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Endoplasmic Reticulum and Protein Synthesis

Jun 4, 2024

Lecture Notes: Endoplasmic Reticulum and Protein Synthesis

Introduction

  • Endoplasmic Reticulum (ER): An organelle that extends from the nucleus.
  • Nuclear Envelope: A double membrane that surrounds the nucleus, and is continuous with the lumen (space) of the ER.

Types of Endoplasmic Reticulum

  1. Smooth Endoplasmic Reticulum (Smooth ER)

    • Functions:
      • Synthesizes lipids (e.g., cell membrane components, steroid hormones).
      • Metabolizes carbohydrates.
      • Detoxifies drugs and toxins.

  2. Rough Endoplasmic Reticulum (Rough ER)

    • Contains ribosomes (purple dots).
    • Functions:
      • Site of protein synthesis.
      • Post-translational modifications (e.g., formation of disulfide bridges).

Protein Synthesis

  • Cytoplasmic Ribosomes: Proteins synthesized here can end up in the nucleus, mitochondria, peroxisomes, or stay in the cytoplasm.
  • Rough ER Ribosomes: Proteins synthesized here can be secreted from the cell, become integral proteins for the cell membrane, or remain within the ER, Golgi apparatus, or lysosomes.

Secretory Pathway

  • Definition: Pathway that proteins follow from synthesis to their final destination (secretion or cell membrane integration).
  • Signal Sequence: Directs proteins to be synthesized in the Rough ER.

Golgi Apparatus

  • Structure: A stack of membrane-bound sacs near the ER.
  • Functions:
    1. Modifies proteins made in the Rough ER.
    2. Sorts and sends proteins to their proper destinations.
    3. Synthesizes certain molecules for secretion.

Processing and Transport of Proteins

  1. Cis Stack (near ER): Initial modification of proteins.
  2. Medial Stack: Further protein modifications.
  3. Trans Stack (far from ER): Final modifications and sorting.

Pathways for Vesicles

  • To Lysosomes: The vesicle merges with the lysosome.
  • To Cell Membrane: The vesicle moves to the cell membrane, merges, and releases its contents or integrates proteins into the membrane.

Secretory Pathway Organelles

  • ER, Golgi Apparatus, Lysosomes, and Cell Membrane: All part of the pathway for transferring proteins among them.

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