Choose the statement that is true concerning hemoglobin.
A. When hemoglobin is not bound to oxygen, it appears blue. B. It can bind a maximum of 3 oxygen molecules. C. It is found in the plasma portion of blood. D. It is composed of 4 protein chains and 4 heme groups.
The Correct Answer and Explanation is :
The correct answer is D. It is composed of 4 protein chains and 4 heme groups.
Explanation
Hemoglobin is a crucial protein found in red blood cells (RBCs) that plays a vital role in transporting oxygen (O₂) from the lungs to the tissues and facilitating the return transport of carbon dioxide (CO₂) from the tissues back to the lungs. Structurally, hemoglobin is composed of four protein chains, typically two alpha (α) and two beta (β) chains in adult humans, and each of these chains is associated with a heme group.
Each heme group contains an iron (Fe) atom at its center, which is responsible for binding oxygen. Therefore, a single hemoglobin molecule can bind up to four oxygen molecules—one to each heme group—enabling efficient oxygen transport throughout the body. This binding is not only crucial for oxygen delivery but also facilitates the release of CO₂, as the binding affinity of hemoglobin for oxygen changes depending on the surrounding environment, including pH and carbon dioxide levels (a phenomenon known as the Bohr effect).
To clarify the other options:
- A. When hemoglobin is not bound to oxygen, it appears blue. This statement is incorrect; deoxygenated hemoglobin appears dark red, not blue. The bluish appearance of blood seen in some veins is due to the way light penetrates the skin and reflects back.
- B. It can bind a maximum of 3 oxygen molecules. This statement is false because hemoglobin can bind up to four oxygen molecules.
- C. It is found in the plasma portion of blood. This is inaccurate; hemoglobin is located within red blood cells, not in the plasma.
In summary, option D is the only accurate statement regarding the composition of hemoglobin, highlighting its essential structure for its function in gas transport.