Smallest Living Cell

Viruses are not considered living cells because they lack the ability to carry out metabolic processes on their own. Among the options provided, mycoplasma is the smallest living cell.

Explanation

Virus:
- Viruses are not considered living cells because they cannot reproduce or carry out metabolic processes on their own. They require a host cell to replicate.

Bacteria:
- Bacteria are single-celled organisms that can carry out metabolic processes independently. They are larger than viruses but still relatively small compared to other organisms.

PPLO (PleuroPneumonia-Like Organisms):
- PPLO are a group of bacteria that are smaller than typical bacteria. They have a simple cell structure and lack a cell wall.

Mycoplasma:
- Mycoplasma are the smallest living cells. They are a type of bacteria that lack a cell wall, which contributes to their small size. They are capable of independent growth and reproduction.

In conclusion, mycoplasma is the smallest living cell among the options provided.