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Wize Grade 12 Biology Textbook > Cell Components: Structure and Function

Endoplasmic Reticulum: Rough and Smooth

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Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a large network of membranous tubules and flattened sacs called cisternae. This organelle is considered to be the biosynthetic factory of the cell.

photo by OpenStax / CC BY


The ER is continuous with the nuclear envelope, and has two distinct, yet connected, regions that differ in structure and function: they are smooth ER and the rough ER.
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(1) Rough Endoplasmic Reticulum (RER)

Structure: The RER is studded with ribosomes, giving it a
rough
appearance under the electron microscope.

adapted from OpenStax / CC BY

Functions:
  1. Protein synthesis: The RER bound ribosomes will translate a mRNA molecule to a polypeptide chain in the lumen of the RER.
  2. Protein Modification: While travelling through the ER channels, the polypeptide chains will get structurally modified and eventually move to a region of the RER called transitional ER where they create pockets.
  • These pockets can then break off as transport vesicles to transport their protein cargo to the Golgi apparatus or the cell membrane.
  1. Membrane synthesis (proteins and phopholipids): Manufactures membrane for itself, as well as for other cell organelles, hence its nickname "membrane factory."
  2. Phospholipid synthesis: Enzymes embedded in the RER membrane assemble phospholipids from cytosolic precursors
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(2) Smooth Endoplasmic Reticulum (SER)

Structure: The SER is not studded with ribosomes, giving it a
smooth
appearance under the electron microscope.


adapted by OpenStax / CC BY


Functions: SER functions vary depending on cell type.

photo by Lilly Cantabile / CC BY
photo by ABHMKDE / CC BY
photo by episy2
  1. Lipid synthesis: Enzymes embedded in the SER membrane help synthesize lipids (oils, steroids, and phospholipids).
  2. Carbohydrate metabolism: Some enzymes in the SER help in the breakdown and built up of carbohydrates.
  3. Calcium ion storage: Ca2+ in the cytosol are pumped into the SER lumen for storage. The release of Ca2+ from the SER is used to trigger different responses depending on cell type.
  4. Detoxification: Some enzymes of the SER help to detoxify poisons, drugs and metabolic waste usually by making them more soluble.

Practice: Endoplasmic Reticulum Structure

What covers the outer membrane of the rough endoplasmic reticulum, giving it its "rough" appearance when viewed using an electron microscope?

Practice: Endoplasmic Reticulum Structure and Function

For each of the following either write RER or SER: