Why are rods more sensitive to light than cones?

• A. Rods have larger surface area
• B. Rods operate at higher temperature
• C. Each rod has more photopigment per disc
• D. Each rod disc contains about 10,000 molecules of rhodopsin

Answer: (D)

Rods are optimized for detecting very low light because their outer segments are packed with a high density of photopigment. When a photon is absorbed, rhodopsin triggers a signaling cascade that leads to a large change in the rod’s membrane potential. Having so much photopigment concentrated in each disc means that a single photon can initiate a substantial amplification, producing a detectable response even in dim light. Cones, by contrast, have less pigment per disc and fewer overall photopigment, so they need more light to reach the same level of signal. This combination—high pigment density and strong amplification per photon—underlies the rods’ superior light sensitivity.