The Endomembrane System*#
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The Endomembrane System
The
The Endoplasmic Reticulum
The endoplasmic reticulum (ER) (see figure above) is a series of interconnected membranous sacs and tubules that collectively
The hollow portion of the ER tubules
Rough ER
The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope (see figure below).
This transmission electron micrograph shows the rough endoplasmic reticulum and other organelles in a pancreatic cell. (credit: modification of work by
Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. These
If the phospholipids or
Since the RER is engaged in
Smooth ER
The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. Functions of the SER include
In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that
The Golgi Apparatus
We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need to
The Golgi apparatus in this white blood cell is visible as a stack of semicircular, flattened rings in the lower portion of the image.
The receiving side of the Golgi apparatus
Finally, the
In another example of form following function, cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundance of Golgi.
In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which
Lysosomes
A macrophage has engulfed (phagocytized) a potentially pathogenic bacterium and then fuses with
Summary of Endomembranes
The
The RER
Possible NB Discussion Point
If you were given a lab and the authority to pursue research in one of the organelles you just learned about, which one would you want to conduct research on? Why? What specific aspect of the organelle would you want to learn more about?