What are mesosomes?

Explanation

### Step-by-Step Solution:

1. **Definition of Mesosomes**: Mesosomes are specialized membranous structures found in certain bacteria, particularly gram-positive bacteria. They are formed by the invagination (folding inward) of the plasma membrane.

2. **Structure Formation**: The process of mesosome formation involves the plasma membrane of the bacterial cell folding inwards. This creates a unique structure that extends into the cytoplasm of the cell.

3. **Location**: Mesosomes are specifically found in gram-positive bacteria, which are characterized by a thick peptidoglycan layer in their cell wall.

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