Molecules pass in and out of cells primarily through the cell membrane, which is a selectively permeable phospholipid bilayer that controls the movement of substances to maintain cellular homeostasis

. There are two main types of transport mechanisms:

• Passive Transport: This process does not require cellular energy (ATP). Molecules move down their concentration gradient (from high to low concentration). It includes:
  • Simple diffusion: Small, nonpolar, or lipid-soluble molecules like oxygen, carbon dioxide, and some small polar molecules (e.g., water) dissolve in and pass directly through the lipid bilayer

* **Facilitated diffusion:** Larger or charged molecules (e.g., glucose, ions) cannot pass directly through the membrane and require specific transport proteins such as carrier proteins or channel proteins to cross without energy expenditure

• Active Transport: This process requires energy (usually from ATP) to move molecules against their concentration gradient (from low to high concentration). Specialized transport proteins, such as ion pumps (e.g., the Na⁺-K⁺ pump), actively transport ions and molecules into or out of the cell, maintaining essential concentration differences

In summary, molecules pass in and out of cells due to the selective permeability of the cell membrane, using passive mechanisms like diffusion and facilitated diffusion when moving down concentration gradients, and active transport mechanisms when moving against concentration gradients, often involving specific transport proteins and energy expenditure