What does hemoglobin molecule bind too?

Hemoglobin molecule binds to oxygen molecules at its heme groups, specifically to the iron ions within the heme. Each hemoglobin molecule has four heme groups, and each heme group can bind one oxygen molecule, meaning that one hemoglobin molecule can carry up to four molecules of oxygen. This enables transport of oxygen from the lungs, where it is loaded onto hemoglobin in the blood to body tissues, where it is unloaded via diffusion of oxygen from the capillaries into the interstitial fluid and then cells.