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Intracellular Compartments and Endomembrane System

Objectives of Unit 6: Intracellular Compartments

  • List intracellular compartments.

  • Describe the function of the endoplasmic reticulum.

  • Describe the function of the Golgi apparatus.

  • Describe the function of lysosomes and peroxisomes.

  • Identify differences between lysosomes and peroxisomes.

Endomembrane System

  • Definition: A system of organelles that functions to:

    • Produce, store, and export biological molecules.

    • Degrade potentially harmful substances.

  • Major components of the endomembrane system include:

    • Nuclear envelope

    • Smooth endoplasmic reticulum (Smooth ER)

    • Rough endoplasmic reticulum (Rough ER)

    • Lysosomes

    • Vacuoles

    • Transport vesicles

    • Golgi apparatus

    • Plasma membrane

Endoplasmic Reticulum (ER)

  • Structure: Comprises interconnected tubes and parallel membranes enclosing cisternae.

  • Connection: Continuous with the nuclear membrane.

  • Types of Endoplasmic Reticulum:

    • Rough ER:

    • Defined by the external surface that is studded with ribosomes.

    • Functions:

      • Manufactures all secreted proteins.

      • Responsible for the synthesis of integral membrane proteins and phospholipids for cell membranes.

    • Smooth ER:

    • Structure: Tubules arranged in a looping network.

    • Functions:

      • In the liver:

      • Lipid and cholesterol metabolism.

      • Breakdown of glycogen.

      • Detoxification of drugs (in collaboration with the kidneys).

      • In the testes:

      • Synthesis of steroid-based hormones.

      • In intestinal cells:

      • Absorption, synthesis, and transport of fats.

      • In skeletal and cardiac muscle:

      • Storage and release of calcium.

Signal Mechanism of Protein Synthesis

  • Process Overview:

    • The mRNA-ribosome complex is directed to the rough ER by a signal-recognition particle (SRP).

    • The SRP is released as the polypeptide chain grows into the cisternae of the ER.

    • The protein folds into a three-dimensional confirmation.

    • The protein is then enclosed in a transport vesicle for movement towards the Golgi apparatus.

  • Key Components:

    • Cytosol

    • mRNA

    • Ribosomes

    • Transport vesicle: Coatomer-coated, carrying the growing polypeptide.

    • ER membrane

    • Signal-recognition particle (SRP): Directs the ribosome to the ER.

Golgi Apparatus

  • Structure: Comprises stacked and flattened membranous sacs (cisternae).

  • Functions:

    • Modification of proteins received from the ER.

    • Concentration and packaging of proteins for transport.

    • Transport vesicles from the ER fuse with the cis face of the Golgi apparatus.

    • Proteins pass through the Golgi apparatus, moving from the cis face to the trans face.

    • Secretory vesicles bud off from the trans face and transport proteins to designated parts of the cell.

  • Functional sides:

    • Cis face: The receiving side of the Golgi apparatus where transport vesicles arrive from the rough ER.

    • Trans face: The shipping side from which secretory vesicles exit the Golgi apparatus.