Which statement describes the structure of oxidized hemoglobin?

The structure of oxidized hemoglobin is characterized by the presence of iron in the ferric state, resulting in a form known as methemoglobin. In this state, the iron in heme is oxidized from the ferrous (Fe²⁺) state to the ferric (Fe³⁺) state. This alteration in the oxidation state prevents methemoglobin from binding to oxygen effectively, hence it cannot carry oxygen. This inability to transport oxygen is a defining feature of oxidized hemoglobin, distinguishing it from normal hemoglobin, which can carry oxygen due to iron being in the ferrous state.

The other statements do not accurately describe oxidized hemoglobin. For instance, while normal hemoglobin does carry oxygen, oxidized hemoglobin in the ferric state does not. Similarly, the notion of carbon dioxide replacing oxygen pertains to a different physiological process and does not imply oxidation of hemoglobin. The mention of carbon monoxide also misrepresents oxidized hemoglobin, as carbon monoxide binding does not equate to oxidation but rather represents competitive binding that affects oxygen transport. Thus, the correct statement aptly captures the essence of oxidized hemoglobin's structure and function.