In which one of the following organisms is the nuclear membrane absent because the cells are prokaryotic in nature?

Introduction / Context:
One of the fundamental distinctions in cell biology is between prokaryotic and eukaryotic cells. Prokaryotic cells lack a true nucleus and nuclear membrane, while eukaryotic cells have a well defined nucleus surrounded by a nuclear envelope. This question asks you to identify which organism among the options lacks a nuclear membrane, indicating a prokaryotic cell organisation.

Given Data / Assumptions:

• Nuclear membrane is present only in eukaryotic cells.
• Prokaryotes, such as bacteria and cyanobacteria, have naked DNA without a nuclear membrane.
• The options include Volvox, Penicillium, Agaricus, Nostoc, and Chlamydomonas.
• Only one of these is a prokaryote; the rest are eukaryotic organisms.

Concept / Approach:
Nostoc is a genus of cyanobacteria (also called blue green algae), which are prokaryotic microorganisms. They possess a single circular chromosome not enclosed by a nuclear envelope. Volvox and Chlamydomonas are green algae with eukaryotic cells that contain true nuclei. Penicillium is a fungus and Agaricus is a mushroom, both clearly eukaryotic. By recalling which group Nostoc belongs to, we can correctly identify it as lacking a nuclear membrane.

Step-by-Step Solution:
Step 1: Classify each organism: Volvox and Chlamydomonas are green algae, Penicillium is a mould, Agaricus is a mushroom, and Nostoc is a cyanobacterium.Step 2: Remember that all algae (except cyanobacteria), fungi, and mushrooms are eukaryotic with a true nucleus and nuclear membrane.Step 3: Recognise that Nostoc is not a eukaryotic alga but a cyanobacterium (blue green alga), which is prokaryotic.Step 4: Since prokaryotes lack a nuclear membrane, Nostoc must be the correct answer.Step 5: Confirm that none of the other listed organisms is prokaryotic.

Verification / Alternative check:
Textbooks on microbiology and plant diversity describe Nostoc as a filamentous cyanobacterium. Cyanobacteria are always classified as prokaryotes, with genetic material found in a nucleoid region rather than inside a membrane bound nucleus. In contrast, green algae, fungi, and higher plants are all eukaryotic. This classification confirms that Nostoc alone fits the description of lacking a nuclear membrane.

Why Other Options Are Wrong:
Volvox is a colonial green alga made of many eukaryotic cells with nuclei surrounded by nuclear membranes.Penicillium is a fungus, and all fungi have eukaryotic cells with true nuclei.Agaricus, the common mushroom, is also a fungus with eukaryotic cellular organisation.Chlamydomonas is a unicellular green alga with a distinct nucleus and nuclear envelope.

Common Pitfalls:
Students sometimes get confused because Nostoc and other cyanobacteria are loosely called blue green algae, leading them to treat them like true algae (which are eukaryotic). Another common mistake is thinking that all microscopic or simple organisms must be prokaryotic, which is not true. To avoid such confusion, remember that cyanobacteria (like Nostoc) are the only algal group that is prokaryotic and therefore lack a nuclear membrane.

Final Answer:
The correct answer is Nostoc.