Final answer:
The key component of rod cells is the protein rhodopsin, which changes chemically in response to light and is essential for vision in low-light conditions and motion detection.
Step-by-step explanation:
The key component of rod cells that make them the photoreceptors of the eye is that they contain the protein rhodopsin. The rod-shaped outer segments of rod photoreceptors have stacks of membrane-bound discs that are laden with rhodopsin. When light falls on the retina, it induces chemical changes in rhodopsin, ultimately leading to a change in the activity of the retinal ganglion cells (RGCs). Unlike cones, rods do not yield color information and are highly sensitive, making them crucial for vision in very dark environments and for detecting motion.