|Endoplasmic reticulum is a network of
tubules, vesicles and sacs that are interconnected. They may serve specialized functions
in the cell including protein synthesis, sequestration of calcium, production of steroids,
storage and production of glycogen, and insertion of membrane proteins. The first part of
this presentation will focus on rough endoplasmic reticulum which gets its name from the
presence of ribosomes on its surface. The text reading for this discussion is
Alberts et al, Molecular Biology of the Cell, third edition, Garland Publishing, 1994, pp
577-588 (Chapter 12) and pp 599-616. All of the figures in these web pages are
linked to a page listing the citation from which the figure was taken. Click on the figure
to learn the citation. If there is no link, the figure came from our own collection
Rough endoplasmic reticulum bears the ribosomes during protein synthesis. The newly synthesized proteins are sequestered in sacs, called cisternae . The system then sends the proteins via small vesicles to the Golgi Complex , or, in the case of membrane proteins, it inserts them into the membrane. As shown in this diagram, rough endoplasmic reticulum may either be vesicular or tubular. Or it may consist of stacks of flattened cisternae (like sheets) that may have bridging areas connecting the individual sheets. The Ribosomes sit on the outer surfaces of the sacs (or cisternae). They resemble small beads sitting in rosettes or in a linear pattern.
|Rough endoplasmic reticulum forms a branched reticulum that expands as the cell becomes more active in
protein synthesis. Sometimes the reticulum branches out. Other times, the cisternae dilate
and form large sacs that fill the cell. This photograph, taken from your text, shows the
reticulum delineated by immunolabeling for newly synthesized protein (labeled fluorescent
blue in this photo).
Where are ribosomes produced and how
do they get to the cytoplasm?
By the end of this unit, you should be able to give examples of how signal sequences and patches help with the sorting and transport of proteins.
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Learn how ribosomes interact with mRNA and tRNA.
Learn how ribosomes help send proteins to the rough endoplasmic reticulum
Study how proteins get to the Golgi Complex
Study the lysosomal sorting and packaging process
Go to the Cell Biology Web page