Use the following information to answer the question.

Nitrogenase, the enzyme that catalyzes nitrogen fixation, is inhibited whenever free oxygen (O2) reaches a critical concentration. Consequently, nitrogen fixation cannot occur in cells wherein photosynthesis produces free O2. Consider the colonial aquatic cyanobacterium, Anabaena, whose heterocysts are described as having "...a thickened cell wall that restricts entry of O2 produced by neighboring cells. Intercellular connections allow heterocysts to transport fixed nitrogen to neighboring cells in exchange for carbohydrates."

Think about this description of the colonial aquatic cyanobacterium, Anabaena. Which of the following question below is important for understanding how nitrogen (N2) enters heterocysts, and how oxygen (O2) is kept out of heterocysts?

Use the following information to answer the question.

Nitrogenase, the enzyme that catalyzes nitrogen fixation, is inhibited whenever free oxygen (O2) reaches a critical concentration. Consequently, nitrogen fixation cannot occur in cells wherein photosynthesis produces free O2. Consider the colonial aquatic cyanobacterium, Anabaena, whose heterocysts are described as having "...a thickened cell wall that restricts entry of O2 produced by neighboring cells. Intercellular connections allow heterocysts to transport fixed nitrogen to neighboring cells in exchange for carbohydrates."

Think about this description of the colonial aquatic cyanobacterium, Anabaena. Which of the following question below is important for understanding how nitrogen (N2) enters heterocysts, and how oxygen (O2) is kept out of heterocysts?

a) If carbohydrates can enter the heterocysts from neighboring cells via the "intercellular connections," how is it that O2 doesn't also enter via this route?
b) If the cell walls of Anabaena photosynthetic cells are permeable to O2 and carbon dioxide (CO2), are they also permeable to N2?
c) If the nuclei of the heterocysts contain the genes that code for photosynthesis, how can these cells fail to perform photosynthesis?
d) If the nuclei of the photosynthetic cells contain the genes that code for nitrogen fixation, how can these cells fail to perform nitrogen fixation?