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Daily RC Article 19

Behavior of Bacteria

Paragraph 1

Although bacteria are unicellular and among the simplest autonomous forms of life, they show a remarkable ability to sense their environment. They are attracted to materials they need and are repelled by harmful substances. Most types of bacteria swim very erratically; short smooth runs in relatively straight lines are followed by brief tumbles, after which the bacteria shoot off in random directions. This leaves researchers with the question of how such bacteria find their way to an attractant such as food or, in the case of photosynthetic bacteria, light, if their swimming pattern consists only of smooth runs and tumbles, the latter resulting in random changes in direction.

Paragraph 2

One clue comes from the observation that when a chemical attractant is added to a suspension of such bacteria, the bacteria swim along a gradient of the attractant, from an area where the concentration of the attractant is weaker to an area where it is stronger. As they do so, their swimming is characterized by a decrease in tumbling and an increase in straight runs over relatively longer distances. As the bacteria encounter increasing concentrations of the attractant, their tendency to tumble is suppressed, whereas tumbling increases whenever they move away from the attractant. The net effect is that runs in the direction of higher concentrations of the attractant become longer and straighter as a result of the suppression of tumbling, whereas runs away from it are shortened by an increased tendency of the bacteria to tumble and change direction.

Paragraph 3

Biologists have proposed two mechanisms that bacteria might use in detecting changes in the concentration of a chemical attractant. First, a bacterium might compare the concentration of a chemical at the front and back of its cell body simultaneously. If the concentration is higher at the front of the cell, then it knows it is moving up the concentration gradient, from an area where the concentration is lower to an area where it is higher. Alternatively, it might measure the concentration at one instant and again after a brief interval, in which case the bacterium must retain a memory of the initial concentration. Researchers reasoned that if bacteria do compare concentrations at different times, then when suddenly exposed to a uniformly high concentration of an attractant, the cells would behave as if they were swimming up a concentration gradient, with long, smooth runs and relatively few tumbles. If, on the other hand, bacteria detect a chemical gradient by measuring it simultaneously at two distinct points, front and back, on the cell body, they would not respond to the jump in concentration because the concentration of the attractant in front and back of the cells, though high, would be uniform. Experimental evidence suggests that bacteria compare concentrations at different times.

Topic and Scope:

The movement of bacteria; specifically, the movement of bacteria toward attractants.  

Purpose and Main Idea:

The author’s purpose is to discuss what scientists know about the process by which bacteria move toward attractants; since this passage is a descriptive one, the author doesn’t have a particular main idea in mind.

Passage Structure:

 Paragraph 1 outlines the topic and scope of the passage: How are bacteria able to overcome their seemingly haphazard swimming pattern—a mix of straight runs and uncontrolled tumbles—to move toward attractants?  

Paragraph 2 begins to answer this question by noting that bacteria have been observed to move along a concentration gradient toward attractants—in other words, to move in a non-random fashion. While this  paragraph  describes the swimming pattern by which bacteria move toward concentrations of attractants, it doesn’t explain how bacteria detect concentrations of attractants in order to be able to move toward them.   

Paragraph 3 takes up the latter issue. There are two possible methods of detection, both of which are discussed in some detail. The last sentence of the passage concludes that scientists believe that bacteria detect changes in the concentration gradient of attractants by measuring concentrations at different points in time, rather than by measuring concentrations along different parts of their bodies at the same point in time.  

The Big Picture:  

  • In passages that are built around a relationship, be sure that you’re absolutely clear about the nature of the relationship. The questions will certainly focus on it.  
  • Although this science passage isn’t especially difficult—all you really have to grasp is the basic relationship between the movement of bacteria and the concentration of attractants—you should probably have left it for later in the section. Why? The author’s descriptive purpose isn’t entirely clear until you’ve read the whole thing.

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