What must materials pass through to enter or leave a cell?

 Materials must pass through the cell membrane (also known as the plasma membrane) to enter or leave a cell. The cell membrane is a semi-permeable barrier that controls the movement of substances into and out of the cell. It is made up of a lipid bilayer with embedded proteins that facilitate the transport of materials. There are several mechanisms by which substances can cross the cell membrane:

1. Simple diffusion: Small nonpolar molecules, such as oxygen and carbon dioxide, can pass directly through the lipid bilayer without the need for transport proteins, moving from areas of higher concentration to areas of lower concentration.

2. Facilitated diffusion: Larger or polar molecules cannot pass through the lipid bilayer on their own and require specific transport proteins to help them move across the membrane. This process also follows the concentration gradient, but does not require energy.

3. Active transport: When substances need to be moved against their concentration gradient (from a lower to a higher concentration), the cell uses energy in the form of ATP to actively transport molecules. This is often carried out by specialized proteins known as pumps.

4. Endocytosis: This is a process where the cell membrane engulfs a substance to bring it into the cell, forming a vesicle. There are different forms of endocytosis, including phagocytosis (for solids) and pinocytosis (for liquids).

5. Exocytosis: Conversely, cells can expel materials by enclosing them in vesicles that fuse with the cell membrane, releasing the contents outside the cell.