24. 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.

"Anabaena is unusual: it is capable of both photosynthesis and nitrogen fixation." This suggests that it is unique among all the bacteria because it can perform photosynthesis and nitrogen fixation at the same time.

25. 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

Cell differentiation is regulated by the amount of nitrogen available in the cell. Because, when it is abundant cells can perform photosynthesis,, but when they are not then special cells can be produced to do nitrogen fixation.


26. 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 passage says that oxygen is produced during photosynthesis, so if heterocysts lack chlorophyll that is necessary for photosynthesis, then option A can be inferred.

27. The author uses the example of Anabaena to illustrate the

(A) the 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) the difficulty of investigating even the simplest unicellular organisms

Anabaena is unique for sure but the passage is not written to highlight this point and it is unique among "photosynthetic bacteria". The author started the passage with saying that "Researchers are finding that an individual bacterium is more analogous to a component cell of a multicellular organism than it is to a free-living". This means that the existing view is lacking this concept.

Having option A&B together in the same question, the GRE is trying to vex the test takers.

However, the rest of the choices are far from being close to the correct answer.

_________________

Hi, could you provide an explanation (particularly for questions 25-27), thank you!

See the explanation above.

Thanks,
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24. According to the passage, which of the following statements is true of bacteria that engage in photosynthesis?
(B) They cannot normally also engage in nitrogen fixation.
Correct: Within a single cell, these two biochemical processes are incompatible

25. It can be inferred from the passage that cell differentiation within Anabaena is regulated by the

(C) amount of fixed nitrogen compounds available
correct: When fixed nitrogen compounds are abundant, Anabaena is strictly photosynthetic

26. The passage supports which of the following inferences about heterocysts?

(A) Heterocysts do not produce oxygen.
correct: heterocysts are produced which lack chlorophyll (necessary for photosynthesis)

27. The author uses the example of Anabaena to illustrate the
POE
(A) the uniqueness of bacteria among unicellular organisms
Incorrect trap answer- Passage does not say bacteria are unquie among unicellular organisms.
It only says Anabena is unusual among photosynthetic bacteria
(B) inadequacy of an existing view of bacteria
Correct- Researchers are finding that in many ways an individual bacterium is more analogous to…………..
than it is to a free-living, autonomous organism.
(C) ability of unicellular organisms to. engage in photosynthesis
Incorrect- This is clearly not the purpose
(D) variability of a freshwater bacteria
Incorrect- it is a freshwater bacteria but that is not the purpose of the author
(E) the difficulty of investigating even the simplest unicellular organisms-
Incorrect- Passage does not mention the difficulty of investigating

For 27, How is the choice B correct, if the author never mentioned about the inadequecy of researchers to view the bacteria which invloved all kinds of bacteria. If the author only talks about photosynthetic bacteria, how can this "illustrate" the inadequecy of researchers for every other bacteria that is not photosynthetic.

This is much an inference question

The researchers found that bacteria in someway are = to multicellular organism

However, among bacteria Anabaena is a particular case : it is able to perform two processes at the same time !!!

Therefore, the actual theories are not sufficient to explain why.

B is the answer