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BIO315H5 Midterm: Test 2 Review - Lecture 14

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Hai- Ying Mary Cheng
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Test 2 Review Lecture 14
o bring something from ECS into the cell -
Involves enclosing the particle with the plasma
membrane fuses, pinches off to form an endocytic
o 2 main classes of endocytosis based on
size of ingested particle & size of vesicle that forms
around it
o Phagocytosis:
Cell Eatig – Large particles are taken in
Vesile that ae foed ae Phagosoes
o Pinocytosis:
Cell Dikig - Small fluid particles such as small
solutes and fluids
Vesiles foed ae Pioti esiles
o Start at PM form endocytic vesicle
o 1st stop is early endosome this vesicle
fuses with early endosome : acts as 1st sorting
station for an ingested particle
o Things can either get trafficked back to PM
directly from endosome or they may be sorted to
aothe opatet alled elig edosoe
then later trafficked back to PM
o But in a lot of cases, ingested particle goes
down route where early endosome - contents are
trafficked closer to interior of cell
o Early endosome matures to late endosome
thee’s seuee of steps iol’d inc. the
formation of
o Intermediate structure called multi-vesicular
body: when you form small vesicles within larger vesicular organelle
o Here membrane pinches in/ invaginates in, pinches off, & forms interluminal vesicles which may contain
Transmembrane proteins or Plasma membrane proteins to be degraded
o From multivesciular body, structure can fuse with late endosomes, late endosomes can fuse with lysosomes to
cause degradation of the ingested particle
o Here, the Trans Golgi network is also involved because it may be brining newly synthesized proteins to these
various structures
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o Pinocytosis (cell-drinking)
o Constitutive process no signal/trigger
required happens constantly at steady rate
o Occurs at clathrin coated pits
o On PM, ~2% of membrane SA contains clathrin
coats on cytosolic side of that membrane
-coats are formed very transiently
-as soon as they form, PM invaginates in & pinches off
o Once vesicles are formed, clathrin coat is shed
and vesicle can fuse with early endosomes
-b/c it is a constitutive process & constant,
o Unless the process is balanced by something
that puts membranes on PM, the cell will get smaller
o Pinocytosis needs to be balanced by a
constitutive form of exocytosis
o Pinocytosis: Can also occur at other regions of
the membrane called Caveolae: little cavities
o Regions are special membrane domains with
special composition of lipids
o Major protein component are caviolins
-most of it faces the cytosol & inserted to PM
-much of proteins jets out into the cytosol
o Contents of endocytic compartments that
come from pinching off (caveosomes) deposited to
PM on opposite side of cell by process of transcytosis
o Difference b/w this form and 1st : clathrin
coated pits are proteins that can be shut whereas
Caveolae is membrane tethered protein (not dealing
with coat proteins that can be shed after the vesicle is
formed), caveaole stays with vesicle, if to fuse with PM
again, caveolae gets deposited on that membrane
o Another form of pinocytosis
o Does not use clathrin
o Unlike the form pinocytosis that does use
clathrin, it requires a trigger it’s ot ostititue
to activate receptors
o Receptor activation causes changes in structure
of plasma membrane (ruffles & folds over)
o Finally, PM Fuses to form a large vesicle called
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“o fa Piotosis that as disussed, ou’e ot seletig fo speific proteins/molecules to be brought into cell Just
what is part of that vesicle that plasma membrane invaginates. In order to have a much more selective mechanism, you
can have receptor-mediated endocytosis
o Specific macromolecules are identified
and internalized would require some specific
interaction with macromolecule & a specialized
o The case for
Cholesterol uptake if cell requires
more, cholesterol is brought in (not free in
blood) organized in a large particle LDL
cholesterol brought in with help of LDL receptor
o Particle contains lots of lipids, lots of
cholesterol molecules embedded within this as
well as inside
o Lipids are organized around a very large
protein called a glycoprotein
-has a domain that is recognized by LDL receptor
o If cell needs to take in cholesterol, it will boost its production of LDL receptors
o LDL receptor gets trafficked to PM, and its going to diffuse in plane of that membrane, when hits clathrin coated
pits, attracted and bound to LDL particles
o It’s going to invaginate in, PM of vesicle pinches off
o Fully formed vesicle and have uncoating of that vesicle
o Then this Vesicle then fuses with early endosome
o The receptor isnt wasted gets trafficked back to the PM for subsequent use
o But LDL gets trafficked to late endosomes and will be digested, cholesterol will be liberated
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