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.
The passage supports which of the following inferences about heterocysts?
- A Heterocysts do not produce oxygen.
- B Nitrogen gas inactivates heterocysts.
- C Chlorophyll increases the productivity of heterocysts.
- D Heterocysts allow nitrogen fixation and photosynthesis to occur in the same cell.
- E Heterocysts are more important for Anabaena's functioning than are photosynthetic cells.