Why do red blood cells lack a nucleus?

Red blood cells lack a nucleus primarily to maximize space for hemoglobin. This adaptation allows them to accommodate a greater amount of hemoglobin, the protein responsible for oxygen transport in the blood. By not having a nucleus and other organelles, red blood cells can also adopt a biconcave shape, enhancing their surface area and flexibility, which improves their ability to traverse narrow capillaries and increases the efficiency of oxygen exchange.

In contrast, the other options are not suitable explanations. While oxygen storage is an important function, the presence of hemoglobin itself, rather than the lack of a nucleus, is what facilitates this storage. The absence of a nucleus does not relate to cell division in the context of red blood cells, as they do not undergo mitotic division. Additionally, nutrient absorption is not a function of red blood cells; rather, this role is more pertinent to cells in the digestive system or epithelial cells. Thus, maximizing space for hemoglobin is the key reason for the absence of a nucleus in red blood cells.