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BIOL 2321 - Cell Biology > Endomembrane > Flashcards

Endomembrane Flashcards

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1
Q

What is the function of the endomembranes?

A

to compartmentalize eukaryotic cells by dividing the cytoplasm

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2
Q

T or F: the endomembrane system is composed of multiple organelles that function as a coordinated unit

A

True

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3
Q

What are the 6 organelles that make up the endomembrane system?

A 
endoplasmic reticulum
Golgi complex
endosomes
lysosomes 
vacuoles 
secretory vesicles
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4
Q

Which 2 organelles are not included in the endomembrane system?

A

mitochondria

chloroplasts

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5
Q

T or F: the combined surface of endomembranes is always less than the surface area of the surrounding plasma membrane

A

False

the combined surface of endomembranes can be MUCH larger than the surface of the plasma membrane

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6
Q

T or F: cells with different functions can have vastly different proportions of each endomembrane

A

true

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7
Q

Approximately how much of an animal cell’s total membrane does the ER constitute? How much of its volume?

A

~half of its membrane

~10% of its volume

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8
Q

How is the ER divided?

A

into ‘smooth’ and ‘rough’ ER

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9
Q

T or F: the smooth and rough ER are discrete and enclose two luminal spaces

A

FALSe FALSE FALSE

They are continuous and enclose a single luminal space

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10
Q

What is the structure of the rough ER referred as?

A

flattened sheets

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11
Q

What is the structure of the smooth ER referred as?

A

tubules

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12
Q

Where do the rough and smooth ER extend?

A

throughout the cytoplasm

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13
Q

Give some examples of the functions of the smooth ER

A

calcium sequestration

membrane lipid synthesis

detoxification of the cell

steroid hormone synthesis

glycogen storage in liver

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14
Q

Which half of the smooth ER bilayer are newly synthesized phospholipids inserted?

A

into the half bilayer facing the cytosol

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15
Q

How does the insertion of the phospholipids into the smooth ER bilayer facing the cytosol affect the orientation of the enzyme that synthesizes these lipids?

A

they are bound to the smooth ER membrane with their active sites facing the cytosol

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16
Q

How are the phospholipids flipped into the opposite leaflet?

A

by flippases

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17
Q

What are functions of the rough ER?

A

biosynthesis and processing of proteins

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18
Q

What types of proteins are included in the rough ER?

A

secretory proteins

membrane-bound proteins

proteins for internal use that require modification (ex. glycosylation, disulfide bridges, etc.)

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19
Q

What is another word for the structure of Rough ER (Sheets)?

A

cisternae

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20
Q

T or F: almost all proteins are initially synthesized on ribosomes within the cytosol

A

true

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21
Q

Where are polypeptides that are fully synthesized on ‘free’ ribosomes located?

A

in the cytosol and they remain in the cytosol

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22
Q

What are 4 proteins that fully synthesize on ‘free’ ribosomes?

A

cytosolic proteins

peripheral membrane proteins

nuclear proteins

proteins incorporated into chloroplasts, mitochondria, and peroxisomes

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23
Q

Where does a polypeptide with an ER signal sequence move to? How does this movement usually occur?

A

into the ER cisternal space through a protein-lined pore

this usually occurs co-translationally

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24
Q

Describe post-translational translocation

A

proteins that are synthesized completely on cytosolic ribosomes and are THEN moved to an organelle (ex. nucleus or peroxisome) AFTER translation

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25
Q

What do almost all proteins produced on membrane-bound ribosomes become?

A

glycoproteins

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26
Q

Describe glycosylation

A

the covalent addition of oligosaccharides

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27
Q

When does glycosylation occur?

A

After a protein is properly folded in the ER lumen

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28
Q

What is the most common type of protein glycosylation that occurs in the ER?

A

N-linked glycosylation

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29
Q

Describe N-linked glycosylation

A

the most common type of protein glycosylation that occurs in the ER

adds sugars to an amino group of the asparagine (Asn) amino acid

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30
Q

What does glycosylation begin with?

A

the addition of an identical 14 sugar core to every glycosylated protein

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31
Q

What is added to every glycosylated protein?

A

an identical 14 sugar core

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32
Q

T or F: the 14 sugar core added to every glycosylated protein is identical

A

true

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33
Q

What are the 3 components of the core sugar on glycosylated proteins?

A

mannose

glucose

N-acetyl-glucosamine

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34
Q

T or F: the core glycosylated protein can be modified to give different oligosaccharides

A

true

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35
Q

Describe dolichol

A

A carrier lipid embedded in the ER membrane which binds to sugars in the core one by one

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36
Q

On which side of the ER membrane are sugars INITIALLY added to dolichol? how are they added?

A

sugars are added enzymatically to the cytosolic side of the ER membrane

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37
Q

What happens to the core dolichol halfway through formation? What is the final product?

A

it is flipped

the final product has the sugars facing the ER lumen

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38
Q

Describe oligosaccharyl transferase

A

a membrane bound enzyme that catalyzes the transfer of the core sugar on dolichol to a protein

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39
Q

Where is the enzyme active site for oligosaccharyl transferase?

A

in the ER lumen

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40
Q

What is the purpose of chaperones in the ER? give an example of a chaperone

A

they ensure proper folding of glycoproteins

they will attempt to refold any misfolded proteins

ex. calnexin

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41
Q

What happens if chaperones are unsuccessful in refolding misfolded proteins?

A

misfolded proteins are tagged by a terminal glucose and translocated to the cytosol and degraded

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42
Q

Describe proteasome

A

A large barrel-shaped protein complex that degrades misfolded glycoproteins in the cytosol

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43
Q

How many key stacks does the Golgi apparatus contain? what are they?

A

3

Cis
Medial
Trans

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44
Q

T or F: the Golgi is unpolarized

A

False, it is polarized

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45
Q

Explain why the Golgi is polarized

A

The cis face is always closest to the ER

the trans face is always closest to the cell surface

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46
Q

Which face of the Golgi is always closest to the ER?

A

cis

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47
Q

Which face of the Golgi is always closest to the cell surface?

A

trans

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48
Q

What are the two networks the Golgi also includes?

A

Interconnected tube and stack structures:

Cis network
Trans network

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49
Q

What does the cis Golgi network include?

A

fused vesicles arriving from the ER

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50
Q

What is the function of the trans Golgi network?

A

it separates proteins into different vesicles depending on their final destination

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51
Q

What happens to the core oligosaccharide in the Golgi? What are some examples?

A

it is modified to produce the unique sugar combinations of glycoproteins

ex. removing mannose

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52
Q

What is another type of glycosylation that occurs in the Golgi?

A

O-linked glycosylation

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53
Q

Describe O-linked glycosylation

A

the addition of oligosaccharides to the hydroxyl group of the serine (or threonine) amino acid

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54
Q

How are materials shuttled through the endomembrane system?

A

in membrane-bound transport vesicles

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55
Q

How are membrane-bound transport vesicles formed?

A

by budding of the donor membrane

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56
Q

How do transport vesicles release their contents in a different location?

A

the vesicles fuse with membranes of an acceptor compartment

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57
Q

T or F: Vesicles maintain their orientation as they move through cell components

A

true

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58
Q

If material was within the lumen of the donor compartment, where will it be relative to the transport vesicle and the target compartment?

A

it will be within the lumen of the transport vesicle and within the lumen of the target compartment

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59
Q

What are the 3 directional routes for vesicular traffic flow between endomembranes?

A
  1. biosynthetic pathway
  2. secretory pathway
  3. endocytic pathway
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60
Q

What kind of proteins use the biosynthetic pathway?

A

proteins destined for certain organelles (ex. lysosomes)

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61
Q

What happens in the biosynthetic pathway?

A

synthesis, modification, and transport of proteins destined for specific organelles

62
Q

What proteins use the secretory pathway?

A

proteins to be discharged from the (secretory proteins)

63
Q

What are the 2 types of secretory pathway?

A

constitutive secretion

regulated secretion

64
Q

What is constitutive secretion?

A

secretion that is continuous

as soon as the protein is in a vesicle, it can fuse the contents can be released

65
Q

When does regulated secretion happen?

A

in response to a stimulus

66
Q

What experiment helped us understand the endomembrane system? What cells did they do the experiment in?

A

autoradiographic ‘pulse chase’ experiments in pancreatic cells

67
Q

Describe the autoradiographic ‘pulse chase’ experiments

A

experiments run in pancreatic cells

followed a secretory protein through the different endomembrane stages

68
Q

What are the 2 models of movement through the Golgi?

A

vesicular transport model

cisternal maturation model

69
Q

describe how vesicles move in the vesicular transport model

A

the vesicles move forward through the complex

70
Q

Describe movement in the Golgi in the Cisternal maturation model

A

individual cisternae move forward and vesicles transport resident proteins backwards

71
Q

In the vesicular transport model, how does cargo move? how does cisternae move?

A

cargo is carried forward by transport vesicles

trick question - cisternae do NOT MOVE, they are stable

72
Q

In the cisternal maturation model, how does cisternae move?

A

each cisterna ‘matures’ as it moves from the cis face to the trans face and eventially disperses in the trans golgi network

73
Q

In the cisternal maturation model, how does cargo move?

A

transport vesicles carry resident golgi enzymes in the reverse direction after the cisterna matures

74
Q

In the cisternal maturation model, how are new cis compartments formed?

A

by membrane vesicles budding and moving forward from the ER

75
Q

T or F: vesicles bud off membranes randomly

A

False!!

76
Q

How do vesicles form?

A

from specific regions of membrane covered with distinctive coat proteins on their cytosolic surface

77
Q

Which membranes form vesicles?

A

membranes covered with coat proteins on their cytosolic surface

78
Q

Where on the membrane are coat proteins located?

A

on their cytosolic surface

79
Q

What are the 3 purposes of coat proteins?

A

select the components to be carried by vesicle

cause the membrane to curve and form a vesicle

target the vesicle to the appropriate destination (coat proteins shed before vesicle fusion)

80
Q

What are the 3 types of coated vesicles?

A

COPII

COPI

Clathrin

81
Q

Describe COPII

A

A coated vesicle that move materials from the ER ‘forward’ to the Golgi complex

82
Q

Describe COPI

A

A coated vesicle that moves materials from Golgi ‘backward’ to the ER, or from the trans Golgi to the cis Golgi cisternae

83
Q

Describe clathrin

A

a coated vesicle that moves materials from the trans Golgi network to endosomes, lysosomes, and plant vacuoles

they are also involved in exo and endocytosis

84
Q

Describe how COPII functions

A

it selects certain cargo for transport in vesicles moving forwards

85
Q

What happens after COPII selects certain cargo for transport in forward moving vesicles?

A

cargo proteins can then bind to transmembrane cargo receptors, then the receptors will bind to COPII proteins

86
Q

What is the purpose of transmembrane cargo receptors with cargo proteins binding to the COPII proteins?

A

it allows specific cargo to concentrate in an area of the lumen that will eventually become a vesicle

87
Q

How many layers does a COPII protein have?

A

2

they have an inner and outer layer

88
Q

What protein is involved in the layers of the COPII protein?

A

Sar1 protein (a G protein)

89
Q

How is Sar1 activated?

A

By a GEF

90
Q

What is the activated state of Sar1?

A

Sar1-GTP

91
Q

What happens when Sar1 is activated?

A

Sar1-GTP (activated) binds to the ER cytosolic leaflet band to recruit Sec proteins

92
Q

What is the structure of Sec proteins?

A

curved

93
Q

What happens when Sar1-GTP and Sec proteins bind together?

A

they help the membrane to form a bud

94
Q

Which proteins are on the outer coat layer?

A

Sec 13/31

95
Q

What type of proteins are on the inner coat layer?

A

Sec 23/24

96
Q

T or F: sec proteins help form only the inner layer of the COPII coat

A

False, sec proteins help form both the inner and outer layer of the COPII coat

97
Q

How is the COPII coat disassembled? When does this happen?

A

When it reaches its target membrane, hydrolysis of GTP turns Sar1 off and the coat disassembles

98
Q

What direction of transport does COPI mediate?

A

backward transport within the golgi or from golgi to ER

99
Q

Why would we need backward transport in the Golgi?

A

ER resident proteins or cis-Golgi/medial Golgi proteins can accidentally get packaged into vesicles and sent forwards out of their home compartment

100
Q

How does the cell identify which proteins might need to be transported backward?

A

a KDEL sequence that is distinct from the original ER signal sequence

101
Q

Describe the structure of clathrin subunits

A

subunits consists of 6 polypeptide chains (3 heavy chains and 3 light chains) that together form a triskelion structure

102
Q

How do triskelions assemble? What is the final product?

A

into a series of hexagons and pentagons to make a misshapen sphere

103
Q

What are clathrin proteins bound to?

A

an inner layer of adaptor proteins

104
Q

When bound to clathrin proteins, what else do adaptor proteins bind to? what does this do?

A

the cargo receptors - this will capture the soluble molecule of interest

105
Q

what does the clathrin coat induce in the membrane?

A

induces membrane curvature and then many other proteins help to pinch off the vescile

106
Q

What happens to the clathrin coat after the transport vesicle is pinched off?

A

it quickly leaves

107
Q

Which two coats will shed quickly after the after the transport vesicles form?

A

clathrin and COPI

108
Q

What happens to the COPII coats after the vesicles form?

A

it remains on the vesicle until they dock with the target membrane and Sar1 is inactivated again

109
Q

What has to happen for the COPII coat to disassemble?

A

the vesicle has to dock on the target membrane and Sar1 must inactivate by GTP hydrolysis

110
Q

What prevents a transport vesicle that has budded off the ER and is moving to the Golgi complex from fusing to the wrong compartment?

A

a tethering and docking process regulated by Rab G-proteins and SNAREs

111
Q

What regulates the tethering and docking process that prevents transport vesicles from fusing to the wrong compartments?

A

Rab G-proteins and SNAREs

112
Q

What is Rab? Where do they reside?

A

the largest family of monomeric GTPases

they reside on the cytosolic surface of many organelles

113
Q

What is the function of Rab-GTP?

A

it is involved in initially tethering an incoming vesicle to the target membrane

114
Q

What is one way Rab-GTP tethers an incoming vesicle to the target membrane?

A

a filamentous effector protein on the target membrane extends into the cytosol and binds to Rab-GTP on the vesicle membrane

115
Q

What is the purpose of Rab effectors?

A

they ensure the target protein stays on the vesicle

116
Q

Where are SNARE proteins located?

A

on both the vesicle and target membranes

117
Q

What do the SNARE proteins do?

A

SNAREs on the vesicle bind to SNAREs on the target membrane to dock the vesicle on the membrane and initiate membrane fusion

118
Q

What does the vesicle docking on the membrane cause?

A

the initiation of membrane fusion

119
Q

How do SNARE proteins exist?

A

as complementary sets = 1 v-SNARe and one organelle specific t-SNARE

120
Q

What are the SNARE sets composed of?

A

1 v-SNARE + 1 organelle-specific t-SNARE

121
Q

T or F: t-SNARES and v-SNARES can exist on their own

A

FALSE, they will ALWAYS be a pair

122
Q

What facilitates membrane fusion?

A

the tight binding of a v-SNARE and t-SNARE pair

123
Q

What needs to be properly incorporated into budding vesicles for proper endomembrane transport?

A

cargo proteins

v-SNAREs

124
Q

How many types of acid hydrolases does the lysosome contain?

A

more than 40

125
Q

What pH do the acid hydrolases in the lysosome work best at?

A

low pH (acidic)

126
Q

Are acid hydrolases initially active or inactive?

A

inactive

127
Q

What do acid hydrolases in the lysosome require to be activated?

A

proteolytic cleavage

128
Q

Most resident lysosomal membrane proteins are ____

A

highly glycosylated

129
Q

Why are most resident lysosomal membrane proteins highly glycosylated?

A

to protect them from the hydrolases in the lumen

130
Q

what establishes the low acidity in the lysosome?

A

a V-type pump

131
Q

Are lysosomes heterogenous or homogenous? why?

A

heterogenous because they receive different cargo from multiple places (Golgi, EC environment, cytosol)

132
Q

What often matures into lysosomes?

A

endosomes

133
Q

How do endosomes mature into lysosomes?

A

they fuse with each other and their contents degrade

134
Q

How are newly synthesized acidic hydrolases targeted to the lysosome?

A

a mannose 6 phosphate ‘tag’

135
Q

Where is the newly synthesized acidic hydrolase targeted for the lysosome tagged? Where do they transport from?

A

the M6P tag is added in the cis Golgi

they transport from the trans Golgi network to the lysosome

136
Q

How can M6P tagged proteins be directed from the trans Golgi network to the lysosome?

A

a mannose 6 phosphate receptor in the trans Golgi network isolates the tagged proteins into budding vesicles cloated in clathrin

137
Q

Where is the M6P-bound protein released? What happens to the receptor?

A

the M6P-bound protein is released at the lower pH of endosomes

the receptor is recycled

138
Q

What is the default endomembrane pathway for constitutive secretion?

A

Movement from the Trans Golgi Network to the cell surface

139
Q

Does directing proteins from the TGN to the plasma membrane require an additional (to the original ER signal sequence) signal? Why?

A

No because movement from the TGN to the cell surface for constitutive secretion is the DEFAULT endomembrane pathway

140
Q

What type of compounds would move through the default endomembrane pathway?

A

Compounds destined for the ECM (ex. proteoglycans)

141
Q

When is regulated secretion used?

A

when certain cell types need to produce high volume if products on demand

ex. hormones, neurotransmitters, digestive enzymes

142
Q

Where are the products of regulated secretion stored? Are they stored in high or low concentrations?

A

stored in high concentrations in secretory granules

143
Q

When are the products of regulated secretion released from secretory granules?

A

when exocytosis is triggered

144
Q

How is the content of a secretory vesicle concentrated?

A

clathrin-coated vesicles retrieve excess membrane and excess luminal content so that what is left is just the cargo

145
Q

How does insulin relate to the secretory pathways? What technique can be used to study this?

A

beta cells of the pancreas secrete insulin via regulatory secretion from the TGN to the cell surface

anti-clathrin antibodies can be bound to gold particles and visualized (immunogold)

146
Q

After cargo is transferred to the cell surface in high concentrations in a mature secretory granule, what must they wait for before they are released?

A

an exocytosis signal

147
Q

T or F: migration in a secretory granule of a neuron from the TGN to the cell surface is a short migration

A

FALSE, the secretory granules are packaged in the CELL BODY and must travel down the AXON to wait at the SYNAPSE

this can be more than a meter

148
Q

Why might it take a long time for the secretory granules of a neuron to reach the cell surface?

A

because they are packaged in the cell body and must travel down the axon to reach the synapse

149
Q

T or F: it is always the same signal that triggers exocytosis

A

false, it can vary (ex. action potential or hormone binding to a receptor)

150
Q

What does the exocytosis signal usually increase?

A

intracellular Ca2+

151
Q

What does Ca2+ control?

A

the SNARE machinery and initiates exocytosis

BIOL 2321 - Cell Biology (26 decks)

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