Here's why:
* Hemoglobin is a protein found in red blood cells. It contains iron, which binds to oxygen molecules in the lungs. This oxygen-rich hemoglobin is then transported throughout the body, where it releases the oxygen to cells that need it for respiration.
* Myoglobin is another oxygen-binding protein, but it's found in muscle cells. Myoglobin acts as an oxygen reservoir, providing oxygen to muscle cells during periods of high activity.
* Hemocyanin is an oxygen-carrying protein found in some invertebrates, like crustaceans and mollusks. It contains copper, which binds to oxygen.
Therefore, while other molecules play roles in oxygen transport, hemoglobin is the primary and most important one in most living organisms.
