Moving forward, we can look now at what's going on inside the cells.
So we have peripheral proteins and we have
membrane embedded proteins or transmembrane proteins
and some of those you'd recall I mentioned
were associated with the cytoskeleton of the cell.
How the cell maintains its structure. Cytoskeleton is not only
about structure, it's about anchoring organelles in place,
keeping these compartments in the location in the cell
where they are most useful,
as well as in moving things around in the cell.
Now why would you need to move things around in the cell?
Well, first of all, recall that eukaryotic cells, our cells,
are much larger than prokaryotic cells.
And so there's some distance that substances need to move.
We could wait for them to diffuse but that would take longer
probably than the cell needs in time to get the materials
to build whatever it is that it's building.
Let's say for example, you're looking at the neuron again. We've got
a long neuron with an axon that reaches all the way down into the toes
or from the toes all the way back up to the spine. And how are we
going to get stuff that's made in the cell body, near the nucleus
all the way down to your tippy toes or vice versa.
And that's where some of the cytoskeletal elements come in
and create highways to allow cell movements stuff within the cell.
In addition, some of these fibers will shorten and contract
in order to allow the cell itself to move.
For example, if we're looking at white blood cell,
a macrophage needing to crawl through tissues,
these cytoskeletal elements
are what allows it to crawl through tissues
simply by shortening on one end and lengthening on the other end,
we can have that forward motion.
Now, we'll take a look at the
three different types of cytoskeletal elements.
We have, first of all, the actin filaments,
or microfilaments we might call them, those are the smallest.
Aand we have the largest which are called microtubules.
Even though they're micro, they're still tubules,
and they're the largest of the cytoskeletal elements. And in
addition to that, we have some intermediate filaments in there.
Exactly that they are intermediate in size.
So let's take a look at what some of these different fibers do.
First of all, we'll start small.
We'll look at the actin filaments.
Actin filaments are often involved in
anything to do with contraction.
So the motility of the macrophages that I introduced you to,
those are acquired because of actin,
lengthening on one end and shortening on the other end
allowing the cell to crawl essentially through its environment.
Macrophages are constantly wandering around in connective tissues,
cleaning things up and taking care of some immune functions.
Also we see actin in muscle cells. So as you contract
your muscles, actin is shortening, also with myosin,
but it causes muscular contraction. So recall actin, actin myosin,
contraction, we might see it involved in pinching of cells,
when we split the cytoplasm during cell division also.
Actin filaments are long and thin and there are two
polypeptide chains that are sort of wrapped around each other
so there's two subunits to each. It's sort of like
a string of pearls or two strings of pearls just twisted up.
Again, they are the smallest,
to give us a little scale, 7nm in diameter.
Really tiny. We definitely cannot see this through the microscope.
The second type of filament that we'll talk about inside the cell
are our element is the intermediate filament.
So they're medium in size. And these are much more structural.
They are the most structurally sound of the cytoskeletal elements.
They function more in a way like our bones were to our body,
holding the whole thing into a solid shape.
For example, keratin, we see in hair,
is what gives hair cells their definitive defined structure.
Finger nails, we see keratin,
so a much more solid network of fibers.
Finally, we'll look at the largest of those fibers
which are the microtubules.
Microtubules are very interesting fibers inside the cell.
These are the ones that sort of act as highways,
And we'll look at the vehicles that
transport things along these fibers shortly
but they act as highways throughout the cell.
They are also involved in attaching to chromosomes during
cell division and yanking them to opposide ends of the cell.
So we're going to revisit microtubules,
a couple more times at least during this course.
Microtubules again are tubular.
We've got lots of repeating subunits,
round and round in this spiral sort of fashion
in order to create this microtubule about 25nm in diameter.
Again, really tiny, but the biggest of the cytoskeletal elements.