Question

In the below diagram, identify the part which connects the peripheral microtubules to the central sheath.

• A. Plasma membrane
• B. Interdoublet bridge
• C. Central microtubule
• D. Radial spoke

Answer 1

The Correct Option is D

In the structure of a cilium or flagellum (e.g., in eukaryotic flagella like sperm tails), the radial spokes are the parts that connect the peripheral microtubule doublets to the central sheath surrounding the central pair of microtubules.

• Plasma membrane (A): The outer boundary, not directly involved in connecting microtubules.
• Interdoublet bridge (B): Connects adjacent peripheral doublets, not the central sheath.
• Central microtubule (C): Part of the central pair, not a connecting structure.
• Radial spoke (D): Correct answer—links peripheral doublets to the central sheath.

Thus, the answer is (D) Radial spoke.

Answer 2

The radial spoke is a crucial structural component found in the axoneme of cilia and flagella. It serves as a mechanical bridge between the nine peripheral microtubule doublets and the central sheath that surrounds the two central microtubules. This connection is essential for regulating the coordinated sliding of microtubules, which enables the bending movement of cilia and flagella. The radial spoke transmits signals to control the activity of dynein arms, ensuring proper motility. Without functional radial spokes, conditions like immotile cilia syndrome can occur, highlighting their biological importance.

Thus, the answer is (D) Radial spoke.