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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.

It can be inferred from the passage that cell differentiation within Anabaena is regulated by the
A.amount of oxygen Anabaena cells produce
B.season of the year
C.amount of fixed nitrogen compounds available
D.number of microscopic channels uniting Anabaena cells
E.amount of chlorophyll in 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.

The author uses the example of Anabaena to illustrate the
A.uniqueness of bacteria among unicellular organisms
B.inadequacy of an existing view of bacteria
C.ability of unicellular organisms to engage in photosynthesis
D.variability of a freshwater bacteria
E.difficulty of investigating even the simplest unicellular organisms

参考答案

正确答案:B C A B