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Read the passage below and asnswer the following questions 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. 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. 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.
It can be inferred from the passage that when describing bacteria as “swimming up a concentration gradient”, the author means that they were behaving as if they were swimming
From an area where the concentration of a substance is weaker to an area where it is stronger.
Away from a substance to which they are normally attracted.
Away from a substance that is normally harmful to them.
From an area where the concentration of a repellent is weaker to an area where it is completely absent.
- The passage describes how bacteria move in response to chemical gradients.
- They travel toward higher concentrations of attractants.
- In the phrase "swimming up a concentration gradient," bacteria move from lower to higher chemical concentrations.
- Option 1: From an area where the concentration of a substance is weaker to an area where it is stronger.
- This fits the described behavior of bacteria moving toward attractants.
- Correct Answer
- Option 2: Away from a substance to which they are normally attracted.
- This contradicts the attractant behavior.
- Option 3: Away from a substance that is normally harmful to them.
- It describes movement away from repellents, not attractants.
- Option 4: From an area where the concentration of a repellent is weaker to an area where it is completely absent.
- It talks about repellents, whereas the context is about attractants.
By: Munesh Kumari ProfileResourcesReport error
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