GRE Reading Comprehension
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Source: 91年
Researchers are finding that in many ways an individual bacterium is more analogous to a component cell of a multicellular organism than it is to a free-living, autonomous organism. Anabaena, a freshwater bacteria is a case in point. Among photosynthetic bacteria, Anabaena is unusual: it is capable of both photosynthesis and nitrogen fixation. Within a single cell, these two biochemical processes are incompatible: oxygen produced during photosynthesis, inactivates the nitrogenase required for nitrogen fixation. In Anabaena communities, however, these processes can coexist. When fixed nitrogen compounds are abundant, Anabaena is strictly photosynthetic and its cells are all alike. When nitrogen levels are low, however, specialized cells called heterocysts are produced which lack chlorophyll (necessary for photosynthesis) but which can fix nitrogen by converting nitrogen gas into a usable form. Submicroscopic channels develop which connect the heterocyst cells with the photosynthetic ones and which are used for transferring cellular (30) products between the two kinds of Anabaena cells.
According to the passage, which of the following statements is true of bacteria that engage in photosynthesis?
- A They eventually become two autonomous cells.
- B They cannot normally also engage in nitrogen fixation.
- C Oxygen normally inactivates them.
- D Cellular products are constantly transferred between such bacteria.
- E They normally lack chlorophyll.