Nitrogenase, the Enzyme That Catalyzes Nitrogen Fixation, Is Inhibited Whenever

Multiple Choice

Nitrogenase, the enzyme that catalyzes nitrogen fixation, is inhibited whenever free O₂ reaches a critical concentration. Consequently, nitrogen fixation cannot occur in cells wherein photosynthesis produces free O₂. Consider the colonial aquatic cyanobacterium, Anabaena, whose heterocytes are described as having "…a thickened cell wall that restricts entry of O₂ produced by neighboring cells. Intracellular connections allow heterocytes to transport fixed nitrogen to neighboring cells in exchange for carbohydrates." Which two questions below arise from a careful reading of this quotation, and are most important for understanding how N₂ enters heterocytes, and how O₂ is kept out of heterocytes?
1) If carbohydrates can enter the heterocytes from neighboring cells via the "intracellular connections," how is it that O₂ doesn't also enter via this route?
2) If the cell walls of Anabaena's photosynthetic cells are permeable to O₂ and 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 heterocytes contain the genes that code for photosynthesis, how can these cells fail to perform photosynthesis?
5) If the cell walls of Anabaena's heterocytes are permeable to N₂, how is it that N₂ doesn't diffuse out of the heterocytes before it can be fixed?
6) If the thick cell walls of the heterocytes exclude entry of oxygen gas, how is it that they don't also exclude the entry of nitrogen gas?

A) 3 and 4
B) 2 and 5
C) 1 and 3
D) 4 and 6
E) 1 and 6