What structure controls the passage of substances into and out of a cell?

The structure that controls the passage of substances into and out of a cell is called the cell membrane. Here are the steps explaining how it works:

1. The cell membrane, also known as the plasma membrane, is a thin layer that surrounds the cell. It separates the cell's internal environment from its external environment.

2. The cell membrane is selectively permeable, which means it allows certain substances to pass through while blocking others.

3. Small, non-polar molecules, such as oxygen and carbon dioxide, can pass directly through the cell membrane. This is because they are able to dissolve in the hydrophobic, non-polar areas of the membrane.

4. Larger, polar molecules, such as glucose, cannot pass directly through the membrane. Instead, they require assistance from transport proteins. These proteins span the membrane and provide channels or pumps for certain molecules to pass through.

5. In some cases, the cell membrane can engulf substances, bringing them into the cell through a process called endocytosis. Conversely, it can also expel substances from the cell through a process called exocytosis.

6. In this way, the cell membrane controls the passage of substances into and out of the cell, maintaining the balance of substances necessary for the cell's functions.