What structure forms a fold in the plasma membrane of bacterial cells?

The mesosome is a structure that forms a fold in the plasma membrane of bacterial cells. It is associated with the internal membranes that arise from invaginations of the plasma membrane. Mesosomes are thought to serve various functions, including involvement in cellular respiration and DNA replication, as they provide a larger surface area for these processes to occur.

In contrast, ribosomes are the cellular machinery responsible for protein synthesis and do not have a structural role in forming folds in the membrane. The capsule is a protective outer layer that surrounds some bacterial cells, offering protection and aiding in adherence but does not involve membrane folding. An endospore is a dormant, tough, and non-reproductive structure formed by some bacteria to ensure survival in harsh conditions, and it does not relate to the composition or structure of the plasma membrane itself. The mesosome's role is specific to the membrane's architecture, highlighting its importance in the overall biology of bacterial cells.