托福考试

【单选题】Listening 5 "BiologyClass"
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What aspect of bacteria is this lecture mainly about
A、How microscopic organisms are measured
B、The use of bacteria for research in genetics

C、Diseases caused by bacterial infections
D、The three major types of bacteria
网考网参考答案：D
网考网解析：
[解析] 23-28 [Narrator] Listen to part of a lecture in a biology class. The professor is talking about bacteria. [Professor] Bacteria is the common name for a very large group of one-celled microscopic organisms that, we believe, may be the smallest, simplest, and perhaps even the very first forms of cellular life that evolved on Earth. Because they’re so small, bacteria must be measured in microns, with one micron measuring about 0.00004 inches long. Most bacteria range from about 0.1 microns to about 4 microns wide and about 0.2 microns to almost 50 microns long. So how can we observe them I’ll give you one guess. Under the microscope, of course. As I said, bacteria are very primitive and simple. In fact, they’re unicellular, Q25 which means that they’re made up of a single cell. We think they probably evolved about three and a half billion years ago. Some of the oldest fossils are bacterial organisms. They’ve been found almost everywhere on Earth, including all the continents, seas, and fresh water habitats, and in the tissues of both plants and animals. Well, since they’re so prevalent, you might ask, how do they reproduce Okay, they grow in Q25 Colonies and can reproduce, quite rapidly, in fact, by a process called fission. In fission, the cell, and remember, there’s only one in bacteria, one cell. So the cell increases in size and then splits in two parts. Fission is also referred to in your text as asexual budding. Now you’ll also read about conjugation, and that’s when two separate bacteria exchange pieces of DNA, so there are two ways that reproduction can occur, but we think that fission is more common. Okay. Bacteria were virtually unknown until about 1600 when microscopes were introduced, and at that time, bacteria were observed and classified into three main types according to their shapes, and that classification hasn’t really changed that much over the years. So that’s what I want to talk about Q23 today—the main types of bacteria. The slides that I’m going to show you are enlargements of bacteria that I observed under the microscope in the lab earlier today. Now, this first slide is an example of bacilli. The bacilli are a group of bacteria that occur in the soil and air. As you can see, they’re shaped like rods, and if you were to see them in motion, they’d be rolling or tumbling under the microscope. Of course, you can’t see that because this is a still visual, but later, when you go into the lab, you’ll see that rolling motion in examples of bacilli. These are kind of a greenish blue, but some are yellow. So don’t try to identify them by their color. Look at the shape. These bacilli are largely responsible for food spoilage. Okay, the next slide is a very different shape of bacteria. Its referred to as the cocci group, and it Q24 tends to grow in clusters or chains, like this example. This specimen is one of the common streptococci bacteria that cause strep throat. Finally, let’s look at the spiral-shaped bacteria called the spirilla. This is the spirilla. They look a little like corkscrews, and they’re responsible for a number of diseases in humans. But I don’t want you to get the wrong idea. It’s true that some species of bacteria do cause diseases, but for the most part, bacteria are benign. There’s a lot of bacteria in this room in fact. We all have it on us. They live 查看试题解析出处>>