Network of interconnected tubules and flat vesicles.
Distributed extensively throughout the cytosol.
Divided into two parts: Rough endoplasmic reticulum and smooth endoplasmic reticulum. Both are connected to each other.
Rough Endoplasmic Reticulum
Many flat vesicles connected to each other.
Membrane is continuous with the outer layer of the nuclear envelope.
Lumen is continuous with the space in between two membranes of the nuclear envelope.
Outer surface contains ribosomes → Gives it rough appearance.
Plays role in protein synthesis → found abundantly in cells synthesizing large quantity of proteins. E.g. cells of the exocrine pancreas producing digestive enzymes.
Ribosomes on the surface of rough endoplasmic reticulum synthesize:
Proteins that are to be secreted out of the cell (Secretory proteins)
Free ribosomes in the cytosol synthesize:
Proteins required in cytosol.
Proteins that are to be secreted:
Synthesized at the rough endoplasmic reticulum
Enter the lumen of rough endoplasmic reticulum
Further modified inside the endoplasmic reticulum & simultaneously travel towards the outer side of the endoplasmic reticulum
Packed in carrier vesicles
Travel towards Golgi apparatus
Further processed in Golgi apparatus
Packed in secretory vesicles
Secreted out of the cell
Membrane proteins follow a similar path but they stay on the membrane. Their final destination is the membrane of the cell or cell organelles.
Smooth Endoplasmic Reticulum
Network of tubules.
No ribosomes → Smooth appearance.
Plays role in: Lipid synthesis, detoxification, and storing of calcium.
e.g. in cells synthesizing steroid hormones.
e.g. in liver cells.
As sarcoplasmic reticulum, a modified form of the smooth endoplasmic reticulum.
e.g. in muscle cells.
Plays a role in intracellular signaling.
As we saw, the secretory and membrane proteins are taken from the rough endoplasmic reticulum to the Golgi apparatus. Now, in the next session, we will see the Golgi apparatus.