Structure and Function of the Plasma Membrane:
The plasma membrane, also known as the cell membrane, is a thin semi-permeable membrane that surrounds the cytoplasm of a cell. It is composed of a phospholipid bilayer with embedded proteins and cholesterol molecules.

Function of the Plasma Membrane:
1. Regulation of substances: The plasma membrane controls the entry and exit of substances into and out of the cell. It helps in maintaining the internal environment of the cell by allowing only specific substances to pass through.
2. Cell signaling: The membrane contains receptor proteins that allow the cell to communicate with its environment. These proteins receive signals from other cells or the environment and initiate cellular responses.
3. Cell adhesion: The plasma membrane plays a crucial role in cell adhesion by forming connections with adjacent cells. This is important for the structural integrity of tissues and organs.

Selective Permeability and Cell Survival:
The selective permeability of the plasma membrane is essential for the survival and function of the cell.
1. Prevention of leakage: The membrane selectively allows only certain molecules to pass through, preventing the leakage of important molecules out of the cell.
2. Regulation of homeostasis: By controlling the entry and exit of substances, the plasma membrane helps maintain the balance of ions and nutrients inside the cell, which is crucial for cell survival.
3. Protection from harmful substances: The selective permeability of the membrane prevents the entry of harmful substances into the cell, thus protecting it from damage.
In conclusion, the structure and function of the plasma membrane, along with its selective permeability, are crucial for the survival and proper functioning of the cell. It helps maintain internal balance, allows for communication with the environment, and protects the cell from external threats.