Describe transport methods that enforce selectivity?

Cells use several transport methods to maintain selectivity—mainly passive transport, active transport, and vesicular transport. 

• Passive transport includes diffusion and facilitated diffusion. These rely on concentration gradients and specialized protein channels that only allow certain molecules, such as glucose or ions, to pass.
• Active transport requires energy to move molecules against their gradient. Protein pumps like the sodium-potassium pump enforce strict control over which ions cross the membrane, ensuring proper nerve and muscle function.
• Vesicular transport—including endocytosis and exocytosis handles bulk movement. The membrane forms vesicles to engulf substances or release them, ensuring only approved materials enter or leave the cell. Each method contributes to maintaining proper ion balance, nutrient uptake, and waste removal. 

These transport systems act like security checks at a border every molecule is screened and handled by specific carriers, preventing harmful materials from disrupting the cell’s internal environment. Together, they ensure cellular efficiency, communication, and survival.