What Holds a Cell Together?
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.'
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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.