Cell Envolop.

In biology, the term "cell envelope" typically refers to the outermost layer or structure that surrounds a cell, particularly in prokaryotic cells like bacteria and archaea. The cell envelope consists of several components, including:

1. **Cell Membrane (Plasma Membrane)**: This is the innermost layer of the cell envelope and is composed of a lipid bilayer. It controls the passage of molecules in and out of the cell and plays a crucial role in maintaining cell integrity.

2. **Cell Wall**: Many prokaryotic cells have a cell wall located just outside the cell membrane. The composition of the cell wall varies among different species. In bacteria, for example, some have peptidoglycan cell walls, while others have different types of cell wall structures.

3. **Capsule or Glycocalyx**: Some bacteria have an additional outer layer called a capsule or glycocalyx. This layer is made up of polysaccharides and helps protect the cell and adhere to surfaces.

4. **S-Layer**: In some archaea and certain bacteria, there is an S-layer composed of proteins or glycoproteins that can surround the cell.

The composition and structure of the cell envelope can vary widely among different types of cells and play essential roles in protecting the cell, maintaining its shape, and interacting with the environment. It is an important concept in microbiology and the study of cell biology.