Final answer:
Cells' small size allows efficient exchange of materials across the plasma membrane due to a large surface-to-volume ratio. The plasma membrane, composed of a phospholipid bilayer with embedded proteins, regulates the movement of substances in and out of the cell.
Step-by-step explanation:
The small size of cells relates to the need to exchange materials across the plasma membrane.
The microscopic size of most cells provides a large surface-to-volume ratio, which allows for efficient exchange of nutrients and waste products.
The plasma membrane, which is composed of a phospholipid bilayer with embedded proteins, controls the movement of substances in and out of the cell.