MCAT Biochemistry Question 36: Answer and Explanation

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Question: 36

6. Which of the following is most likely to be found bound to a protein in the body?

  • A. Sodium
  • B. Potassium
  • C. Chloride
  • D. Calcium

Correct Answer: D

Explanation:

Ions that are not readily accessible in the cytoplasm or extracellular space are likely to be bound to a binding protein. Classically, calcium and magnesium are protein-bound. Without this background knowledge, the question can still be answered. Sodium, choice (A), and potassium, choice (B), must exist in their free states to participate in action potentials. Chloride, choice (C), is readily excreted by the kidney, which would not be true if it were protein-bound. Calcium must be sequestered in both the bloodstream and intracellularly because calcium is used for muscle contraction, exocytosis (of neurotransmitters and other signals), and many other cellular processes that must be tightly regulated.

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