Which of the following structure contains a polar nodule on both ends

The correct answer is heterocyst.

Let us understand the structure and function of a heterocyst step-by-step:

1. What is a Heterocyst?
A heterocyst is a specialized, nitrogen-fixing cell formed by certain filamentous cyanobacteria (blue-green algae), such as Anabaena and Nostoc, during times of nitrogen starvation. They provide an anaerobic environment necessary for the functioning of the oxygen-sensitive enzyme nitrogenase, which fixes atmospheric nitrogen.

2. Structural Features of a Heterocyst:
Heterocysts are larger, thicker-walled, and paler than vegetative cells. One of their key anatomical characteristics is the presence of polar nodules (also known as polar plugs).
- The polar nodules are formed from cyanophycin (a nitrogen-storing polypeptide) at the points of connection between the heterocyst and the adjacent vegetative cells.
- If a heterocyst is situated in the middle of a filament (intercalary heterocyst), it is connected to two vegetative cells, one on each side. Therefore, it contains a polar nodule on both ends.
- If a heterocyst is terminal (at the end of a filament), it has only one polar nodule connecting it to the single adjacent vegetative cell.

3. Analysis of Other Options:
- Hormogonia: These are multicellular fragments of filaments that break off from the main colony to help in asexual reproduction and motility. They do not possess polar nodules.
- Akinetes: These are thick-walled, dormant spore-like cells that help cyanobacteria survive unfavorable conditions. They do not have polar nodules.

Thus, the structure that contains a polar nodule on both ends (specifically when intercalary) is the heterocyst.