Endoplasmic Reticulum
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.
Protein synthesis
Ribosomes on the surface of rough endoplasmic reticulum synthesize:
Membrane proteins
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.
Lipid Synthesis
By enzymes.
e.g. in cells synthesizing steroid hormones.
Detoxification
By enzymes.
e.g. in liver cells.
Store Calcium
As sarcoplasmic reticulum, a modified form of the smooth endoplasmic reticulum.
e.g. in muscle cells.
Plays a role in intracellular signaling.
Next >>
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.