Mikrotubuluksista

What Holds a Cell Together?

https://study.com/academy/lesson/microtubules-definition-functions-structure.html

Just as our skeletons give our bodies' structure and shape, the cytoskeleton gives cells structure and shape. The cytoskeleton is responsible for lots of important cellular functions:

• It allows cells to move
• Engulf particles
• Brace themselves against pulling forces
• Transport vesicles through the cytosol
• Separate chromosomes during cell division
• Allows our muscles to contract

Clearly, things just wouldn't be the same without the cytoskeleton.
In eukaryotic cells, the cytoskeleton is made up of three major kinds of filaments: actin filaments, intermediate filaments (IF), and microtubules. Each of these filaments is a polymer, meaning that it is made up of many single subunits, like a child's building blocks snapped together to form a long chain. The subunits are called monomers, and each type of cytoskeletal filament is built out of a different kind of monomer.
The polymeric structure of cytoskeletal filaments means that they can be disassembled and rearranged at any time. This means that the cell can respond to signals in its environment and rapidly change its shape, motion, or attachment accordingly. You can imagine it like this: if the buildings in a city were made out of easily rearranged monomers, it would be easy to take them down and make new buildings in different places. We usually don't need to do this, but our cells do!
In this lesson, we'll focus on one type of cytoskeletal filament, microtubules, and learn about their structure and functions within the cell.

Microtubule Structure

Microtubules are the largest cytoskeletal filaments in cells, with a diameter of 25 nanometers. They are made out of subunits called tubulin. Each tubulin subunit is made up of one alpha and one beta tubulin that are attached to each other, so technically tubulin is a heterodimer, not a monomer. As you can see, it really does look like a tube, hence the name micro'tubule.'
In a microtubule structure, tubulin monomers are linked both at their ends and along their sides (laterally). This means that microtubules are quite stable along their lengths. Imagine that you have some plastic building blocks that are all identical and can attach to each other both at their ends and laterally. If you arranged them into a microtubule structure, and then wanted to take the structure apart, you can imagine that it would be really hard to take it apart somewhere in the middle, because how would you get the first block out? If you wanted to take it apart, you'd have to start at the ends. And indeed, this is how microtubules are assembled and disassembled, only from their ends.

Plus and Minus Ends

Since the tubulin subunits are always linked in the same direction, microtubules have two distinct ends, called the plus (+) and minus (-) ends. On the minus end, alpha tubulin is exposed, and on the plus end, beta tubulin is exposed.
Microtubules preferentially assemble and disassemble at their plus ends. An important consequence of this fact is that microtubule minus ends can be clustered together in a so-called microtubule-organizing center, or centrosome. The centrosome stays stable as the plus ends of the microtubules grow and shrink.
Microtubules are used in many important cellular functions.