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7.3B: The Endoplasmic Reticulum

  • Page ID
    3211
  • [ "article:topic", "authorname:kaiserg" ]

    Skills to Develop

    1. Briefly describe rough endoplasmic reticulum and state its functions.
    2. Briefly describe smooth endoplasmic reticulum and state its functions.

    The endoplasmic reticulum or ER is a maze of parallel membranous tubules and flattened sacs surrounding the nucleus that connects with the nuclear membrane and runs throughout the cytoplasm (Figure 33). The ER functions to:

    1. provide a surface area for protein and lipid synthesis;
    2. form a pathway for transporting molecules within the cell; and
    3. provide a storage area for molecules the cell has synthesized.

    The endoplasmic reticulum connects to the pores of the nuclear envelope. The pores in the nuclear membrane allow ribosomal subunits and mRNA transcribed off genes in the DNA to leave the nucleus, enter the cytoplasm, and participate in protein synthesis. There are two distinct regions of the ER: the rough ER and the smooth ER.

    Figure 33: Role of the Endoplasmic Reticulum and Golgi Apparatus in the Movement of Molecules within and from Eukaryotic Cells. The genes in the DNA are transcribed into mRNA molecules that enter the cytoplasm through pores in the nuclear membrane. Ribosomal subunits attach to the mRNA molecules and the genetic message is translated into protein. Ribosomes attached to mRNA molecules coding for proteins to be secreted from the cell or enter lysosomes attach to receptors on the endoplasmic reticulum (ER). These proteins then enter the lumen of the ER where they can be transported elswhere within the ER. The proteins typically enter the smooth endoplasmic reticulum where they are placed in transition vesicles. The transition vesicles fuse with the Golgi complex where the proteins may be modified, sorted, and placed in secretion vesicles. The secretion vesicles, in turn, fuse with the cytoplasmic membrane releasing the proteins from the cell.

    Rough Endoplasmic Reticulum

    ER with ribosomes attached is called rough endoplasmic reticulum (see Fig. 31, Fig. 30, and Fig. 33) and is involved in protein synthesis, production of new membrane, modification of newly formed proteins, and transport of these proteins and membrane to other locations within the cell.

    Ribosomal subunits and mRNA molecules transcribed off genes in the DNA leave the nucleus through pores in the nuclear membrane, enter the cytoplasm, and participate in protein synthesis. Ribosomes attached to mRNA molecules coding for proteins to be secreted from the cell or enter lysosomes attach to receptors on the ER. The ribosomes are tightly attached to the rough ER and contain a tunnel that connects to a pore in the ER called a translocon. The proteins that are synthesized by the ribosomes can then pass through the translocon and enter the lumen of the ER where they can be transported to other locations within the ER. Proteins secreted from the cell by exocytosis or destined for lysosomes are synthesized by the ribosomes on the surface of the rough ER (Figure 3.B.2). Proteins for use within the eukaryotic cell or intended for organelles such as mitochondria, chloroplasts, and peroxisomes are synthesized by mRNA molecules attached to ribosomes in the cytoplasm.

    Figure 3.B.2: Transmission electron micrograph of a thin section of the ribosome-studded rough endoplasmic reticulum of guinea pig pancreas. The ribosomes (small dots) were originally called Palade particles. Image made available by James D. Jamieson and the Department of Cell Biology, Yale University School of Medicine (CC-NY-SA-3.0).

    Smooth Endoplasmic Reticulum

    ER without ribosomes is called smooth endoplasmic reticulum (see Fig. 31 and Fig. 33) and contains enzymes for lipid biosynthesis, especially the synthesis of phospholipids and steroids.The smooth endoplasmic reticulum forms transition vesicles to transfer molecules produced in the rough ER to the Golgi complex. (see Fig. 31 and Fig. 33).

     

    Concept map for Eukaryotic Cell Structure

    Summary

    1. The endoplasmic reticulum or ER is a maze of parallel membranous tubules and flattened sacs surrounding the nucleus that connects with the nuclear membrane and runs throughout the cytoplasm.
    2. ER with ribosomes attached is called rough endoplasmic reticulum and is involved in protein synthesis, production of new membrane, modification of newly formed proteins, and transport of these proteins and membrane to other locations within the cell.
    3. ER without ribosomes is called smooth endoplasmic reticulum and contains enzymes for lipid biosynthesis, especially the synthesis of phospholipids and steroids. The smooth endoplasmic reticulum forms transition vesicles to transfer molecules produced in the rough ER to the Golgi complex.

    Questions

    Study the material in this section and then write out the answers to these questions. Do not just click on the answers and write them out. This will not test your understanding of this tutorial.

    1. Match the following:

      _____ Coated with ribosomes. (ans)

      _____ Lacks ribosomes. (ans)

      _____ Formstransition vesicles to transfer molecules produced in the rough ER to the Golgi apparatus and other parts of the cell. (ans)

      1. smooth endoplasmic reticulum
      2. rough endoplasmic reticulum

    Contributors

    • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)