The bacterial structure that acts as a selective barrier allowing nutrients to enter the cell and wastes to leave the cell is the cell membrane, also known as the plasma membrane.

Here's why:

* Selective Permeability: The cell membrane is a phospholipid bilayer with embedded proteins. This structure allows it to control which substances can pass through it.

* Nutrient Entry: The cell membrane has mechanisms to transport essential nutrients like sugars, amino acids, and ions into the cell. This can happen through passive diffusion (movement down a concentration gradient) or active transport (requiring energy).

* Waste Removal: Similarly, the cell membrane facilitates the removal of waste products like carbon dioxide and other byproducts of metabolism from the cell.

The cell membrane is crucial for the survival of bacterial cells, ensuring the right balance of substances within the cell while maintaining its integrity.