In the gradient described, in which direction does the Ca2+ ion tend to move?

• A. From outside to inside
• B. From inside to outside
• C. No movement
• D. Only during depolarization

Answer: (A)

Calcium is kept at a much higher concentration outside the cell than inside, creating a strong chemical gradient that drives Ca2+ inward when the membrane is permeable to it. The interior is also negatively charged, so the electrostatic component favors inward movement of the positively charged Ca2+ when channels open. In the described gradient, this makes Ca2+ tend to move from outside to inside. This inward flux is the basis for Ca2+-triggered processes such as neurotransmitter release and muscle contraction when calcium channels open. Movement outward would require an opposite gradient or active transport, and movement would not occur if the membrane were completely impermeable or if there were no gradient to drive diffusion.