The plasma membrane is a thin, flexible and selectively permeable boundary that separates the cell from its external environment. It is crucial for the survival and functioning of the cell. The following are the structure and function of the plasma membrane:

Structure of Plasma Membrane
The plasma membrane is composed of various molecules such as lipids, proteins, and carbohydrates. The lipid bilayer is the major component of the plasma membrane, which consists of two layers of phospholipids. The phospholipids have a hydrophilic head and a hydrophobic tail, which makes the plasma membrane selectively permeable.

Function of Plasma Membrane
The plasma membrane performs various functions, some of which are as follows:

1. Selective Permeability: The plasma membrane allows the selective passage of molecules in and out of the cell. Small and non-polar molecules such as oxygen and carbon dioxide can pass through the membrane easily, while larger and polar molecules require the assistance of membrane proteins.

2. Cell Recognition: The plasma membrane has glycoproteins and glycolipids on its surface that help in cell recognition. These molecules act as markers that identify the cell's identity and prevent the immune system from attacking the body's own cells.

3. Cell Signaling: The plasma membrane has receptor proteins that bind to specific molecules such as hormones and neurotransmitters. This triggers a signal transduction pathway that leads to the activation of various cellular responses.

4. Structural Support: The plasma membrane gives structural support to the cell and helps maintain its shape.

Conclusion
The plasma membrane is a vital component of the cell that performs various functions such as selective permeability, cell recognition, cell signaling, and structural support. Its structure and composition make it an efficient barrier that separates the cell from the external environment.

The cell membrane or plasma membrane has fine pores through which substances may enter or leave the cell. It is composed of lipo-protein. The permeability of the plasma membrane is selective .i.e it allows only certain substances to pass through while it prevents others.