IB Biology Exam Prep

Category - Biology

Which of the following best describes the mechanism by which hemoglobin loads and unloads oxygen?
  1. Production of CO2 during cellular respiration lowers blood pH, which decreases the affinity of hemoglobin for oxygen and facilitates the unloading of oxygen to cells.
  2. Oxygen molecules binding to one subunit of a hemoglobin molecule change its shape, and the other three subunits also change shape and accept oxygen more readily.
  3. Blood arriving at the lungs has a lower PO2 pressure than the air inside the lungs, which favors the diffusion of oxygen into the blood from the lungs.
  4. Carbon dioxide in the blood as a result of increased cellular respiration causes the heart's pacemaker to speed up and increase the transport of oxygen to cells.
Explanation
Correct Response: A. Loading and unloading of oxygen from hemoglobin is sensitive to the pH of the blood. As blood becomes more acidic due to the release of carbon dioxide as a byproduct of cellular respiration, the affinity of hemoglobin for oxygen decreases, and oxygen is released into the surrounding tissues. The affinity of hemoglobin for oxygen does increase after the initial oxygen molecule binds (B), but this mechanism does not control the loading or unloading of the initial oxygen molecule onto hemoglobin. Blood arriving at the lungs does have a lower PO2 than the air inside the lungs (C), causing oxygen to diffuse into the blood from the lungs; however, this does not explain the mechanism by which hemoglobin loads and unloads oxygen. Although the heart rate does increase in response to increased levels of carbon dioxide in the blood (D), this does not describe the mechanism by which hemoglobin loads and unloads oxygen.
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