Final answer:
The end of a growing microtubule is rich in β-tubulin molecules.
Step-by-step explanation:
The end of a growing microtubule is rich in β-tubulin molecules.
Microtubules are small hollow tubes composed of polymerized dimers of α-tubulin and β-tubulin. The walls of the microtubule are made up of these globular proteins. The plus end of a microtubule, where growth occurs, is rich in β-tubulin molecules.