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 Orientation
Alberts 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, nonspecific. 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-cell
Fig. 15.28
The End
Transport 3
Vessicular traffic/Bulk transport
Endocytosis & Exocytosis