Slide 1
Eukaryotic cells are highly compartmentalized cells - this is in
comparison to prokaryotic cells which lack the subcellular specialization that
we see in the eukaryotes.
This compartmentalization seen in the eukaryotes is the result of an internal membrane system - eukaryotes have not only a cell membrane but a nuclear membrane and a number of membrane-bound organelles. These structures play important roles in the normal functioning of cells.
Slide 2
We will start our tour of the cell with the nucleus. When
you look at a cell under a simple light microscope - the nucleus is usually
the only structure you can clearly see. In this image, we see the nuclei
of onion cells stained blue.
Slide 3
The nucleus houses the majority of the DNA in a cell (remember
there is also DNA in the mitochondria and chloroplasts of cells).
DNA replication takes place in the nucleus before a cell divides and transcription of a gene into mRNA takes place in the nucleus during protein synthesis.
The nucleus also houses a structure known as the nucleolus - the nucleolus is the structure that produces the ribosomal RNA.
Another important feature of the nucleus and other membrane bound organelles is that there are pores in the membrane. These pores allow molecules to move in and out of the nucleus. Remember the mRNA has to leave the nucleus and travel to the ribosomes and other molecules have to move into the nucleus to help regulate gene expression.
Slide 4
Once the mRNA leaves the nucleus - it travels to a ribosome so
translation can take place.
There are two basic types of ribosomes - free and bound.
Free ribosomes are ribosomes that are suspended in the cyoplasm of the cell and not anchored to the endoplasmic reticulum.
Bound ribosomes are bound to the rough endoplasmic reticulum.
The free ribosomes produce proteins that are going to stay inside the cell while the bound ribosomes make proteins that are going to be exported outside of the cell.
Ribosomes are in constant movement and what was once a free ribosome can become a bound ribosome and then become a free ribosome again.
Slide 5
The system of membrane bound organelles within a eukaryotic cell
is known as the endomembrane system. it include the nucleus, the
endoplasmic reticulum, the golgi apparatus, the lysosomes, and the cell
membrane.
The endoplamic reticulum is series of membranous spaces that is either rough or smooth.
Rough endoplasmic reticulum is rough because it is studded with ribosomes. Its job is to make proteins for export outside of the cell.
Smooth endoplamic reticulum is smooth because there are no ribosomes attached. Its job is to help synthesize some fats and lipids. It also plays a very important role in helping us detoxify toxins in our body. The cells of the liver have a great deal of smooth ER. When you take a tylenol or drink alchohol - the smooth ER in your liver metabolizes the active compounds in these substances and helps your body flush them out. Overdosing on the active ingredient in tylenol - acetaminophen - is the number one cause of liver failure in the United States. People often don't recognize that cold medications and cough syrups also have acetaminophen and it can be easy to take too much and overload the smooth ERs ability to detoxify it and remove it from the body.
Slide 6
When the rough ER makes a protein for export from the cell - the
cell must have a way to recognize it as an export protein.
This is achieved through a process known as protein targeting. Protein targeting is achieved because newly synthesized proteins that are destined for export from the cell have a unique signal sequence that directs them to the rough endoplasmic reticulum.
When a growing polypeptide with this signal sequence is being built at a ribosome (and notice - it is starts out being built by a free ribosome) then a signal recognition particle (SRP) binds to the growing polypeptide.
The SRP helps to move the ribosome to the rough ER and the whole complex docks at a pore in the RER.
The growing polypeptide is then inserted through the pore and translation continues - note that the ribosome is now a bound ribosome.
The final polypeptide is now inside the RER and it is on its way to being exported to the outside of the cell.
Slide 7
From the RER, the polypeptide will move to the golgi
apparatus. While inside the golgi - the protein will be modified by
having sugars added to it. This is a process known as
glycolysation. This process of glycolysation is essentially packaging
and labeling the protein for export.
Slide 8
From the golgi apparatus the protein will be packaged into a
vesicle. This vesicle will either travel to the cell membrane and fuse
with the membrane thus releasing the protein to the outside of the cell.
Or it will travel to a lysosome and deliver enzymes to the lysosome - we will learn more about this later.