Final answer:
The plasma membrane's structure, with a phospholipid bilayer, provides selective permeability. This allows certain substances to freely pass through while preventing others from doing so.
Step-by-step explanation:
The plasma membrane, also known as the cell membrane, is a selectively permeable barrier between the cytoplasm and the external environment. It is primarily composed of phospholipids arranged in a bilayer, with the hydrophobic tails on the interior and the hydrophilic heads pointing outwards. This structure provides the first level of control over selective permeability.
Substances that can pass through the plasma membrane unaided are those that meet certain criteria. For example, relatively small and nonpolar molecules, such as oxygen and carbon dioxide, can easily diffuse through the lipid bilayer. However, larger and charged molecules require specialized transport proteins to facilitate their passage.
Overall, the plasma membrane's selective permeability allows the cell to control the movement of substances in and out of the cell, ensuring that essential materials are obtained and harmful substances are kept out.