CELS191 Lecture 4: Endomembrane System & Bulk Transport Processes

Overview of CELS191 Lecture 4: Endomembrane System & Bulk Transport Processes

• Course Context

  • Focus on anatomical science education, first-year experience, and educational technology.

• Key Topics

  • Endomembrane system definition
  • Functions of endomembrane components
  • Bulk transport processes (endocytosis and exocytosis)
  • Role of lysosomes

Endomembrane System

• Definition:
  • A network of membranes and organelles that work together to modify, package, and transport lipids and proteins.
• Components:
  • Nuclear envelope
  • Endoplasmic reticulum (smooth and rough)
  • Golgi apparatus
  • Vesicles
  • Lysosomes
  • Vacuoles
  • Plasma membrane
• Functionality:
  • Interconnected by physical contact or via vesicle transport.

Bulk Transport Processes

• Exocytosis:
  • Process of transporting materials out of the cell.
  • Types:
  • Constitutive Exocytosis:
    • Releases extracellular matrix proteins.
  • Regulated Exocytosis:
    • Releases hormones and neurotransmitters.
• Endocytosis:
  • Process of taking materials into the cell; can be divided into three types:
  • Phagocytosis:
    • "Cell eating"; uptake of large particles (food).
    • Forms a phagocytic vacuole, digested by lysosomes.
  • Pinocytosis:
    • "Cell drinking"; uptake of extracellular fluid and solutes.
    • Non-selective process forming small vesicles.
  • Receptor-mediated Endocytosis:
    • Specific uptake of substances at low concentrations using receptor proteins.

Endoplasmic Reticulum (ER)

• Types:
  • Smooth ER (sER):
  • Functions:
    • Metabolism of carbohydrates.
    • Lipid synthesis.
    • Detoxification.
    • Calcium ion storage.
  • Rough ER (rER):
  • Involved in protein synthesis due to ribosomes on its surface.
  • Processes secreted and membrane-bound proteins for further transport.

Golgi Apparatus

• Structure:
  • Series of stacked membrane sacs and vesicles.
• Functions:
  • Receives proteins from rER, modifies and sorts them for transport.
  • Has polarity; vesicles enter at cis face and exit at trans face.
  • Glycosylation:
  • Involves adding carbohydrates to proteins for surface or secreted proteins.
  • Sorting:
  • Adds tags (molecular markers) for vesicle targeting after budding from the trans face.
  • e.g., phosphorylated sugars for lysosomal enzyme identification.

Lysosomes

• Functionality:
  • Membrane-bound organelles containing hydrolytic enzymes.
  • Degrade biomolecules (proteins, lipids, carbohydrates, nucleic acids).
  • Crucial for autophagy and programmed cell death (apoptosis).
  • Defects can lead to lysosomal storage diseases.

Vacuoles

• Characteristics:
  • Large vesicles in plant cells, absorbing water, allowing growth without increasing cytoplasm volume.
  • Involved in phagocytosis through food vacuoles.

Summary of Key Processes

• Bulk Transport Outcomes:
  • Exocytosis for releasing materials.
  • Endocytosis for cellular intake of nutrients and signaling.
• Integrating Functions:
  • The endomembrane system coordinates the synthesis, sorting/tagging, and delivery of proteins and other molecules, demonstrating a complex cellular organization critical to cell function and health.

Review Questions and Practice

1. What are components of the endomembrane system?
2. How do the sER and rER differ in structure and function?
3. Describe the roles of the Golgi complex.
4. What is the distinction between constitutive and regulated exocytosis?
5. List and explain the three types of endocytosis.
6. Why is the interior of a lysosome acidic?