5. Cell Structure And Organization - part 06 - Endoplasmic Reticulum

 


5. Cell Structure And Organization - part 06 - Endoplasmic Reticulum


Endoplasmic Reticulum (ER):

  • This little network within the cytosol is present in all eukaryotic cells except ova and mature red blood corpuscles. 
  • Under the electron microscope, it appears like network of membranous tubules and sacs called cisternae.
  • It forms more than 50% of the total membrane of a eukaryotic cell. 
  • This divides the cytoplasm in two parts viz; 
  1. one within the lumen of ER called, laminal cytoplasm and 
  2. non-laminal cytoplasm that lies outside ER.
  • Membrane of Endoplasmic reticulum is continuous with nuclear envelope at one end and extends till cell membrane.
  • It thus acts as intracellular supporting framework and helps in maintaining position of various cell organelles in the cytoplasm.
  • The outer surface of endoplasmic reticular membrane may or may not be studded with ribosomes. Accordingly, it is called rough or smooth ER. 
  • Smooth and rough ER differ in their functions.
Smooth ER :
  • It is involved in various processes in different cells. 
  • Depending on cell type, it helps in - 
  1. synthesis of lipids (ex. steroid secreting cells of cortical region of adrenal gland, testes and ovaries)
  2. detoxification of drugs and poisons (liver cells) and 
  3. storage of calcium ions (muscle cells).

Rough ER : 
  • It is primarily involved in protein synthesis. (e.g. pancreatic cells that secrete insulin). 
  • These proteins are secreted by ribosomes attached to rough ER and are called secretory proteins. 
  • These proteins get wrapped in membrane that buds off from transitional region of ER. Such membrane bound proteins depart from ER as transport vesicles. 
  • Rough ER is also involved in formation of membrane for the cell. 
  • The ER membrane grows in place by addition of membrane proteins and phospholipids to its own membrane. 
  • Portions of this expanded membrane are transferred to other components of endomembrane system.

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