Final answer:
3) Methemoglobin is the form of hemoglobin that naturally occurs in red blood cells and must be converted for oxygen delivery since it cannot effectively bind oxygen due to its ferric iron state.
Step-by-step explanation:
The form of hemoglobin that occurs naturally in plasma and red blood cells, but must be converted back to hemoglobin for oxygen delivery, is Methemoglobin. Methemoglobin has iron in the ferric (Fe3+) state rather than the ferrous (Fe2+) state, which is found in normal hemoglobin capable of binding oxygen. Methemoglobin cannot bind oxygen effectively, which leads to a disturbance in the oxygen delivery function of hemoglobin. A naturally occurring peptide in red blood cells called glutathione helps to prevent a significant increase in methemoglobin levels. Higher levels of methemoglobin can cause symptoms such as cyanosis (blue skin) and dyspnoea (labored breathing), typically treated by reducing oxidation of hemoglobin or by using methylene blue.