Test: Prokaryotic Cells

• Prokaryotes and eukaryotes are the two different types of cells. 
• Eukaryotic cells contain membrane-bound organelles, such as the nucleus, endoplasmic reticulum, mitochondria while prokaryotic cells do not but the ribosome is the only organelle that can be seen in both prokaryotic and eukaryotic cells. 
• Prokaryotic and eukaryotic ribosomes perform the same functions that is protein synthesis, however, eukaryotic ribosomes are much larger than prokaryotic ones.

Prokaryotic cells are cells that do not have a true nucleus or most other cell organelles. Organisms that have prokaryotic cells are unicellular and called prokaryotes. Bacteria and archaea are prokaryotes.

Fig: Prokaryotic cell

Nostoc is a prokaryote. Prokaryotic cells lack membrane bound organelles and well organized nucleus, that is nuclear envelope is absent in these cells.

Most prokaryotes carry a small amount of genetic material in the form of a single molecule, or chromosome, of circular DNA. The DNA in prokaryotes is contained in a central area of the cell called the nucleoid, which is not surrounded by a nuclear membrane.

The distinction is that eukaryotic cells have a "true" nucleus containing their DNA, whereas prokaryotic cells do not have a nucleus.

Similar to the cytoplasm of a cell (protoplasm), the nucleus contains 'nucleoplasm' (nucleus sap) or karyoplasm. The nucleoplasm is one of the types of protoplasm, and it is enveloped by the nuclear membrane or nuclear envelope. The nucleoplasm includes the chromosomes and nucleoli.

Nucleoplasm:

• As in prokaryotes, centrioles are absent.
• Centrioles play an important part in mitosis during spindle fibre formation. so mitosis is not seen in them, thus they use binary fission.

50S, roughly equivalent to the 60S ribosomal subunit in eukaryotic cells, is the larger subunit of the 70S ribosome of prokaryotes. The 50S subunit is primarily composed of proteins but also contains single-stranded RNA known as ribosomal RNA (rRNA).

The plasma membrane of eubacteria resembles that of the eukaryotic cell. It is made of phospholipid, protein and some amount of polysaccharides. However, it lacks sterol, the characteristic of the eukaryotic cell membrane. Instead, there is sterol like hopanoid.

Fig: Eubateria

• Glycocalyx or pericellular matrix is a carbohydrate-enriched coating of glycoprotein and a glycolipid that covers the cell membranes of a few bacteria, epithelial cells and other cells. 
• It is mainly used in the recognition of the cell and also regulates the endothelial vascular tissues.
• The prokaryotic cell envelope consists of a three-layered structure – the outermost glycocalyx, followed by a cell wall and then the plasma membrane.

Fig: Bacterial cell

So, the correct answer is 'Plasma membrane'.