Hemoglobin is:

Hemoglobin is a crucial protein found within red blood cells, and its primary function is to transport oxygen from the lungs to the tissues throughout the body. This protein binds to oxygen molecules, allowing red blood cells to effectively carry and deliver the necessary oxygen for cellular respiration and energy production. The structure of hemoglobin, which consists of four subunits, enables it to pick up oxygen in areas of high concentration (like the lungs) and release it in areas where it is needed (like the tissues).

The other options describe functions or substances that do not relate to hemoglobin. For instance, enzymes involved in digestion have different roles in breaking down food and are not connected to oxygen transport. White blood cells play a vital role in the immune system and are distinct from hemoglobin's function in red blood cells. Lastly, while the liver is involved in cholesterol metabolism, hemoglobin is synthesized in the bone marrow and is not produced by the liver. Understanding these distinctions highlights the significance of hemoglobin in respiratory physiology.