Microtubule

Microtubules are protein structures found within cells. They have diameter of ~ 24 nm and varying length from several micrometers to possible millimeters in axons of nerve cells.

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Microtubules are polymers of tubulin. They are hollow cylinders, which can be thought of as 13 protofilaments of tubulin arranged into a cylinder, or as a single spiral with 13 subunits in one helical turn of the spiral. Microtubules have a polarity: each microtubule has a (+) and a (-) end. Gain or loss of tubulin subunits from the (+) end can change the length of a microtubule.

Protein: A protein (in Greek πρωτεϊνη = first thread) is a complex, high-molecular-weight organic compound that consists of amino acids joined by peptide bonds. Proteins are essential to the structure and function of all living cells and viruses....

Cell: A cell is a single unit or compartment, enclosed by a border or wall. A cell is usually part of a larger structure. More specific meanings depend on the context in which the work is used....