The process by which oxygen moves into the cell across the membrane when the outside has a higher concentration is called

• A. Osmosis
• B. Active transport
• C. Diffusion
• D. Cellular respiration

Answer: (C)

Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration due to their random motion. When oxygen is more concentrated outside the cell, it naturally diffuses across the membrane into the cell until the concentrations balance. This is a passive process and requires no energy from the cell. Oxygen’s small size and nonpolar nature allow it to diffuse directly through the lipid bilayer (though some membranes may use channels as well). Osmosis specifically refers to water movement, not oxygen. Active transport would move substances against their gradient and needs energy. Cellular respiration is a metabolic process, not a transport mechanism.