Final answer:
When hemoglobin is in the R state, it has a higher affinity for oxygen and is most likely bound to oxygen (O2). Each hemoglobin molecule can bind four oxygen molecules, facilitated by the iron (Fe2+) in its heme groups.
Step-by-step explanation:
The question asks which molecules are bound to hemoglobin when it is in the R state. The R state, also known as the relaxed state, is the conformation that hemoglobin adopts when it has a higher affinity for oxygen (O2). Thus, when hemoglobin is in the R state, it is most likely to be bound to oxygen.
In detail, oxygen transport in the blood is primarily achieved through binding to hemoglobin within red blood cells. Each molecule of hemoglobin can bind up to four oxygen molecules, one per each of its four heme groups. The heme group contains an iron (Fe2+) ion that is critical for the binding with O2. The binding of oxygen results in a conformational change to the R state, increasing hemoglobin's oxygen affinity and making it easier for additional oxygen molecules to bind. Upon reaching tissues with lower oxygen concentration, hemoglobin releases the bound oxygen, providing essential O2 for cellular respiration.