Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologymediummcq1 pt

In skeletal muscle, calcium ions sit at millimolar levels inside the endoplasmic reticulum yet remain micromolar or lower in the resting cytosol. What maintains this enormous imbalance?

A.An ion pump burns ATP to push calcium into the ER against its concentration gradient
B.Permanently open channels let calcium diffuse inward until both sides equalize
C.Osmosis carries dissolved calcium along with water drawn into the lumen
D.Facilitated diffusion loads calcium whenever luminal levels dip below cytosolic levels

Explanation

Core Concept

SERCA-type calcium pumps hydrolyze ATP and change shape to move Ca2+ from the cytosol into the ER lumen, uphill against its concentration. Storing calcium this way primes rapid release through channels during contraction. Passive processes cannot build such a steep gradient, so active transport must run continuously at rest.

Correct Answer

AAn ion pump burns ATP to push calcium into the ER against its concentration gradient

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