Bulk Transport/Vessicular Traffic

• Bulk transport refers to the transport of solutes enclosed in vesicles. Many of these solutes are too large to diffuse across the membrane or be transported across it by carriers or by pumps.

• These vesicles originate from the ER, the Golgi or from the plasma membrane. They move along tracks formed by microtubules and are pushed along these tracts by motor proteins, dyneins and kinesins.

• In the cytoplasm, there is heavy traffic of vesicles inward and outward

Vessicular Traffic Fig. 15.18

Significance of Vessicular Traffic

• Vesicular traffic allows for: Communication/exchange among members of the endomembrane

system. Allows for delivery of specific proteins, glycoproteins to selected

organelles. Allows for export or delivery of large concentration of hormone or

neurotransmitter or extracellular matrix protein out of the cell by exocytosis

Allows for import of solutes, particles or whole cells into the by endocytosis

Vessicular Traffic Inward & Outward

• Inward: Solutes (or cells or particles) in the ECF are enclosed in vessicles that

originate from the plasma membrane (PM). From the PM the vesicle move to early endosomes -> to late endosomes

to -> lysosomes• Outward:

Solutes (usually proteins & glycoproteins) from the RER lumen move to -> Cis-Golgi network to -> Medial cisternae to -> Trans-Golgi network -> Plasma membrane.

• This is common for PM protein and for protein hormones & neurotransmiters An alternative route is from the Trans-Golgi network to -> Early

endosomes to -> late endosomes -> to lysosomes• This is common for autophagy and recycling of intracellular proteins &

organelles

Autophagy and Endocytosis(Fig. 15.36)

Some Facts About Endosomes

• Endosomes are members of the endomembrane system of the cell and they form a compartment called the endosomal compartment.

• This compartment is the main “sorting station” for the inward vessicular traffic (just like the GA is the main sorting station for outward vessicular traffic)

• The lumen of endosomes has pH 5 to 6• Hydrogen ion gradient across the endosome membrane is

maintained by a V-type ATPase.• Early endosomes mature into late endosomes which

mature into lysosomes.

Some Facts About Vesicles

• The luminal surface of the vesicle membrane is topologically equivalent to the extracellular surface of the PM.

• The cytosolic surface of the vesicle membrane is topologically equivalent to the cytosolic surface of the plasma membrane (next slide illustrates

• The cytosolic surface of vessicles is “coated” with a variety of coat proteins, of which there are 3 Classes Clathrin, COP I and COP II

• Other proteins on the vessicle surface are the Rab protein, Adaptin, Dynamin, and V-SNAREs

• (V-SNAREs will snap into corresponting T-SNARES on target membrane)• Significance of each vesicle protein ON YOUR OWN!

Membranes Maintain Their OrientationAlberts Fig 11.19

Endocytosis of Egg Yolk Protein (Fig 15.19a)Note “Clathrin Coat”

Clathrin-Coated Vessicle (Fig 15.19b)

Endocytosis

• Endocytosis is the process for bulk import/transport of: Macromolecules Particles (charcoal dust, asbestos dust…) Whole cells or cell fragments

• Classified as: Phagocytosis Pinocytosis or Receptor - mediated endocytosis

on basis of a) size of solute and b) specificity

On Phagocytosis

• This is for whole cells, bacteria and particles• Carried out by specialized scavenger cells found in the blood

(neutrophils, eosinophils) and in connective tissue (macrophages)• These cells are loaded with lysosomes, produce highly reactive

oxidants (like pechlorate and H2O2)• The imported material is enclosed in a vesicle called a phagosome,

which will eventually be fused with a lysosome.• It is a specific process, that is triggered by solute/cell/particle/ binding

to receptors (Fc receptors) on the surface of the cell. Binding to the receptors activate and intracellular signal that brings about phagocytosis

• Ligands for the Fc receptors include complement proteins, antibodies, oligosaccharides on surface of microorganisms or phospholipids on surface of cells.

Phagoctosis of Bacterium & Plastic Bead(Alberts Fig. 15.32)

On Pinocytosis

• It is sometimes called “cellular drinking”.• It is carried out by “clathrin-coated pits” and vesicles.• It is an indiscriminate (not picky) process, therefore, non-

specific. Any solutes trapped during the invagination step will be enclosed in the vesicle and delivered to an endosome (after shedding the clathrin coat)..

• The shed clatrhins are recycled to the plasma membrane

On Receptor-Mediated Endocytosis

• This is a specific process mediated primarily by clathrin-coated pits and vessicles. The pits (which are precursors to the vessicles) contain receptors for specific cargo solutes.

• It allows for specific uptake of a particular solute against its concentration gradient.

• The vessicle is usually delivered to an endosome and eventually to a lysosome. In the endosome, the receptors dissociate from the cargo. The receptors are recycled to the plasma membrane and the cargo delivered to lysosomes.

• Examples of material taken up by Receptor-mediated endocytosis LDL (low density lipoproteins (composed of phospholipids, cholesterol

esterified to fatty acids and proteins) Transferrin (Apotransferrin-Fe complex HIV (Human immunodefficiency virus)

Endocytosis of Egg Yolk Protein (Fig 15.19a)Note “Clathrin Coat”

Receptor-Mediated Endocytosis (I)Fig. 15.20

Endocytosis of LDL(Fig. 15.33)

Exocytosis

• This is sometimes called the secretory pathway. • It is for delivery of concentrated solutions of hormones,

neurotransmitters, hydrolytic enzymes and mucus directly into the extracellular fluid, the blood or the extracellular matrix.

• Carried out by many exocrine glands (pancreatic acinar cells, goblet cells) and endocrine glands (b-cells of Islets of Langerhans, somatotrophs in the pituitary.

• Examples of proteins secreted by exocytosis: Insulin, Glucagon, Growth hormone, Trypsinogen

• Exocytosis may be classified as: a) Constitutive or b) Regulated Constitutive is the default system (for secretion of immunoglobulins,

collagen, elastin. Regulated is usually for hormone in response to a given signal.

Exocytosis: Constitutive and Regulated(Fig 15.27)

Exocytosis of Insulin From beta-cellFig. 15.28

The End

Transport 3Vessicular traffic/Bulk transport

Endocytosis & Exocytosis