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Tab HELP CENTER ncorrect Question 14 0 / 4 pts Muscle cells require calcium ions (Cast) to contract. When the concentration of calcium in the intracellular fluid decreases, a muscle cell relaxes. When the concentration of calcium in the cytoplasm increases, a muscle cell contracts. To prevent unwanted contraction, a muscle cell stores most of its Cast in a specialized compartment within the cell, called an organelle. An organelle has its own membrane, similar in structure to the membrane that surrounds the cell. The concentration of Cast in the intracellular fluid depends on the activity of transport proteins in the cell's membrane and organelle's membrane. The figures show a system of transport proteins that regulate the movement of Cast in a cell. The top figure illustrates the function of the transport proteins in this system. The lower left-hand portion of the bottom figure shows a muscle cell as the concentration of calcium in the intracellular fluid decreases, leading to relaxation. The lower right-hand portion of the bottom figure shows a muscle cell as the concentration of calcium in the intracellular fluid increases, leading to contraction. Ca2+ channels Ca2+ ATPase Na*/ Ca2+ Na*/ K* ATPase (closed - open) antiporter (pump) Na Na ATP ADP + P ATP K K