Endocytosis Flashcards
Describe endocytosis
a process in which cells take up plasma membrane components, fluids, solutes, macromolecules and small particles
How is the ingested material packaged before ingestion?
in a small portion of plasma membrane
How does the plasma membrane enclose the to be ingested material?
plasma membrane buds inward and pinches off to form an endocytic vesicle
What is an endocytic vesicle?
the plasma membrane buds inwards to enclose material that will be ingested by a cell and pinches off to form the vesicle
Where does an endocytic vesicle move through?
the endocytic pathway
What are the 3 types of endocytosis?
phagocytosis
bulk pinocytosis
receptor-mediated endocytosis (also pinocytosis)
Describe phagocytosis
‘cellular eating’
large particles are ingested NON-specifically by packing into large vesicles called phagosomes
T or F: phagocytosis is ingestion of specific particles
false! phagocytosis is non-specific
T or F: phagocytosis is the ingestion of large particles
true
Describe phagosomes
the large vesicles that form by enclosing large particles to be ingested by a cell via phagocytosis
What organisms depend on phagocytosis for feeding?
single-cell protists (ex. amoebas) trap small microorganisms as food particles
How do most animals use phagocytosis?
as a protective mechanism
specialized cells (macrophages/neutrophils) move through tissues to engulf dead or damaged cells
What will phagosomes fuse with? what occurs when this happens?
phagosomes will fuse with lysosomes
the ingested particle will be degraded
Describe pinocytosis
‘cellular drinking’
fluid and molecules are ingested by packing into small vesicles called pinocytic vesicles
Describe pinocytic vesicles
the small vesicles that form by enclosing fluid or molecules to be ingested by a cell via pinocytosis
T or F: pinocytic vesicles are large
False, they are small
What size are pinocytic vesicles limited to?
150 nm in diameter
What size can phagosomes be as big as?
0.5 um in diameter
What are the 2 kinds of pinocytosis?
bulk endocytosis
receptor-mediated endocytosis
Which type of pinocytosis is specific? which is non-specific?
bulk endocytosis = non-specific
receptor-mediated endocytosis = specific
Describe bulk pinocytosis
the pinocytic vesicles will fuse with the lysosome and any complex molecules will be degraded
Describe receptor-mediated endocytosis
A type of pinocytosis
specific ligands (macromolecules) bind to their receptors on the EC surface of the plasma membrane and are brought into the cell
Describe the receptors involved in receptor-mediated endocytosis
membrane-bound proteins with ligand-binding domains on one side of the membrane and a functional domain on the other side
T or F: all endocytic vesicles have coats
False! Not all of them do
If an endocytic vesicle has a coat, what type will it always be?
clathrin
What type of endocytic vesicle always uses a coat?
receptor-mediated endocytosis
Where are the receptors for receptor-mediated endocytosis always concentrated?
receptors are highly concentrated near the clathrin coat
What technique would be good to visualize clathrin coats on membranes?
Freeze fractured membranes visualized with an SEM
What side of the membrane would the ligands be concentrated (in relation to a clathrin coat)?
extracellular side of the membrane
Before the cargo sinks in during receptor-mediated endocytosis, what structure would the clathrin coat have? Where would this be located?
on the cytoplasmic side of the membrane, the clathrin lattice would be flat
As the cargo sinks in during receptor-mediated endocytosis, what structure would the clathrin coat have? Where would this be located?
the clathrin coat invaginates and gets more spherical - still on the cytoplasmic side of the membrane
Briefly describe the steps involved in the receptor-mediated endocytosis of a yolk protein
- membrane receptors for the protein bind to the protein
- the receptors bound to the proteins migrate to one area of the membrane
- a clathrin coat forms on the cytoplasmic side to form a coated pit
- the coated pits invaginate and pinch off as coated vesicles
What is required to pinch off the clathrin coated vesicles from the membrane?
dynamin protein
How does dynamin pinch off the clathrin coated vesicle from the membrane?
it self-assembles to form a helical collar under the budding clathrin-coated vesicle
GTP hydrolysis causes dynamin to constrict and cut off the coated pit = free vesicle
What structure does dynamin take on? How does it do this?
dynamin self-assembles to form a helical collar
What is required for dynamin to function?
GTP hydrolysis generates the mechanical force of dynamin
What have experiments with a non-hydrolysable analog of GTP shown?
dynamin will polymerize and form long neck but will never be able to sever the vesicle head off
T or F: endocytosis is not very common
FALSE it can be very common in some cell types
ex. macrophages ingest 3% of their own plasma membrane every minute
What process often occurs at a similar rate to endocytosis?
exocytosis
Why does exocytosis have to occur at a similar rate to endocytosis?
to ensure the membrane surface area remains the same
Where would endocytosis and exocytosis be especially linked? give an example
in areas with high membrane turnover
ex. synaptic terminal bulb
What happens to the clathrin coat if a vesicle has one after its been pinched off?
the clathrin coat is shed off once the membrane vesicle is pinched off and enters the cytoplasm
What forms when vesicles fuse?
a compartment called the early endosome
Describe the early endosome
a compartment formed when endocytic vesicles fuse
What is the function of an early endosome?
sorting occurs here
From the early endosome, what happens to empty vesicles?
they go back to the plasma membrane
From the early endosome, what happens to vesicles with remaining cargo?
they move to a second compartment called the late endosome
What does the late endosome contain?
all the cargo from the endocytic vesicle fusion of the early endosome - very concentrated
T or F: the late endosome is very concentrated
true
How does an endosome mature? What happens to the remaining contents?
from an early lysosome to a late endosome
they are committed to degradation
Where may the vesicles move from the late endosome?
to the lysosome or the Golgi
What is the main reason for the connection between the TGN and the late endosome?
to recycle the mannose 6 phosphate receptor
T or F: as the vesicles move toward the lysosome, the conditions get more and more acidic
true because the lysosome is very acidic
How re V-type pumps involved in endocytosis?
they pump H+ into the lumen and create the acidic conditions required to separate the cargo from the receptor
What separates the cargo from the receptor in receptor-mediated endocytosis?
acidity
Where does the cargo move to in receptor-mediated endocytosis?
the late endosome
Where does the receptor move to in receptor-mediated endocytosis?
back to the plasma membrane
