12.+Endomembrane+System-Spring2025

Unit 2: Cell Biology Lecture 12 – Endomembrane System

Page 1: Overview of Animal Cell Structures

  • Key Structures in Animal Cells:

    • Rough Endoplasmic Reticulum (Rough ER): Involved in protein synthesis and processing.

    • Smooth Endoplasmic Reticulum (Smooth ER): Functions in lipid synthesis and detoxification.

    • Centrosome: Organizes microtubules and plays a role in cell division.

    • Cytoskeleton: Provides structural support and facilitates intracellular transport.

      • Microfilaments: Composed of actin, involved in cell movement.

      • Microtubules: Composed of tubulin, important for maintaining cell shape and transport.

      • Intermediate Filaments: Provide mechanical support.

    • Peroxisome: Contains enzymes for lipid metabolism and detoxification.

    • Lysosome: Contains digestive enzymes for waste processing (only in animal cells).

    • Golgi Apparatus: Modifies, sorts, and packages proteins.

    • Ribosomes: Sites of protein synthesis.

    • Nucleus: Contains nucleolus and chromatin; houses genetic material.

    • Plasma Membrane: Encloses cell contents and regulates entry/exit of substances.

Page 2: Endomembrane System Details

  • Components of the Endomembrane System:

    • Nuclear Envelope: Encases the nucleus, interconnected with other membranes.

    • Single Membrane Structures: Each with specialized functions:

      • Endoplasmic Reticulum (ER):

        • Rough ER: Studded with ribosomes, synthesizes proteins.

        • Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

      • Golgi Apparatus: Sorts and modifies proteins.

      • Lysosomes: Contain digestive enzymes for breakdown of macromolecules.

      • Microbodies: Include peroxisomes and glyoxysomes, involved in lipid metabolism.

      • Vacuoles: Present in plant cells, involved in storage (food and contractile vacuoles).

Page 3: Golgi Apparatus and Protein Modification

  • Functions of the Golgi Apparatus:

    • Protein Modification:

      • Glycosylation: Addition of carbohydrates (oligosaccharides) to proteins.

      • Myristylation: Addition of lipids to proteins.

    • Sorting and Shipping:

      • Proteins enter through the cis face and exit via the trans side within vesicles.

Page 4: Mechanism of Protein Trafficking

  • Overview of Protein Trafficking:

    • Process:

      • Proteins synthesized in the Rough ER are packaged into vesicles.

      • Vesicles transport proteins to the Golgi Apparatus for further processing.

      • Proteins are directed to their final destinations via secretory vesicles.

    • Target Sequences: Determine the destination of proteins.

Page 5: Targeting Proteins to Chloroplasts

  • Post-Translational Import:

    • Proteins are imported into chloroplasts across two membranes post-translation.

    • Translocons: TOC and TIC facilitate movement across outer and inner chloroplast membranes, respectively.

Page 6: Lysosomes and Microbodies

  • Lysosomes:

    • Membrane-bound sacs containing hydrolytic enzymes.

    • Function: Breakdown of food substances and damaged organelles at a pH of 5.

  • Microbodies:

    • Peroxisomes:

      • Break down fatty acids and detoxify reactive oxygen species through enzymes like peroxidase and catalase.

    • Glyoxysomes: Specialized peroxisomes in plants, convert stored lipids into sugars.

  • Vacuoles:

    • Central Vacuoles in Plants:

      • Can occupy up to 90% of plant cell volume, involved in storage and maintenance of turgor pressure.

      • Tonoplast: Membrane surrounding the central vacuole.

    • Food Vacuoles: Involved in the storage of nutrients in animal cells.

    • Contractile Vacuoles: Present in protists, regulate water balance.