The answer is B 1 and 6, but I don\'t know why. Can somebody please axplain why

Question

The answer is B 1 and 6, but I don't know why. Can somebody please axplain why is it 1 nd 6?

Think about this description of the colonial aquatic cyanobacterium, Anabaena. What two questions below are important for understanding how nitrogen (N2) enters heterocysts, and how oxygen (02) is kept out of heterocysts? 1. If carbohydrates can enter the heterocysts from neighboring cells via the "intracellular connections," how is it that 02 doesn't also enter via this route? 2. If the cell walls of Anabaena photosynthetic cells are permeable to 02 and carbon dioxide (CO2), are they also permeable to N2? 3. If the nuclei of the photosynthetic cells contain the genes that code for nitrogen fixation, how can these cells fail to perform nitrogen fixation? 4. If the nuclei of the heterocysts contain the genes that code for photosynthesis, how can these cells fail to perform photosynthesis? 5. If the cell walls of Anabaena heterocysts are permeable to N2, how is it that N2 doesn't diffuse out of the heterocysts before it can be fixed? 6. If the thick cell walls of Anabaena heterocysts exclude entry of oxygein gas, how is it that they don't also exclude the entry of nitrogen gas? A) 1 and 3 B) 1 and (6 C) 2 and 5

Explanation / Answer

1. The Heterocyst is made up of different layers,that permit carbohydrates,whereas on the other hand they stop the atmospheric oxygen to come inside. This is a damage-control mechanism for the enzyme nitrogenase, as the nitrogenase is sensitive to O2 and cold, and cannot function in the presence of O2.

Moreover the Photosystem II (PS II), is also absent from the Heterocyst, because PS (II) does the Photolysis of water and generates free O2 gas.

That is why only PS (I) is present here, which generates assimilatory powers ATP, which helps in nitrogen fixation. Had there been PS (II) also in the Heterocyst, then it would have done the Photolysis of water in the Heterocyst and free oxygen gas (O2) would have stopped the functioning of nitrogenase enzyme, thus ultimately checking the whole cyanobacterial nitrogen fixation. Absence of PS (II) helps in maintaining the O2 (-), or H2 (+) internal environment of Heterocyst.

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