In carrying out normal activities, cells use oxygen and produce carbon dioxide. the concentration of oxygen is higher in the blood than inside the cell, so oxygen moves into the cell. similarly, carbon dioxide moves out of the cell into the blood because the concentration of carbon dioxide inside the cell is greater than the concentration outside the cell. how are the small molecules of oxygen and carbon dioxide moving through the cell membrane? a. passive transport by diffusion b. active transport by endocytosis c. passive transport by osmosis d. active transport by carrier proteins