Read the text make up its plan and answer the questions

1. What types of bacteria do you know?

Bacterium, plural bacteria, any of a group of microscopic organisms that are prokaryotic, i.e., that lack a membrane-bound nucleus and organelles. Bacteria are unicellular (one-celled) and may have spherical (coccus), rod-like (bacillus), or curved (vibrio, spirillum, or spirochete) bodies. Different bacteria inhabit virtually all environments, including soil, water, organic matter, and the bodies of eukaryotes (multicellular animals). Some bacteria are known to be beneficial to humans and the higher animals, while many others are harmful; bacteria are the chief cause of infectious diseases in humans.

2. What kind of cell walls do they have?

On average, bacteria are about 1 micrometre long and 0.5 micrometre in diameter. All bacteria are surrounded by a lipid membrane that regulates the flow of materials in and out of the cell. A rigid cell wall completely surrounds the bacterium and lies outside the membrane. Gram-positive bacteria are stained blue by the gram stain, because their cell walls have a relatively thick and meshlike structure that traps the dye. In gram-negative bacteria, the cell wall is thin and releases the dye readily when washed with an alcohol solution.

3. What are flagellae?

Outside the cell wall, some species of bacteria also have a capsule made up of polysaccharides. Such capsules have many functions, including protecting the bacterium from phagocytes and from dessication (drying). Many species of bacteria swim by means of flagellae, i.e., hairlike structures whose whiplike lashing provide propulsion.

4. What structures are distributed about the bacterial cytoplasm?

The DNA of most bacteria is found in a single circular chromosome and is distributed throughout the cytoplasm rather than in a membrane-bound nucleus. Smaller circular auxiliary DNA strands, called plasmids, are also found in the cytoplasm. There are a number of other structures distributed about the bacterial cytoplasm, including ribosomes.

5. How do bacteria reproduce?

When applied to bacteria, the term growth refers to an increase in the size of a population rather than in that of an individual microorganism. Bacteria usually reproduce through binary fission, an asexual process in which the mother cell increases in size until it divides into two identical daughter cells. There are also bacteria that reproduce through budding, through chains of spores, and through the segmentation of elementary units. Bacteria do not reproduce sexually, but there are several mechanisms by which DNA is exchanged in a one-way transfer between them.

6. What are heterotrophs and autotrophs?

All bacteria require carbon, nitrogen, sulfur, phosphorus, inorganic salts, and micronutri-ents. Bacteria that use an organic compound as their source of carbon are called heterotrophs, while those that use an inorganic source are called autotrophs. In addition, some bacteria use photosynthesis to generate energy in the form of the compound ATP; these are called phototrophs. Some species of bacteria are parasitic and can grow only within a living host cell; examples include the genera Rickettsia and Chlamydia, both of which are parasites in eukaryotic cells. Those bacteria that require oxygen, such as Bacillus, are called aerobes; anaerobes, such as Clostridium, can not survive in the presence of oxygen.

7. What types of bacteria do you know?

Various types of bacteria that are present in water can cause disease in humans, and water-purification plants are designed to destroy these microorganisms. Bacteria from industrial wastes may also act as pathogens, or agents of disease. Conversely, some types of bacteria act as cleansing agents in water, and water-treatment facilities utilize some such bacteria to break down the organic matter that is present in sewage.

Various types of bacteria contaminate foods and can cause food poisoning in humans. Pasteurization is routinely used to neutralize bacteria that may be present in milk, for example. Other sterilization techniques include high temperature, radiation, ethylene oxide, and other antiseptics and germicides.

8. What is virulence?

The ability of a bacterium to cause a disease is called virulence. One contributing factor to the degree of virulence is the type of capsule a bacterium has. Some bacteria have a specificity for various parts of the body: meningo-coccal bacteria infect the brain membranes, and tubercule bacteria infect the lungs. Some are slightly more generalized: staphylococcal bacteria can infect the skin, causing boils; the bloodstream, causing blood poisoning; and the bones, causing the condition known as osteomyelitis.

9. Are all bacteria harmful?

Although human interest in bacteria frequently focuses on their harmful effects, most bacteria are harmless to human beings, and many of them are actually beneficial. Saprophytic bacteria, for example, perform an ecologically indispensable role in the breakdown of dead organisms and organic wastes; with out such agents of decomposition, the cycling of various elements vital to living organisms (including nitrogen, carbon, and phosphorus) would cease in the biosphere.

10. How are bacteria used?

Bacteria also form highly beneficial associations with animals. For instance, the bacterial inhabitants of the ruminant stomach break down cellulose; this enables cows, sheep, and other ruminants to digest grass. Humans also harbour beneficial bacteria, such as those in the lower intestine that synthesize vitamin K. Bacteria are also used in various industrial processes, especially in the food industry; the production of buttermilk, yogurt, cheeses, pickles, and sauerkraut are all dependent upon bacterial action.

11. What is the scientific classification of bacteria?

The scientific classification of bacteria is in transition, particularly at the higher taxonomic levels. The bacteria form the only prokaryotic kingdom, that of the Monera. Within this kingdom, at least two groups have been distinguished, the eubacteria and archaebacteria. DNA hybridization studies of ribosomal RNA have proven useful in defining these groups.

5. Read the text and find English equivalents to the Russian sentences given below.

Fish

Fish are vertebrates (animals with a backbone) that are completely adapted for life in water. They breathe under water and have a streamlined body for swimming. They have fins to propel, balance, and steer themselves. Fish are found in both freshwater and marine habitats. Sharks and rays belong to an ancient group of fish that existed 200 million years before dinosaurs. They are called cartilaginous fish because they have a skeleton made of cartilage, a kind of gristle. Their big, oily liver helps them float, and they have a skin with toothlike scales.

1. У них есть плавники, при помощи которых они передвигаются, держат равновесие и управляют направлением движения.

2. Их называют хрящевыми рыбами, так как у них скелет состоит из своего рода хрящей.

3. Они дышат под водой и имеют обтекаемое тело для плавания.

4. Их большая жирная печень помогает им держаться на воде, а кожа покрыта зубчатой чешуей.

5. Средой обитания для рыб может как пресная, так и морская вода.

6. Read the text, tell about characteristic features of amphibians and the differences between frogs and toads; find English equivalents to the Russian expressions:

Выпученные глаза;

Припадающий к земле корпус;

Перепончатые лапы;

Бородавчатая кожа.

Date: 2015-04-23; view: 801; Нарушение авторских прав