Final answer:
A standard microtubule is made up of 13 protofilaments that form a hollow cylindrical structure, playing crucial roles in maintaining cell shape, organelle movement, and cell division.
Step-by-step explanation:
Microtubules are a critical part of the cell's cytoskeleton, providing structural support as well as being involved in various cellular processes, including organelle movement and cell division. The basic building block of a microtubule is the tubulin molecule which exists in two forms: alpha-tubulin and beta-tubulin. These form heterodimers which then line up in rows to create protofilaments. A standard microtubule has 13 protofilaments arranged parallel to each other, forming a tube. This tubular structure, with an estimated diameter of 25 nm, is hollow.
Microtubules are dynamic, which means they can swiftly assemble and disassemble based on the needs of the cell. They are most dense near the centrosome, an area close to the nucleus. Not only do they maintain the cell's shape, but they also play an essential role in cell motility, the positioning of organelles, and the segregation of chromosomes during cell division. Parts of the microtubules are also constituents of specialized cell structures like cilia and flagella, which help in cellular locomotion or the movement of fluids across the cell surface.