A cell has a membrane potential of –100 mV (more negative inside than outside) and has 1,000 times more calcium ions outside the cell than inside. Which of the following best describes a mechanism by which Ca²⁺ enters the cell?

• A. Movement of Ca²⁺ into the cell through an ion channel down its concentration gradient
• B. Passive diffusion of Ca²⁺ into the cell down its electrochemical gradient
• C. Cotransport of Ca²⁺ into the cell with Cl^–
• D. Movement of Ca²⁺ into the cell through a carrier protein down its electrical gradient
• E. Facilitated diffusion of Ca²⁺ into the cell down its electrochemical gradient

Answer: (E)

Verified step by step guidance

1. Understand the concept of membrane potential: The membrane potential is the difference in electric potential between the inside and outside of a cell. A negative membrane potential indicates that the inside of the cell is more negatively charged compared to the outside.
2. Recognize the concentration gradient: The problem states that there are 1,000 times more calcium ions (Ca2+) outside the cell than inside. This creates a concentration gradient where Ca2+ naturally tends to move from an area of high concentration (outside) to low concentration (inside).
3. Consider the electrochemical gradient: The electrochemical gradient is the combined effect of the concentration gradient and the electrical gradient. Since the inside of the cell is negatively charged, positively charged ions like Ca2+ are attracted to move into the cell.
4. Identify the mechanism of facilitated diffusion: Facilitated diffusion is a type of passive transport that allows ions to move across the cell membrane through specific ion channels or carrier proteins without the use of energy, driven by the electrochemical gradient.
5. Apply the concept to the problem: Given the negative membrane potential and the higher concentration of Ca2+ outside the cell, facilitated diffusion allows Ca2+ to move into the cell down its electrochemical gradient, utilizing ion channels or carrier proteins.