University of California
Organic Structures:the Architecture of Life
Article discusses how objects in nature are held together through balanced forces. Scroll midway down to find info on microtubules and microfilaments. Links to excellent electron micrographs.
University of Arizona
University of Arizona: Cytoskeleton Tutorial
Complete descriptions of microtubules and microfilaments. Flip to the following pages to go through a cytoskeleton tutorial with graphics of cross-sections and diagrams. You can test yourself at the end.
University of Hamburg
University of Hamburg: Microtubules
Discussion of the size and structure of the microtubules in a cell. Links within the text take students to images and schematics.