Endomembrane System

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The Endomembrane system includes

  • Endoplasmic reticulum
  • Golgi complex
  • Lysosomes and
  • Vacuoles.

The functions of these organelles are well coordinated.

 

Endoplasmic reticulum

  • The Endoplasmic reticulum is a network of membrane bound cavities, vesicles and tubules distributed throughout the cytoplasm.
  • It is the cytoskelteon of the cell.
  • The name Endoplasmic reticulum was coined by Porter.
  • ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.
  • It is absent in prokaryotes, egg and embryonic cells

Image result for endoplasmic reticulum components

  • It consists of three components
    • Cisternae: These are long, flattened, un-branched sac like structures arranged as parallel bundles. They have ribosomes on their surface and are normally found in secretory cells
    • Vesicles: These are rounded or ovoidal structures.
    • Tubules: These are smooth walled and highly branched tubular spaces having diverse forms. They normally occur in non-secretory cells.
  • The membrane of E.R. is continuous with plasma membrane, Golgi membrane and nuclear membrane.
  • The lumen of ER acts as a passage for the intracellular transport of secretory products.

ER is classified into two types.

Granular or Rough Endoplasmic Reticulum (RER)

  • In some ER, spherical granular structures called ribosomes are attached on the surface.
  • This type of ER is called Rough ER
  • It occurs in almost all cells which are actively engaged in protein synthesis

Agranular or Smooth Endoplasmic Retuculum (SER)

  • Here ribosomes are not attached with the ER and the surface is smooth.
  • It is developed in cells that synthesize steroid hormones.
  • The SER is the major site for synthesis of lipid.
  • The ER present in retinal cells is called myeloid bodies.
  • The ER present in muscle cells are called sacroplasmic reticulum.

Functions

  • It functions as a cellular circulatory system.
  • It synthesizes several steroid hormones, enzymes and cholesterol.
  • The rough ER synthesizes protein with the help of ribosomes
  • It produces microbodies.
  • Metabolic waste products are detoxified by smooth ER.
  • It helps in the formation of middle lamella and nuclear membrane