Final answer:
Oxygen and carbon dioxide primarily cross the plasma membrane through diffusion and facilitated diffusion. Diffusion does not require energy and occurs along the concentration gradient, while facilitated diffusion involves the aid of transport proteins.
Step-by-step explanation:
Oxygen, carbon dioxide, as well as other small molecules, cross the plasma membrane primarily through the process of diffusion and facilitated diffusion. Simple diffusion allows these molecules to move along the concentration gradient, which means they move from an area where they are more concentrated to an area where they are less concentrated, without the need for energy. This is how oxygen and carbon dioxide can freely enter and exit cells.
For molecules that cannot directly pass through the lipid bilayer due to their size or polarity, facilitated diffusion is utilized. In facilitated diffusion, transport proteins in the cell membrane, such as channel proteins or carrier proteins, provide assistance for these molecules to cross the membrane with the concentration gradient.
Mechanisms like osmosis are specifically for the diffusion of water across the cell membrane, while active transport requires energy to move substances against their concentration gradient, and endocytosis and exocytosis are involved in the trafficking of large particles into and out of the cell, respectively.