Which proteins use iron in their structure to transport oxygen?

Hemoglobin and myoglobin are the two proteins that specifically use iron as an integral part of their structure to transport oxygen. Hemoglobin, found in red blood cells, binds oxygen in the lungs and carries it to tissues throughout the body for cellular respiration. The presence of iron in its heme group is crucial for this function, as it allows hemoglobin to reversibly bind to oxygen molecules.

Myoglobin, which is found in muscle cells, also contains a heme group with iron and serves a similar purpose on a smaller scale, storing and releasing oxygen as needed during muscle contraction. This specialized ability of both hemoglobin and myoglobin to use iron highlights their essential roles in oxygen transport and storage, making option B the correct choice.

In contrast, the other options consist of proteins that either do not play a direct role in oxygen transport or do not utilize iron in the same functional context. For example, fibrinogen and collagen are primarily involved in blood clotting and providing structural support in tissues, respectively. Albumin is a transport protein in the blood that carries various substances but does not transport oxygen directly, while transferrin is involved in iron transport but does not play a role in oxygen transport itself. Keratin and casein are structural proteins