Here's why:

* Selectively Permeable: The cell membrane is selectively permeable, meaning it allows some substances to pass through while blocking others. This controlled movement is crucial for maintaining the cell's internal environment.

* Phospholipid Bilayer: The cell membrane is composed of a phospholipid bilayer, which acts as a barrier. The hydrophilic heads of the phospholipids face outward, interacting with the watery environment, while the hydrophobic tails face inward, forming a barrier against water-soluble molecules.

* Proteins: Embedded within the membrane are various proteins that play important roles in transport:

* Channel Proteins: Act like tunnels, allowing specific molecules to pass through.

* Carrier Proteins: Bind to specific molecules and transport them across the membrane.

* Receptor Proteins: Bind to signaling molecules, triggering specific responses within the cell.

In addition to the cell membrane, plant cells have a cell wall, which provides structural support and protection. However, the cell wall doesn't actively control what goes in and out of the cell. Instead, it acts as a filter, preventing the passage of large molecules.