4.5.1 Study Notes The Endomembrane System

A. The endomembrane system 

• a series of intracellular membranes that compartmentalize the cell.

  • It consists of the nuclear envelope, the membranes of the endoplasmic reticulum, the Golgi apparatus, and several types of vesicles. 

B. Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is a system of membrane channels and saccules (flattened vesicles) continuous with the outer membrane of the nuclear envelope

  • Rough ER is studded with ribosomes on the cytoplasm side

    •  it is the site where proteins are synthesized

    • proteins enter the ER interior for processing and modification

  • Smooth ER is continuous with rough ER but lacks ribosomes

    •  it is a site of various synthetic processes, detoxification, and storage

    • smooth ER forms transport vesicles

C. The Golgi Apparatus 

• It is named for Camillo Golgi, who discovered it in 1898

• consists of a stack of slightly curved saccules

  • cis face is directed towards the ER

  • trans face is directed toward the plasma membrane.

• receives protein-filled vesicles that bud from the rough ER and lipid-filled vesicles from the smooth ER

• Enzymes within the Golgi apparatus modify the carbohydrates that were placed on proteins in the ER

  • proteins and lipids are sorted and packaged

• Vesicles formed from the membrane of the outer face of the Golgi apparatus move to different locations in a cell

  • at the plasma membrane they discharge their contents as secretions a process called exocytosis because substances exit the cell.

D. Lysosomes  

• membrane-bounded vesicles produced by the Golgi apparatus 

• contain powerful digestive enzymes and are highly acidic

• Macromolecules enter a cell by vesicle formation

  • lysosomes fuse with vesicles and digest the contents of the vesicle

    • White blood cells that engulf bacteria use lysosomes to digest the bacteria.

• Tay-Sachs disease is a lysosomal storage disease

  • enzyme that is missing in these individuals digests a fatty substance that helps insulate nerve cells.

  • the fatty substance accumulates and causes nerve cells to die.

E. Endomembrane System Summary

• Proteins produced in rough ER and lipids from smooth ER are carried in vesicles to the Golgi apparatus.

• The Golgi apparatus modifies these products and then sorts and packages them into vesicles that go to various cell destinations.

• Secretory vesicles carry products to the membrane where exocytosis produces secretions.

• Lysosomes fuse with incoming vesicles and digest macromolecules.