Download Transcript of Notes for The Cell Note Sheet Part VI CYTOSKELETON

Transcript of Notes for The Cell Note Sheet Part VI
CYTOSKELETON
Because of its functions dealing with cell shape and cell movement, the cytoskeleton is thought of as the
cell’s organizer. It is simply a network of fibers extending throughout the entire cytoplasm as shown in
the picture.
Cell Shape:
One of the functions of the cytoskeleton is to provide shape to the cell. It does this by pushing out on
the cell membrane from inside the cell. It is much like a camping tent. Before the tent has been set up,
it is just a pile of fabric on the ground and has no recognizable shape. However, once the poles have
been set up inside the tent, they push out on the fabric of the tent creating a classic camping tent
shape. The cytoskeleton fibers inside the cell are like the camping tent poles, creating shape by pushing
out from the inside.
Transport Vesicle Tracks:
In addition to cell shape, the cytoskeleton also provides a “train track” or “monorail track” along which
transport vesicles are carried. Remember when we were learning about the formation of a protein in the
RER? We just took for granted that the transport vesicle could bleb off of the RER and float through
the cytoplasm to the Golgi. In fact, it’s not floating through the cytoplasm at all. It goes to the Golgi
because it travels along a cytoskeleton fiber that leads to the Golgi, much the same way that a train
ends up at the train station because the train tracks lead to the station.
Cleavage Furrow:
The cytoskeleton is also involved in creating a cleavage furrow to split a cell into two (i.e. cell division).
Let’s break down the phrase… To cleave means to cut. Think of a meat cleaver. It looks like a miniature
ax for the kitchen that cuts meat. Furrow indicates a trench or indent. Think about furrowing your
brow. When you furrow your brow, you create a little indent in the spot on your forehead between your
eyebrows. So what does this have to do with cells? Well, a cleavage furrow is an indent (created by
cytoskeleton) that cuts or divides the cell in half as the picture indicates. We can also draw our own
picture to show the series of events. Notice in our drawing that the cytoskeleton fibers are pulling the
cell membrane in from the inside creating two indents that will eventually pinch off, creating two
separate cells.
Changes in Cell Shape such as pseudopodia
In this picture, we see an amoeba engulfing (or phagocytosing) a smaller prey cell. Again we took for
granted that the pseudopodia could just emerge from the amoeba, but it is really the cytoskeleton that
causes these changes in cell shape. The cytoskeleton fibers push out on the cell membrane causing the
two “arms” or pseudopodia to form.
Component of Cilia and Flagella
Cytoskeleton is a major component of cilia and flagella. Cilia and flagella are structures that extend
from the outside of a cell and move (either whip around or beat in particular direction). It is because of
the cytoskeleton inside the cilia and flagella that they have enough rigidity to make these types of
movements.
Cilia:
Cilia are hair-like projections on the outside of cells. They are shorter and more numerous than flagella.
If they are located on the surface of a single-celled organism, such as the paramecium in the diagram,
they will function to move the cell in a particular direction. If a paramecium wants to move in a certain
direction, it simply beats its cilia in that direction and they will propel the paremecium through the fluid,
much like swimming.
If the cilia are located on the surface of a fixed cell that is part of a multicellular organism, then the
cilia function to move fluid along the surface of the cell instead of moving the cell itself. For example,
we have cells in our own body that have cilia! As the picture shows, some of the cells within our airway
(or trachea) have cilia. To make this more clear, let’s draw a picture of you, your trachea and your lungs!
The trachea is a tube that connects the back of your throat and nasal passages with your lungs.
As the picture indicates, when you breathe in air, you’re breathing in a lot more than just air. Along with
the air, comes dirt, dust, bacteria, viruses, and even mold spores. These are not things you want to get
caught in your lungs! If they did get down into your lungs, they could cause an infection called
pneumonia. So your airway (trachea) has two mechanisms to protect the lungs from the dirt, dust,
bacteria, etc. that comes in with the air. To make this clearer, let’s draw a close-up of the trachea.
The trachea is a tube that is part of your body, and since your body is made up of cells, the trachea
itself is also made up of cells. We see the trachea cells with their nuclei along the sides of the tube.
Some of these trachea cells secrete mucus, so there is a layer of mucus covering the entire inside of the
trachea tube. When you breathe in things like dust and bacteria, they will get trapped in the mucus
while the air can move on towards your lungs.
Now, you haven’t quite solved the problem because now you have a layer of mucus inside your trachea
that is filled with dust, dirt, bacteria, viruses and mold spores! Good thing some of the trachea cells are
ciliated (covered in cilia). The cilia are constantly beating in an upward motion towards the back of your
throat. So the mucus (filled with trapped dust, bacteria, etc.) is pushed up to the back of your throat
all day long. That means that every single time you swallow, you are actually swallowing down that
trachea mucus! It’s a pleasing thought, I know! You usually don’t feel the mucus when you swallow it, but
sometimes, if you are sick or having allergies, you may make extra mucus. In this case, you may feel the
lump of mucus as you swallow it. So why is it ok to put this mucus into your stomach, but not okay to put
it into your lungs? … I hope you were thinking that it’s because the stomach has stomach acid! The
stomach acid can take care of all that bad stuff that might hurt the lungs.
Flagella:
Flagella are similar to cilia in that they are located on the surface of a cell and are involved with motion.
While cilia are generally numerous on the cell surface, we generally only find 1-3 flagella on a single cell.
In addition, flagella lager and longer than cilia.
As we can see in the picture, human sperm cells have a single flagellum which propels the sperm forward.
In addition, algae sperm as well as the sperm of some aquatic plants have sperm to help propel the sperm
through the water.
As we can see in the second picture, there are single-celled organisms that have flagella that allow them
to move. This example is a bacterium that has three flagella.
CELL WALL:
The cell wall is an extracellular component of plant cells. “Extra” means “outside of.” So “extracellular”
means “on the outside of the cell.” The cell wall is ONLY found in PLANTS and is made up of rigid
cellulose. Recall from biochemistry that cellulose is the rigid polymer of glucose monomers that
“massages” our intestines when we eat it allowing us to have easier bowel movements.
You can see the cell wall In this picture as the lighter colored border around the plant cell. Note that
the green colored organelles are chloroplasts (the chlorophyll makes them green.)