Lecture Notes on Intracellular Compartments and Transport

Learning Outcomes

  • Definitions

    • Endocytosis: Process by which cells internalize proteins/molecules from the outside by membrane budding to form vesicles.

    • Exocytosis: Process where vesicles containing proteins fuse with the plasma membrane, secreting their contents outside the cell.

    • Golgi apparatus: Stack of membranes that processes and sorts proteins and lipids.

    • LDL vs HDL: LDL (low-density lipoprotein) carries cholesterol to cells; HDL (high-density lipoprotein) carries cholesterol to the liver.

  • Understand the movement of vesicles through organelles.

  • Differentiate between types of endocytosis and exocytosis:

    • Constitutive vs Regulated Secretion

    • Types of endocytosis: Phagocytosis, Pinocytosis, Receptor-mediated endocytosis.

Endomembrane System and Vesicular Transport

  • Comprised of membranes and organelles working together to package, modify, and transport proteins.

  • Process Overview:

    1. Proteins synthesized in the ER (endoplasmic reticulum).

    2. Proteins transported to the Golgi apparatus for processing and sorting.

    3. Transport vesicles carry proteins to their final destination (plasma membrane or lysosome).

Exocytosis

  • Process:

    • Involves vesicles traveling from ER -> Golgi -> secretory vesicles -> plasma membrane.

    • Types:

    • Constitutive Secretion:

      • Occurs continuously in all eukaryotic cells.

      • Releases proteins and supplies the plasma membrane with new lipids/proteins.

      • Example: Zona Pellucida in mammalian oocytes for sperm-egg recognition.

    • Regulated Secretion:

      • Occurs in specialized cells (e.g., pancreatic beta cells, neurons).

      • Proteins concentrated and stored in secretory vesicles, only released upon receiving signals like hormones.

Endocytosis

  • Uptake of extracellular materials via plasma membrane internalization, forming vesicles.

  • Types:

    • Phagocytosis: Internalization of large particles (e.g., bacteria) by specialized cells like macrophages and neutrophils.

    • Pinocytosis: Ingestion of small droplets of liquid.

    • Receptor-mediated endocytosis: Involves receptor proteins binding and capturing specific target molecules (e.g., LDL).

Receptor-mediated Endocytosis of LDL

  • LDL is recognized by specific receptors on the cell surface and internalized as a coated vesicle.

  • Steps:

    1. LDL binds to its receptor and is internalized.

    2. The vesicle merges with an endosome, becoming a lysosome.

    3. Cholesterol is released for use in cell functions.

    4. The receptor is recycled back to the membrane.

Golgi Apparatus Structure and Function

  • Structure: Series of flattened membrane-enclosed structures (cisternae) with three regions: cis, medial, and trans.

  • Main Functions:

    • Modifies proteins and lipids (e.g., glycosylation).

    • Sorts them for transport to their respective destinations.

Lysosomes

  • Organelles involved in intracellular digestion, containing hydrolytic enzymes functioning at pH 4-5.

  • Functions:

    • Degrade endocytic materials (e.g., bacteria).

    • Process damaged internal organelles, such as mitochondria.

Vesicular Traffic

  • Three main steps in vesicular transport:

    1. Budding: Cargo binds to receptors, adaptins and clathrin form a bud, and dynamin pinches off the vesicle.

    2. Travel: Vesicles move through the cytoplasm to their target.

    3. Fusion: Vesicles dock and fuse with target membranes, mediated by SNARE proteins (v-SNARE and t-SNARE).

Exam Preparation Tips

  • Understand the mechanisms of vesicular transport and the functions of each protein involved (adaptins, clathrin, dynamin).

  • Familiarize oneself with the differences between types of secretion and endocytosis processes.

  • Be able to illustrate the pathways of exocytosis and endocytosis involving Golgi apparatus and lysosomes.