questions:

1. What are bacteria?

2. Where do bacteria live?

3. How do bacteria differ from each other?

4. What coccus types are there?

5. What bacillus type causes tuberculosis?

6. Why must the food of all bacteria be soluble?

7. What is there in some bacteria?

8. How do bacteria usually reproduce?

Date: 2015-12-13; view: 675; Нарушение авторских прав