Final answer:
Oxygen enters the cells through simple diffusion, a type of passive transport, as it can readily diffuse across the cell membrane from an area of higher concentration to an area of lower concentration. The correct option is A.
Step-by-step explanation:
The question How does oxygen cross into the cell? can be answered by understanding the process of passive diffusion. Oxygen molecules enter the cells through a process known scientifically as simple diffusion. This mechanism allows oxygen to move across the cell membrane from an area of higher concentration to an area of lower concentration without the aid of transport proteins or energy consumption. In contrast to other substances, oxygen can readily diffuse through the hydrophobic phospholipid bilayer of the cell membrane.
Ion pumps and phagocytosis are both examples of active transport mechanisms that require energy input. These processes differ from passive diffusion, as active transport involves the movement of substances against their concentration gradient, which passive diffusion does not.
Facilitated diffusion is similar to simple diffusion in the sense that it occurs along the concentration gradient; however, it does require a transport protein for the process to occur. Large or hydrophilic molecules and charged ions often use facilitated diffusion to cross the cell membrane. Examples of facilitated diffusion include the movement of glucose or various ions through specific transmembrane proteins.