SAT Subject Biology Practice Question 284: Answer and Explanation

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Question: 284

14. Hemoglobin is a protein in red blood cells that binds and carries oxygen and some carbon dioxide. Its affinity for oxygen changes as blood travels from the lungs to the body tissues and back to the lungs again. One could expect hemoglobin to have

A. a high carbon dioxide affinity in the lungs and a low oxygen affinity in the tissues
B. a low carbon dioxide affinity in the lungs and a high oxygen affinity in the tissues
C. a high oxygen affinity in the lungs and a low oxygen affinity in the tissues
D. a low oxygen affinity in the lungs and a high oxygen affinity in the tissues
E. a high oxygen affinity in the lungs and a high carbon dioxide affinity in the lungs

Correct Answer: C

Explanation:

C The job of the blood is to carry oxygen from the lungs, where it is plentiful, to the tissues, where it is not. Thus hemoglobin should have a high affinity for oxygen in the lungs so it can bind oxygen (choices A, B, and D could be eliminated) and a low oxygen affinity in the tissues (so it can release the oxygen where it is needed). The reverse is true for carbon dioxide. Hemoglobin has a high carbon dioxide affinity in the tissues and a low carbon dioxide affinity in the lungs.

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