What is the primary process involved in oxygen diffusion?

The primary process involved in oxygen diffusion is indeed diffusion through cellular membranes, which allows oxygen to move from areas of higher concentration to areas of lower concentration, a fundamental principle of passive transport. This process relies on the concentration gradient, where molecules naturally flow towards areas where they are less concentrated.

Oxygen enters cells, for example, because it is more concentrated in the blood compared to the lower concentration within the cells, enabling it to diffuse easily. This phenomenon is essential for various biological functions, such as respiration in all aerobic organisms. The cellular membrane's structure, consisting of a lipid bilayer, facilitates this diffusion, allowing small and nonpolar molecules like oxygen to passively move across without the need for energy, thus highlighting the role of simple diffusion in biological systems.

The other choices do not correctly describe the process of oxygen diffusion in the context of biological systems. Movement from high pressure to low pressure may relate to specific scenarios in gas exchange but is not the main mechanism of oxygen diffusion. Active transport implies an energy-requiring process, which is not the case for the passive diffusion of oxygen.