Endocytosis

Question 1

Describe endocytosis

Answer 1

a process in which cells take up plasma membrane components, fluids, solutes, macromolecules and small particles

Question 2

How is the ingested material packaged before ingestion?

Answer 2

in a small portion of plasma membrane

Question 3

How does the plasma membrane enclose the to be ingested material?

Answer 3

plasma membrane buds inward and pinches off to form an endocytic vesicle

Question 4

What is an endocytic vesicle?

Answer 4

the plasma membrane buds inwards to enclose material that will be ingested by a cell and pinches off to form the vesicle

Question 5

Where does an endocytic vesicle move through?

Answer 5

the endocytic pathway

Question 6

What are the 3 types of endocytosis?

Answer 6

phagocytosis

bulk pinocytosis

receptor-mediated endocytosis (also pinocytosis)

Question 7

Describe phagocytosis

Answer 7

‘cellular eating’

large particles are ingested NON-specifically by packing into large vesicles called phagosomes

Question 8

T or F: phagocytosis is ingestion of specific particles

Answer 8

false! phagocytosis is non-specific

Question 9

T or F: phagocytosis is the ingestion of large particles

Answer 9

true

Question 10

Describe phagosomes

Answer 10

the large vesicles that form by enclosing large particles to be ingested by a cell via phagocytosis

Question 11

What organisms depend on phagocytosis for feeding?

Answer 11

single-cell protists (ex. amoebas) trap small microorganisms as food particles

Question 12

How do most animals use phagocytosis?

Answer 12

as a protective mechanism

specialized cells (macrophages/neutrophils) move through tissues to engulf dead or damaged cells

Question 13

What will phagosomes fuse with? what occurs when this happens?

Answer 13

phagosomes will fuse with lysosomes

the ingested particle will be degraded

Question 14

Describe pinocytosis

Answer 14

‘cellular drinking’

fluid and molecules are ingested by packing into small vesicles called pinocytic vesicles

Question 15

Describe pinocytic vesicles

Answer 15

the small vesicles that form by enclosing fluid or molecules to be ingested by a cell via pinocytosis

Question 16

T or F: pinocytic vesicles are large

Answer 16

False, they are small

Question 17

What size are pinocytic vesicles limited to?

Answer 17

150 nm in diameter

Question 18

What size can phagosomes be as big as?

Answer 18

0.5 um in diameter

Question 19

What are the 2 kinds of pinocytosis?

Answer 19

bulk endocytosis

receptor-mediated endocytosis

Question 20

Which type of pinocytosis is specific? which is non-specific?

Answer 20

bulk endocytosis = non-specific

receptor-mediated endocytosis = specific

Question 21

Describe bulk pinocytosis

Answer 21

the pinocytic vesicles will fuse with the lysosome and any complex molecules will be degraded

Question 22

Describe receptor-mediated endocytosis

Answer 22

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

Question 23

Describe the receptors involved in receptor-mediated endocytosis

Answer 23

membrane-bound proteins with ligand-binding domains on one side of the membrane and a functional domain on the other side

Question 24

T or F: all endocytic vesicles have coats

Answer 24

False! Not all of them do

Question 25

If an endocytic vesicle has a coat, what type will it always be?

Answer 25

clathrin

Question 26

What type of endocytic vesicle always uses a coat?

Answer 26

receptor-mediated endocytosis

Question 27

Where are the receptors for receptor-mediated endocytosis always concentrated?

Answer 27

receptors are highly concentrated near the clathrin coat

Question 28

What technique would be good to visualize clathrin coats on membranes?

Answer 28

Freeze fractured membranes visualized with an SEM

Question 29

What side of the membrane would the ligands be concentrated (in relation to a clathrin coat)?

Answer 29

extracellular side of the membrane

Question 30

Before the cargo sinks in during receptor-mediated endocytosis, what structure would the clathrin coat have? Where would this be located?

Answer 30

on the cytoplasmic side of the membrane, the clathrin lattice would be flat

Question 31

As the cargo sinks in during receptor-mediated endocytosis, what structure would the clathrin coat have? Where would this be located?

Answer 31

the clathrin coat invaginates and gets more spherical - still on the cytoplasmic side of the membrane

Question 32

Briefly describe the steps involved in the receptor-mediated endocytosis of a yolk protein

Answer 32

1. membrane receptors for the protein bind to the protein 2. the receptors bound to the proteins migrate to one area of the membrane 3. a clathrin coat forms on the cytoplasmic side to form a coated pit 4. the coated pits invaginate and pinch off as coated vesicles

Question 33

What is required to pinch off the clathrin coated vesicles from the membrane?

Answer 33

dynamin protein

Question 34

How does dynamin pinch off the clathrin coated vesicle from the membrane?

Answer 34

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

Question 35

What structure does dynamin take on? How does it do this?

Answer 35

dynamin self-assembles to form a helical collar

Question 36

What is required for dynamin to function?

Answer 36

GTP hydrolysis generates the mechanical force of dynamin

Question 37

What have experiments with a non-hydrolysable analog of GTP shown?

Answer 37

dynamin will polymerize and form long neck but will never be able to sever the vesicle head off

Question 38

T or F: endocytosis is not very common

Answer 38

FALSE it can be very common in some cell types ex. macrophages ingest 3% of their own plasma membrane every minute

Question 39

What process often occurs at a similar rate to endocytosis?

Answer 39

exocytosis

Question 40

Why does exocytosis have to occur at a similar rate to endocytosis?

Answer 40

to ensure the membrane surface area remains the same

Question 41

Where would endocytosis and exocytosis be especially linked? give an example

Answer 41

in areas with high membrane turnover ex. synaptic terminal bulb

Question 42

What happens to the clathrin coat if a vesicle has one after its been pinched off?

Answer 42

the clathrin coat is shed off once the membrane vesicle is pinched off and enters the cytoplasm

Question 43

What forms when vesicles fuse?

Answer 43

a compartment called the early endosome

Question 44

Describe the early endosome

Answer 44

a compartment formed when endocytic vesicles fuse

Question 45

What is the function of an early endosome?

Answer 45

sorting occurs here

Question 46

From the early endosome, what happens to empty vesicles?

Answer 46

they go back to the plasma membrane

Question 47

From the early endosome, what happens to vesicles with remaining cargo?

Answer 47

they move to a second compartment called the late endosome

Question 48

What does the late endosome contain?

Answer 48

all the cargo from the endocytic vesicle fusion of the early endosome - very concentrated

Question 49

T or F: the late endosome is very concentrated

Answer 49

true

Question 50

How does an endosome mature? What happens to the remaining contents?

Answer 50

from an early lysosome to a late endosome they are committed to degradation

Question 51

Where may the vesicles move from the late endosome?

Answer 51

to the lysosome or the Golgi

Question 52

What is the main reason for the connection between the TGN and the late endosome?

Answer 52

to recycle the mannose 6 phosphate receptor

Question 53

T or F: as the vesicles move toward the lysosome, the conditions get more and more acidic

Answer 53

true because the lysosome is very acidic

Question 54

How re V-type pumps involved in endocytosis?

Answer 54

they pump H+ into the lumen and create the acidic conditions required to separate the cargo from the receptor

Question 55

What separates the cargo from the receptor in receptor-mediated endocytosis?

Answer 55

acidity

Question 56

Where does the cargo move to in receptor-mediated endocytosis?

Answer 56

the late endosome

Question 57

Where does the receptor move to in receptor-mediated endocytosis?

Answer 57

back to the plasma membrane