The reason oxygen (O2) and carbon dioxide (CO2) can pass through the plasma membrane is largely due to their small size and nonpolar nature. The plasma membrane, primarily composed of a phospholipid bilayer, has hydrophobic (water-repelling) interior regions that act as a barrier to most substances.
Oxygen is essential for cellular respiration, while carbon dioxide is a byproduct of this process. Both gases are small enough to diffuse across the membrane without the need for specific transport proteins. This process is known as simple diffusion.
In simple diffusion, molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached. Since O2 is often more concentrated outside the cell (where it is abundant in the atmosphere) and CO2 is more concentrated inside the cell (due to metabolic processes), both gases naturally move in the direction that balances their concentrations.
Additionally, because O2 and CO2 are nonpolar, they are not repelled by the hydrophobic region of the plasma membrane, which allows them to pass through more easily compared to polar or charged molecules.