Final answer:
Sodium ion is responsible for maintaining the dark current in photoreceptor cells. It moves into the cell through passive diffusion.
Step-by-step explanation:
The ion responsible for maintaining the dark current in photoreceptor cells is sodium (Na+). In the absence of light, there is a continuous flow of Na+ ions into the cell through ion channels. This inward movement of Na+ ions generates a depolarizing current, which is essential for the cell to be able to respond to light when it is detected.
The movement of Na+ ions into the cell is facilitated by passive diffusion. The concentration of Na+ ions is higher in the extracellular environment compared to the intracellular environment, creating a concentration gradient that drives the movement of Na+ ions into the cell.