Mesosomes are areas in the cell membrane of prokaryotic (bacterial) cells that fold inward. They play a role in cellular respiration, the process that breaks down food to release energy.
In Eukaryotes, the majority of this process occurs in mitochondria. The third, and final, step of cellular respiration (electron transport chain) occurs in the space between the two membranes of the mitochondria. This step is critical to the cell as most of the energy from food is released during this stage. Since Prokaryotes do not contain membrane bound organelles, they need a different approach. Instead, they use the mesosomes as a site for the electron transport chain.
Mesosomes are part of the structure of the plasma membrane. You'll find them lining the cell wall. They are clumped and folded together, to maximize their surface area. This is important because it is needed for cell respiration, which is a function of the mesosomes. These folds can become distinct vesicles, which can then be used to contain material, keeping it distinct from the remainder of the cell.
Mesomes are the principal sites of respiratory enzymes, can easily be demonstrated in gram positive bacteria, are essential in bringing about cell division, are convoluted invaginations of the plasma membrane and are also called as chondroids.