What structure acts as a barrier for the cell and has selective permeability?

The cell membrane is essential in maintaining the integrity of the cell and plays a vital role in its functionality. It acts as a selective barrier, controlling what enters and exits the cell. This selective permeability is crucial for the cell's homeostasis, allowing necessary nutrients to enter while keeping out harmful substances and helping to expel waste products. The structure of the cell membrane consists of a phospholipid bilayer with embedded proteins, which contribute to its ability to selectively allow materials to pass through based on size, charge, and solubility.

This function is fundamental for all living cells, enabling them to communicate with their environment and respond appropriately to changes. Other structures mentioned, while important in their own right, either serve different primary functions (such as the cytoplasm, which houses organelles), provide protection (like the cell wall in plant cells), or separate the nucleus from the cytoplasm (the nuclear envelope), but do not perform the selective barrier function in the way the cell membrane does.