intracellular trafficking and membrane transport Flashcards
1
Q
what is the organelle type in eukaryotic cells
A
membrane bound organelles
2
Q
what is the membrane composed of in eukaryotic cells & name x2 characteristics if it
A
= phospholipid bilayer
- hydrophilic polar heads = interact with the aqueous environment both inside and outside of a cell
- hydrophobic hydrocarbon tails that interact to exclude water
3
Q
what is the role of the cell membrane x1
what are the roles of organelle membrane x2
A
- prevent contents of the cell from escaping & mixing with its surrounding environment
- lipid membranes enclosing organelles act as containers for proteins & other soluble molecules = preventing them from freely mixing with proteins & molecules within the cytoplasm
- organelle in membrane enables the cell & individual organelles to maintain different conditions (e.g., concentration gradients or different pH)
4
Q
define ectoplasmic face
A
faces outward environment
5
Q
define cytosolic face
A
faces into the cell
6
Q
what does transport of molecules inside & outside of cell depend on
A
size, charge, polarity & permeability of the membrane
7
Q
why do cells need access from the outside environment x3
A
- need materials for biosynthesis
- need materials for energy production
- excrete waste
8
Q
characteristic of the interior & exterior of cell membrane
A
- hydrophobic interior
2. hydrophilic exterior
9
Q
can hydrophilic molecules cross the hydrophobic membrane interior without help
A
no
10
Q
what facilitates the transport of the hydrophilic substances into membrane & out
A
= highly selective transporter & channel proteins that span the bilayer and allow these substances (e.g. proteins, molecules, ions) to be imported or exported
11
Q
2 types of diffusion across membrane
A
- simple diffusion
2. facilitated diffusion
12
Q
what does the rate of simple diffusion rely on
A
depends on its relative hydrophobicity and the size of the molecule
13
Q
can ions cross with simple diffusion
A
no
14
Q
order in terms of the rate of speed that these substances cross via simple diffusion:
- large uncharged polar molecules
- small uncharged polar molecules
- hydrophobic molecules
- ions
A
- hydrophobic molecules travel the fastest through
- next small uncharged polar molecules but some can’t get in
- slowest is large uncharged polar molecules and some can’t get in
- ions can’t travel through via simple diffusion at all
15
Q
define facilitated diffusion
A
membrane transport proteins facilitate the movement of hydrophilic solutes across the bilayer membrane without them needing to interact with the hydrophobic interior
16
Q
what are the two types of facilitated diffusion
A
protein transporters & channels
17
Q
two types of transport through the membrane transporters & channels
A
passive or active (input of energy) transport
18
Q
define passive transport
A
All channels and many transporters allow solutes to spontaneously cross the membrane bilayer and travel down a concentration gradient
19
Q
define active transport
A
pumping of substances against the concentration gradient
20
Q
does active transport occur in channels and transporter proteins?
A
no only transporter proteins
21
Q
3 types of binding sites on a transporter
A
- Outward open: the binding sites for the solute are exposed to the outside
- Occluded: the sites are not accessible from either side
- Inward open: the binding sites are exposed on the inside of the bilayer
22
Q
how does the transition between outward & inward states in transporter proteins occur & what’s it called?
A
when solute concentrations are higher outside of the cell, more solute will bind to the transporter in the outward conformation, when the transporter switches conformation, there will be a net transport of solute down its concentration gradient into cell
= spontaneous
23
Q
how is the energy supplied in active transport
A
supplied via the hydrolysis of ATP, or through coupled pumps
24
Q
define coupled transporters (type of active transport)
A
combine the movement of one solute to that of another.
= electrochemical gradient of one solute moving down its concentration gradient, is used to drive the active transport of another solute that is moving against its concentration gradient (e.g. sodium molecule travelling down concentration gradient drags glucose molecule with it)
25
define symporter
transporter pump moves both solutes in the same direction
26
define antiporter
two solutes are transported in opposite directions e.g. sodium potassium channel
27
example of a coupled transporter
Sodium-Glucose Symporter
28
where is a Sodium-Glucose Symporter found
= found on the apical membrane of the epithelial cells of the small intestine
29
what is transported in Sodium-Glucose Symporter
- as sodium moves into the epithelial cells, it travels down its electrochemical gradient (sodium concentration lower inside)
- energy stored within sodium electrochemical gradient is used to drag glucose into the epithelial cell against its concentration gradient (glucose concentration higher inside cell)
30
The greater the electrochemical gradient for Na+ the.... (Sodium-Glucose Symporter)
the more Na+ moves into the epithelial cell, simultaneously bringing more glucose into the cell
31
how does it transition between states in Sodium-Glucose Symporter
When Na+ binds to the pump in the outward open state, glucose will also bind. The occluded intermediate state will only be formed when both substrates are bound, before transitioning to the inward open state
32
what drives movement in ATPase pumps active transport)
hydrolyse ATP to drive the transport of a solute against an electrochemical gradient
33
what are the four types of ATPase (active transport)
1. P-type pumps = phosphorylate themselves during the pumping cycle --> transports Na+, K+, H+, and Ca2+
2. ABC transporters = pump small molecules across cell membranes = largest fam
3. V-type pumps = turbine-like protein, multiple subunits, pump H+ into organelles e.g. lysosomes, to acidify the interior of these organelles.
4. F-type ATPases (ATP synthases) similar to V pumps structurally
- found in mitochondria
- They work in reverse by using a H+ gradient across the membrane to drive the synthesis of ATP from ADP and phosphate
34
what does ABC transporters stand for
ATP-Binding Cassette transporters
35
example of P-type (active transport)
Sodium-Potassium ATPase Pump
36
how does the Sodium-Potassium ATPase Pump work & where is it found
- found on the plasma membrane of a cell
1. the hydrolysis of ATP induces a conformational change in the transporter
2. drives the exchange of 3x Na+ out of the cell, against its concentration gradient, with the pumping of 2x K+ into the cell, against its concentration
= action creates a negative environment within the cell and contributes to the membrane potential.
37
define channel proteins
form transmembrane pores across the bilayer and allow the passive movement of small water soluble molecules
38
what do channels facilitate the transport of
inorganic ions e.g. Na+, K+, Ca2+, or Cl–
39
are ion channels selective
yes & dependent on the diameter, shape of the ion channel & distribution of the charged amino acids which line the inner surface of the channel
40
types of ion channels x4
1. voltage-gated
2. ligand-gated (extracellular ligand)
3. ligand-gated (intracellular ligand)
4. mechanically gated
41
what is faster: ion channels or transporter proteins
ion channels because they don't need to undergo any conformational changes
42
when do voltage gated ion channels open
they open when there is a change in voltage across the membrane
43
when do ligand-gated ion channels open
when a molecule binds to it
44
when do mechanical gated channels open
opens in response to vibrations or pressure
45
what creates a membrane potential
Differences in ion concentration inside and outside of the cell creates a membrane potential, with the inside of the cell being more negatively charged than the exterior
46
what makes a resting membrane potential
when the exchange of cations (+ ions) and anions (- ions) across the membrane is balanced and the voltage difference remains unchanged
47
what changes membrane potential
when ions flow through open ion channels is the basis of electrical signalling in many cells, particularly nerve & muscle cells
48
what charge are channels at resting potential & why
negative
potassium concentration higher inside the cell = slowly moves outside
49
define depolarisation
a shift in the membrane potential to be less negative inside
50
what triggers action potential
depolarisation
51
what triggers voltage-gated sodium channels to open in skeletal muscles
depolarisation = allows influx of positive sodium into the cell as it travels down its electrochemical gradient = further opening of channels = then when net flow of sodium is zero channels automatically inactivate = voltage gated potassium channels open to restore the membrane potential to initial neg value
52
how does the depolarisation travel
as one sodium channel opens the next one does cause a mexixan wave like action, the depolarisation travels along the nerves and repolarisation after
53
what happens when the depolarisation reaches a nerve terminal in a presynaptic cell
it stimulates the terminal to release its neurotransmitter
54
what does the neurotransmitter bind to & what does this cause
binds to ligand gated ion channels on the postsynaptic target opens these channels at the synapse
= resulting ion flows alter the membrane potential of the postsynaptic membrane, thereby transmitting a signal from the excited nerve
55
define aquaporins
specialised channel proteins that facilitate the movement of water across the membrane (help osmosis)
56
how does the structure of aquaporins benefit them
allow the passive diffusion of uncharged water molecules while blocking the movement of ions through the channel
57
can water travel through ion channels
allows the movement of ions while blocking water from flowing through
58
2 stimulus that open gated channels
change in membrane potential (voltage-gated channels) or the binding of a neurotransmitter to the channel (ligand-gated channels).
59
three mechanisms of protein transport between organelles
1. movement of proteins from the cytosol into the nucleus
2. movement of proteins from cytosol to mitochondria
3. movement of proteins from the cytosol to ER
= use pores and translocator proteins to enable transport
3. vesicular transport
= movement of proteins from the ER to the plasma membrane, and from the plasma membrane to lysosomes.
60
role of vesicles
collect cargo (e.g. soluble proteins, carbohydrates) from one compartment, and transport them to another compartment
61
are vesicles selective of cargo & target membrane
yes
62
the types of vesicle pathways
1. secretory pathway
| 2. endocytic pathway
63
what happens in secretory pathway
protein molecules are transported from the ER to the plasma membrane or (via endosomes) to lysosomes
64
what happens in endocytic pathway
molecules are ingested in endocytic vesicles derived from the plasma membrane and delivered to early endosomes and then (via late endosomes) to lysosomes.
65
what is the principle of vesicular transport = the simple process of transporting proteins from one organelle to another
Transport vesicles containing proteins (cargo) from the lumen of an organelle bud off from that membrane (donor) and fuse with the membrane of another organelle (target) & when they fuse with the target membrane they release their cargo into the lumen
66
how are transport vesicles formed
formed from special coated regions of membrane that bud off as coated vesicles, & have distinctive proteins covering their cytosolic surface
67
what proteins are in the inner coat layer of a membrane
adapter proteins
68
role of adapter proteins & what is their action
determine the selection of cargo molecules, and what part of the membrane will be turned into a vesicle
= they do this by binding both the coat protein and the membrane-bound cargo receptors
69
what is the action of the outer coat layer and what protein is recruited
assembles into a curved lattice that deforms the membrane patch and which leads to the formation of a coated bud, or a coated pit if it is in the plasma membrane
- protein dynamin is recruited
70
dynamin
membrane-bending protein = recruited to the neck of the budding vesicle, where a sharp membrane curvature is introduced, and the vesicle buds off
71
do coat proteins remain
rapidly lost shortly after the vesicle buds off as fusion of the vesicle to the cytosolic face of the target membrane, require the membranes to closely interact
72
what are the three types of coat proteins used for vesicle transport
Clathrin coated vesicles
COPI coated vesicles
COPII coated vesicles
73
role of Clathrin coated vesicles
mediate transport from the Golgi and from the plasma membrane = endocytosis
74
role of COPI coated vesicles
mediate transport from the Golgi to the ER (retrograde direction)
75
role of COPII coated vesicles
mediate transport from the ER and to Golgi cisternae (anterograde direction)
76
what is the role of Coat-recruitment GTPases
control the assembly of clathrin, COPI ,and COPII coats on endosome, Golgi, and ER membranes respectively
77
where are Coat-recruitment GTPases found & what state
- high concentration in the cytosol
| - GDP-bound inactive state
78
what are some Coat-recruitment GTPases & their role
ARF proteins = responsible for the assembly of COPI and clathrin coats at Golgi membranes
Sar1 protein = responsible for the assembly of COPII coats at the ER membrane
79
role of monomeric GTpases
GTP bound =
GDP bound =
catalyse GTP hydrolysis and act as molecular switches that activate and inactivate proteins based on whether GTP, or GDP following hydrolysis, is bound.
GTP = active
GDP = inactive
80
what regulates binding GTP
Guanine exchange factors (GEFs)
81
role of Guanine exchange factors (GEFs)
promotes GTP binding and activation, and GTPase-activating proteins which promote GTP hydrolysis to GDP and inactivation
82
proteins composing COPII coated vesicles
= composed of 5 seperate proteins
The inner coat = Sar 1, Sec 23 and Sec 24 proteins
outer coat = Sec13 and Sec31 proteins
83
role of sar1 protein
is a GTPase which controls the assembly of COPII coats at the ER membrane.
84
steps in assembly of the COPII coated vesicles by GTPases x8
inner coat then outer coat
1. Inactive, soluble Sar1-GDP binds to a Sar1-GEF in the ER membrane, causing the Sar1 to release its GDP and bind GTP.
2. A GTP triggered conformational change in Sar1 exposes an amphiphilic helix, which inserts into the cytoplasmic leaflet of the ER membrane, initiating membrane bending
3. GTP-bound Sar1 binds to a complex of two COPII adaptor coat proteins, called Sec23 and Sec24, which form the inner coat.
= inner coat
4. The entire surface of the complex that attaches to the membrane is gently curved, matching the diameter of COPII-coated vesicles.
4. Sec13 and Sec 31 form the outer shell of the coat. they assemble on their own into symmetrical cages with appropriate dimensions to enclose a COPII-coated vesicle.
5. Membrane-bound, active Sar1-GTP recruits COPII adaptor proteins to the membrane.
6. They select certain transmembrane proteins and cause the membrane to deform.
7. The adaptor proteins then recruit the outer coat proteins which help form a bud.
8. A subsequent membrane fusion event involving the GTPase protein, dynamin, facilitates the pinching off of the coated vesicle.
85
define Rab proteins
they are GTPases found on both transport vesicles and target membranes that help to guide transport vesicles to their target membranes.
86
SNARE proteins
catalyse membrane fusion
87
the process of vesicles transporting to their target membranes = how they are guided
involves: Rab proteins & SNARE proteins
1. Rab effector proteins interacts with active Rab proteins (Rab-GTPs) located on the target membrane to establish the first connection between the two membranes that are going to fuse.
2. SNARE proteins on the two membranes pair, dock the vesicle to the target membrane and catalysing the fusion of the two lipid bilayers.
When lipid bilayers of two membranes are close together (<1.5nm), lipids can move from one to the other and fuse.
3. V-SNAREs on the vesicle and t-SNAREs on the target membrane interact to form a trans-SNARE complex which holds the vesicle tightly to the target membrane so that membrane fusion can occur.
4. During docking and fusion, a Rab-GAP induces the Rab protein to hydrolyse its bound GTP to GDP, causing the Rab to dissociate from the membrane and return to the cytosol as Rab-GDP, where it is inactive.
88
are SNARE interactions highly specific
yes
89
where are proteins made and modified
ER
90
where are proteins further modified and sorted
golgi apparatus
91
define glycosylation and where it occurs
short oligosaccharide side chains are attached to protein in ER
92
reason for glycosylation
1. protect protein from degradation
2. hold the protein in the ER until properly folded
3. used as a transport signal to help guide it to the appropriate organelle
93
what proteins are retained within the ER
missfoled proteins
they are then bound to chaperones for proper folding or sent to cytosol where degraded
94
protein transport out of cell pathway
1. ER proteins are first packaged into COPII-coated transport vesicles.
2. After transport vesicles have budded from the ER and have shed their coat, they begin to fuse with one another to form vesicular tubular clusters.
3. The vesicular clusters move along microtubule tracks to the Golgi, where they fuse with one another to form the cis Golgi network.
95
why do vesicular tubule clusters seperate from ER
Vesicular tubule clusters are separate from the ER as they lacks many of the proteins that function in the ER.
96
define anteograde
(forward) transport is mediated by COPII vesicles - ER to Golgi eg newly synthesised secretory or transmembrane proteins
97
define retrograde
(reverse) transport is mediated by COPI vesicles – eg recycling of membrane lipids and proteins such as SNARE proteins
98
what are the steps in movement from the golgi apparatus
1. Soluble proteins and membrane enter the cis Golgi network through vesicles derived from the ER.
2. proteins are sorted and travel through the cisternae via vesicles that bud from one cisternae and fuse with the next.
3. As they move through the Golgi cisternae, proteins are modified through the addition of oligosaccharide chains.
4. At the trans Golgi network, proteins are sorted according to their destination, and exit through the trans face packaged in Golgi derived vesicles.
99
what are golgi cisternae
ordered collection of flattened membrane enclosed sacs in the golgi apparatus
100
two faces of golgi cisternae
1. entry/cis face which is adjacent to the ER
| 2. exit or trans face which points towards the plasma membrane.
101
steps for newly synthesised proteins destined for lysosomes from the golgi
1. transported from the lumen of the ER
2. through the Golgi network
3. exit the trans Golgi network within clathrin-coated transport vesicles (= endosomes),
4. before being transported to lysosomes
102
define exocytosis
transport from the Golgi to the plasma membrane
103
two secretory pathway of exocytosis
1. a constitutive pathway
| 2. regulated secretory pathway.
104
define constitutive pathway x2
1. soluble proteins are continuously secreted from the cell
| 2. pathway also supplies the plasma membrane with newly synthesised membrane lipids and proteins
105
define regulated secretory pathway
where soluble proteins and other substances are initially stored in secretory vesicles for later release by exocytosis
106
where is regulated secretory pathway found & what is secreted through this pathway
1. found in cells specialised for secreting products when extracellular signals stimulate their secretion.
2. Hormones, neurotransmitters, and digestive enzymes are often secreted via this pathway
107
where does vesicle formation begin in endocytosis
at clathrin-coated pits
108
what are at clathrin-coated pits
specialized patches at the plasma membrane that concentrate receptors, curve to form an invagination and bud off with their receptor cargo
109
what does the cargo in vesicles contain
membrane components and soluble molecules from the lumen of each compartment
110
define endosome
a vesicle formed by the invagination and pinching off of the cell membrane during endocytosis
111
difference between early endosome and late endosme
late are more mature and found closer to the nucleus
112
what happens to some endocytosed molecules
1. retrieved from early endosomes and returned (some via recycling endosomes) to the cell surface for reuse
2. retrieved from the early and late endosomes and returned to the Golgi, and some are retrieved from the Golgi apparatus and returned to the ER
113
define endocytosis
cells take substances from outside of the cell and bring them into the cell
114
Endocytosed cargo can include...
receptor– ligand complexes, macromolecule nutrients, extracellular matrix components, cell debris, bacteria and viruses, and even other cells.
115
define pinocytosis
the material progressively enclosed by a small portion of the plasma membrane, which first invaginates and then pinches off to form a vesicle containing the ingested substance or molecule = formation of endocytic vesicles called pinocytic vesicles
116
how does the cell's surface area remain unchanged after endocytosis
same amount of membrane being removed by endocytosis is being added to the cell surface by exocytosis
117
steps in endocytosis
1. the clathrin-coated pit invaginates into the cell and pinches off to form a clathrin-coated vesicle
2. coated vesicles promptly shed their clathrin coat and fuse with the early endosome where the internalised cargo is sorted
3. As the early endosome moves from the cell periphery to a location closer to the nucleus it matures into a late endosome.
4. During this transition, changes to the protein composition of the endosome membrane occurs as it stops recycling material to the plasma membrane and commits its remaining contents to degradation.
5. Late endosomes fuse with one another and with lysosomes to form endolysosomes which degrade their contents.
118
how are lysosomes formed
When most of the endocytosed material within an endolysosome has been digested so that only resistant or slowly digestible residues remain, these organelles become classical lysosomes
119
how are endolysomes formed
newly synthesised lysosomal hydrolases fuse with late endosomes & this fuses with existing lysomes
120
what are the four pathways that feed substances into lysosomes for digestion
1. Endocytosis: the taking up of macromolecules from extracellular fluid via vesicles.
2. Phagocytosis of large particles and microorganisms by macrophages and neutrophils
3. Macropinocytosis: nonspecific uptake of fluids, membrane, and particles attached to the plasma membrane.
4. Autophagy: self-digestion of the cell’s own cytosol and worn-out organelles
121
what are the steps in receptor mediated endocytosis
- of low density lipoproteins
1. An LDL receptor on the cell surface binds to an LDL particle
2. Clathrin coated pits on the plasma membrane containing the LDL receptor-ligand complex bud inward to form a coated transport vesicle
3. The clathrin coat is then shed becoming an early endosome. The early endosome fuses with a late endosome
4. The acidic pH in this compartment causes a conformational change in the LDL receptor that leads to the release of the bound LDL particle.
5. Late endosome fuses with a lysosome and the proteins and lipids contained within the LDL particle are broken down by enzymes in the lysosome.
6. The LDL receptor is recycled back the plasma membrane where it can bind another LDL particle.
