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 (O2 ) is kept out of heterocysts?

Question

1. If carbohydrates can enter the heterocysts 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 photosynthetic cells are permeable to O2 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 oxygen 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
D) 4 and 6

Answer 1

B