• Hollow rods • 25 nm diameter • 200 nm – 25 μm long • Constructed from tubulin– Globular, α-tubulin and β- tubulin • Microtubules grow by adding tubulin • Tubulin can also be removed and used to build microtubules elsewhere in cell • Because of arrangement of the 2 subunits of tubulin – One end can grow and shrink faster than the other; this is the PLUS END – Plus end indicates rates of growth/shrinking are higher

Microtubule functions: • Shape and support cell • Resist compression • Serve as tracks organelles with motor proteins can move along – Ex: vesicle going from ER to Golgi, Golgi to plasma membrane – Involved in separation of chromosomes during cell division

• Animal cell microtubules grow from a centrosome – A region often near the nucleus • Within centrosome is a pair of centrioles • Centrioles in centrosomes help organize microtubules in animal cells – Plant cells organize them in other ways • Microtubules are responsible for the beating of FLAGELLA and CILIA

Flagellum – Long appendage for locomotion – 1 or 2 per cell • Sperm have flagella

Cilium – Short appendage for locomotion or moving fluid – Occur in large numbers • Cilia line the trachea to move mucus and trapped debris out of lungs • act as an antenna – Receive signals for the cell – Often only 1 per cell – Non-motile

• Unicellular eukaryotes use cilia or flagella to move through water • Flagella and cilia both have a group of microtubules sheathed in extension of plasma membrane