The Role of the Ribosomes, ER, Golgi Apparatus, and Nucleus in Production of Proteins

Let's do a little review of basic cell structure.  The three main parts of the cell are: the plasma membrane, the nucleus, and the cytoplasm (with its associated organelles).

       In the enter of the cell is the nucleus, which holds DNA- the genetic material.  Every cell is contained within a membrane.  Plant cells also have a cell wall around the outside of the membrane while animal cells do not.  The cell wall provides protection to the plant cell as plants lack a skeleton. Inside the plasma membrane there is the cytoplasm (also called the cytosol).  The cytoplasm is not just a fluid, but rather a complex system of fibers, tubes, membranes, particles, and organelles (little organs), all of which have very specific functions.
   
    The relationship between three of these organelles- the ribosomes, the ER, and the Golgi- is very intricate.  Lets look a their functions one at a time.

     Ribosomes
            These are relatively tiny organelles. There are two kinds of ribosomes; free ribosomes (which float free in the cytoplasm), and bound ribosomes (which are attached to the endoplasmic reticulum).  Ribosomes are so important because they assemble amino acids into proteins.  If a protein is going to be used inside the cytoplasm of the cell, it is generally built by free ribosomes and not by ones bound to the endoplasmic reticulum.

     Endoplasmic Reticulum
        This brings us to an organelle called the Endoplasmic Reticulum.  Scientists call it ER for short.  There are two different kinds of ER,  Smooth ER and Rough ER.

 
            Smooth ER is not involved in protein synthesis.  It is the site of the synthesis of fatty acids and phospholipids.  It is called "smooth" because it has no ribosomes on its surface.  It is shaped like a network of little tubes.

            Smooth ER creates steroids and stores ions so that the cell can have them when its needs them.  Smooth ER's enzymes catalyze many different reactions.  It detoxifies poisonous substances in the liver, for example.  Smooth ER is associated with drug tolerance.  Alcoholics have a lot more smooth ER than people who don't drink.  As people consume more alcohol, they realize they have to drink more to get the same effect.  This happens because more inactivating enzymes are being produced.  These inactivating enzymes are responsible for helping to detoxify the liver.  A body's production of more smooth ER is one example of how your body tries to maintain homeostasis by protecting you from the toxic effects of alcohol.

            Rough ER gets its name because it is studded (covered) with ribosomes,which makes it look "rough" under a microscope.  These ribosomes are said to be "bound" to the ER.  The largest number of bound ribosomes is found in cells that produce enzymes to be secreted, which means they are made for delivery outside of the cell into the body.

             Once a protein has been synthesized (made), rough ER creates a bubble around it by pinching off a portion of its own membrane.  This bubble is called a transition vesicle.  The transition vesicle then moves to either the cell membrane or to the Golgi apparatus.

            The Golgi Apparatus was named for Camillo Golgi, the Italian physician who discovered it.  It is also referred to as the Golgi Body, the Golgi Complex, or simply the Golgi to avoid confusion.  The Golgi looks like a loose stack of pancakes.  Plant cells contain many of these stacks, while animals cells contain less.

         The Golgi takes simple molecules it gets from the Rough ER and makes them larger.  Remember the vesicles?  These vesicles or 'transporter packages' have their contents modified inside the Golgi.  After these little packages get modified, they get 'addressed' for delivery to their next destination.  The packages that now carry the modified contents are  called secretory vesicles. The Golgi is the final link in the relationship between the ribosomes, ER, and Golgi.

             IN SUMMARY:
              - Proteins are made in ribosomes.
              - Rough ER sends simple protein molecules to the Golgi Apparatus in transition vesicles.
              - The Golgi absorbs these transition vesicles through one side of its membrane.
              - The Golgi takes the simple molecules and converts them into larger macromolecules.
                      The larger molecules get packaged into secretion vesicles.
               - The Golgi then releases these secretion vesicles out of the other side into the
                      cytoplasm.
               -  From there, the secretion vesicles move onto the cell membrane and are released out
                     of the cell.