Coloured portion of the nostoc cell is

The correct option is outer chromoplasm.

Let us break down the cellular structure of a Nostoc cell to understand why this is the correct answer:

1. What is Nostoc?
Nostoc is a genus of blue-green algae (cyanobacteria). As prokaryotes, their cells lack membrane-bound organelles (like chloroplasts or mitochondria) and a true membrane-bound nucleus.

2. Differentiation of the Protoplast:
Despite being prokaryotic, the protoplasm of a cyanobacterial cell like Nostoc is distinctly differentiated into two regions:
• Outer Chromoplasm (Peripheral Region): This is the outer, colored part of the cytoplasm. It contains flattened, membranous sacs called thylakoids. Photosynthetic pigments such as chlorophyll a (green), c-phycocyanin (blue), and c-phycoerythrin (red) are located on these thylakoids, giving this region its characteristic color.
• Inner Centroplasm or Central Body (Central Region): This is the inner, colorless part of the cell. It houses the genetic material (DNA or genophore) and is devoid of photosynthetic pigments and thylakoid membranes.

Thus, the colored portion of the Nostoc cell is the outer chromoplasm.