Questions: Cilia are composed of microtubules. How are microtubules different from the other cytoskeletal filaments? Microtubules are large bundles composed of the protein myosin. They are generally 15 nanometers in size. Microtubules are hollow tubes with a diameter of 25 nanometers and are composed of the protein tubulin. Microtubules are the smallest cytoskeletal element with a diameter of 5 nanometers, and they are composed of the protein actin. Microtubules vary in size from 9 to 11 nanometers and have different protein compositions depending upon the type of cell.

The answer is the second one: Microtubules are hollow tubes with a diameter of 25 nanometers and are composed of the protein tubulin.

Explanation for each option:

1. Microtubules are large bundles composed of the protein myosin. They are generally 15 nanometers in size.

   • This is incorrect. Microtubules are not composed of myosin; they are made of tubulin. Myosin is a motor protein associated with actin filaments, not microtubules.

2. Microtubules are hollow tubes with a diameter of 25 nanometers and are composed of the protein tubulin.

   • This is correct. Microtubules are indeed hollow tubes with a diameter of approximately 25 nanometers and are composed of tubulin proteins.

3. Microtubules are the smallest cytoskeletal element with a diameter of 5 nanometers, and they are composed of the protein actin.

   • This is incorrect. Microtubules are not the smallest cytoskeletal elements; they are the largest. Actin filaments, not microtubules, are composed of actin and are smaller in diameter.

4. Microtubules vary in size from 9 to 11 nanometers and have different protein compositions depending upon the type of cell.

   • This is incorrect. Microtubules have a consistent diameter of about 25 nanometers and are universally composed of tubulin, regardless of the cell type.

In summary, microtubules are distinct from other cytoskeletal filaments due to their structure as hollow tubes made of tubulin, with a consistent diameter of 25 nanometers.