Vesicular Transport

VESICULAR TRANSPORT

• Transports macromolecules/large protein molecules in & out of cells, transported as a vesicle

3 mechanisms –

• Endocytosis
• Exotoxsis.
• Transcytosis

1. Endocytosis

• Cell takes contents in (ie., from ECF to cytoplasm).

Steps:

• On contact with large molecule, cell membrane invaginates forming vesicle, including macromolecule.
• Vesicle is pinched within cell & restoring cell membrane.
• Requires energy, Ca⁺⁺ & contractile elements in cell.

Types of endocytosis:

1a. Constitutive endocytosis:

• Non-specific continuous process.
• Carrier protein involved – No.
• Energy required – Yes
• Concentration gradient – N/A

Pinocytosis/cell drinking: 

• Endocytosis of liquid.

Phagocytosis/ceIl eating: 

• Endocytosis of solid particle (bacterium/dead tissue).
• Eg: As in WBCs & tissue macrophages.

1b. Receptor-mediated endocytosis:

• Carrier protein involved – Yes
• Energy required – Yes
• Concentration gradient – N/A

Subtypes:

I) Clathrin-mediated endocytosis/absorptive pinocytosis.

Step 1:

• Process of internalizing molecules by inward budding of plasma membrane vesicles containing macromolecules (ligand) with receptor sites specific to molecules internalizing.

Step 2:

Coated pits:

• Specific receptors as small pits on outer surface of cell membrane.

Inner surface coating:

• Clathrin – Fibrillar protein coating inner surface pits
• Also coated by contractile filaments actin & myosin.

Step 3:

• Formation of “coated/pinocytic vesicle“:
• Found inside cytoplasm.

Contents of vesicle:

• Receptors, ligand (macromolecule) & layer of clathrin protein.

Step 4:

Vesicular fusion with endosome:

• Low pH within endosome breaks receptor-ligand/macromolecule complex releasing its contents.
• Contents include receptor, macromolecule, clathrin & membrane fragments.
• Receptors & clathrin transferred back to cell membrane for reusage.
• Macromolecule/ligand transported to lysosome for degradation via late endososome.

II) Exocytosis/emiocytosis/reverse pinocytosis:

• Reverse of endocytosis.
• Process of extrusion of secretory granules from cell.
• On contact of secretory vesicle with cell membrane, vesicular fusing & subsequent vesicular content extrusion takes place à Extruded as secretory granules.

Requirements:

• Carrier protein involved – No
• Energy required – Yes
• Concentration gradient – N/A

Mechanisms:

1. Non-constitutive/regulated pathway:

• Processing & storage before release occurs.
• Eg: Release of mature hormones from Golgi apparatus.

2. Constitutive pathway:

• Direct release without storage/processing occurs.
• Eg: Neurotransmitter release from presynaptic vesicles.

III) Transcytosis:

• Process of transporting macromolecules across cell interior
• Endocytosis occurs at one end & exocytosis at opposite end, after passage via cell interior.

Eg:

• Seen in epithelial cells, endothelial cell lining of blood capillaries.
• Less common sites – Neurons osteoclasts & M cells of intestine. 

Don’t Forget to Solve all the previous Year Question asked on VESICULAR TRANSPORT