Final answer:
An oxygen molecule crosses the cell surface membrane only once to enter a red blood cell, where it binds to hemoglobin for transport.
Step-by-step explanation:
An oxygen molecule crosses the cell surface membrane only once as it moves from the air into a red blood cell (erythrocyte). This process is facilitated by the protein hemoglobin, which is packed within erythrocytes. Hemoglobin contains four iron-containing heme molecules that each bind one oxygen molecule, enabling each hemoglobin molecule to transport up to four oxygen molecules. The erythrocytes, through passive simple diffusion, pick up oxygen from the alveoli where its concentration is higher and deliver it to body tissues where it is lower. Thus, the total amount of oxygen that a single red blood cell can carry is immense, thanks to the high concentration of hemoglobin within each cell.