These are osmotically inactive

The correct answer is mycoplasma.

To understand why mycoplasmas are osmotically inactive (or insensitive to osmotic changes), let us break down the structure and characteristics of these organisms:

1. Role of the Cell Wall in Osmoregulation:
In typical bacteria (such as Nostoc, which is a cyanobacterium, and other common bacteria), a rigid cell wall composed of peptidoglycan surrounds the cell membrane. This cell wall provides structural support and protects the cell from bursting (lysis) when placed in a hypotonic environment, where water rushes into the cell due to osmotic pressure.

2. Unique Characteristics of Mycoplasmas:
Mycoplasmas are a unique group of bacteria that completely lack a cell wall. Instead, their cell boundary is limited only by a flexible, three-layered cell membrane.

3. Absence of Osmotic Lysis (Osmotic Inactivity):
Because mycoplasmas lack a rigid cell wall, they do not build up turgor pressure in the same way typical bacteria do. Furthermore, their cell membranes contain sterols (which they acquire from their growth medium), providing them with stability and flexibility to withstand osmotic variations without lysing. Consequently, they are described as osmotically inactive or highly resistant to osmotic shock compared to typical walled bacteria.

Therefore, while Nostoc and typical bacteria have rigid cell walls that govern their osmotic responses, mycoplasma is the correct choice due to its lack of a cell wall and unique membrane properties.