What is included in the outer segment of photoreceptors?

• A. Mitochondria
• B. Discs containing photopigments
• C. Ca2+ channels
• D. Nucleic acids

Answer: (B)

The outer segment is the light-detecting part of the photoreceptor, built as stacks of membranous discs that house photopigments like rhodopsin in rods and opsins in cones. These discs maximize the surface area available for photon catching and initiate the phototransduction cascade when light is absorbed, leading to a change in the cell’s electrical signal. Mitochondria and most other metabolic machinery sit in the inner segment, providing energy for the cell’s activities. Ca2+ channels are located mainly in the synaptic region for neurotransmitter release, not in the outer segment. Nucleic acids reside in the nucleus and other cellular compartments, not as a defining feature of the outer segment. So discs containing photopigments are the key component here.