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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

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Enter the lumen of rough endoplasmic reticulum

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Further modified inside the endoplasmic reticulum & simultaneously travel towards the outer side of the endoplasmic reticulum

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Packed in carrier vesicles

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Travel towards Golgi apparatus

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Further processed in Golgi apparatus

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Packed in secretory vesicles

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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.

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