Intracellular Compartments and the Endomembrane System

Question 1

how did eukaryotic cells evolve to have membrane-enclosed organelles

Answer 1

through a process of membrane expansion

Question 2

each organelle is separated from the cytoplasm by what

Answer 2

at least one phospholipid bilayer

Question 3

the compartments of membrane-enclosed organelles aid in what

Answer 3

the organization of metabolic processes

Question 4

do the number and relative volumes of organelles vary between cell type

Answer 4

yes

Question 5

organelle populations and characteristics are determined by what

Answer 5

cell specialization and function

Question 6

organelles enclose what

Answer 6

• specific molecules
• including proteins which facilitate a variety of functions

Question 7

how are ions and small molecules transported across membrane-enclosed organelles

Answer 7

• diffusion
• channels
• carrier proteins

Question 8

how are proteins created

Answer 8

transcription and translation

Question 9

where does transcription take place

Answer 9

nucleus

Question 10

where does translation take place

Answer 10

cytosol

Question 11

how do proteins reach their target destination

Answer 11

protein sorting

Question 12

proteins are transported to organelles by which 3 mechanisms

Answer 12

• pores
• protein translocators
• transport vesicles

Question 13

what are pores

Answer 13

selective gates that actively transport specific macromolecules and allow free diffusion of smaller molecules

Question 14

what do protein translocators do

Answer 14

transport proteins (typically unfolded) into organelles

Question 15

what do transport vesicles do

Answer 15

pinch off from the membrane of one compartment and then fuse w another

Question 16

proteins are targeted to specific organelles by what

Answer 16

AA sequences called signal peptides or signal patches

Question 17

what do signal peptides do

Answer 17

targets proteins to specific organelles

Question 18

are there distinct signal sequences for each organelle

Answer 18

yes

Question 19

what do signal sequences do

Answer 19

direct proteins to the correct destination

Question 20

what is a normal signal sequence vs a relocated signal sequence

Answer 20

• normal Proteins destined for the ER possess an N-terminal signal sequence that directs them to that organelle, whereas those destined to remain in the cytosol lack any such signal sequence
• relocated Recombinant DNA techniques can be used to change the destination of the two proteins: if the signal sequence is removed from an ER protein and attached to a cytosolic protein, both proteins are reassigned to the expected, inappropriate location.

Question 21

describe the structure of the nucleus

Answer 21

• nucleus is surrounded by 2 membranes (nuclear envelope)
• outer membrane is continuous w the ER
• envelope is perforated by nuclear pores

Question 22

describe the structure of nuclear pores

Answer 22

• comprised of a complex of ~30 proteins
• they act as gates that allows small molecules through, but selectively controls the transport of larger molecules

Question 23

what travels in and out of the nucleus

Answer 23

• RNA and ribosomal subunits need to leave nucleus
• proteins formed in cytoplasm need to enter

Question 24

how do proteins enter the nucleus

Answer 24

• directed by a nuclear localization signal
• nuclear import receptors bind to the signal sequence and guide the protein through the pore

Question 25

what drives the directional transport of nuclear proteins

Answer 25

hydrolysis of GTP by Ran

Question 26

what do GTPases do

Answer 26

hydrolyze GTP into GDP and Pi

Question 27

what does Ran do

Answer 27

cycles between GDP and GTP bound forms, w the aid of other enzymes (Ran-GAP, Ran-GEF)

Question 28

what happens when GTP is hydrolyzed in protein srtingg

Answer 28

- it is released - proteins w a nuclear localization signal can then bind

Question 29

what binds to the nuclear import receptor to allow the nuclear protein to be released

Answer 29

Ran-GTP (once in the nucleus)

Question 30

most mitochondrial proteins are encoded in the __________ and imported from the __________

Answer 30

- nuclear genome - cytosol

Question 31

proteins are transported into the mitochondria through what

Answer 31

- import receptor proteins - protein translocators embedded in the outer and inner membranes

Question 32

what are the steps to protein sorting to the mitochondria

Answer 32

1. proteins binds to import receptors 2. it diffuses across the outer membrane until it meets a translocator in the inner membrane 3. proteins are then moved into the mitochondrial matrix and the signal peptide is cleaved

Question 33

what is the most extensive membrane system in a eukaryotic cell

Answer 33

the ER

Question 34

what does the ER do

Answer 34

- involved in the sythesis of proteins (both for themself and other locations) - protein translocation occurs during translation at the rough ER

Question 35

what translates RNA into protein

Answer 35

ribosomes

Question 36

what happens to ribosomes that are translating proteins w an ER localization signal

Answer 36

they are directed to the ER membrane

Question 37

what is used to synthesize all proteins encoded by the nuclear genome

Answer 37

a common pool of ribosomes

Question 38

does translocating proteins w an ER signal take energy? why/why not?

Answer 38

- **no additional energy** - because they are being translocated as theya re being made

Question 39

what does SRP stand for

Answer 39

signal recognition particle

Question 40

how does protein sorting to the ER work

Answer 40

- SRPs bind to the signal sequence and ribosome, slowing translation - SPR binds to a receptor associated to the ER - ribosome is passed to a protein translocator and protein synthesis resumes at its normal rate - as the protein is synthesized, it enters the ER - eventually the polypeptide chain is released into the lumen as a soluble protein - the signal peptide is cleaved off and remains in the memebrane to be degreaded

Question 41

when do proteins enter the ER

Answer 41

while they are being synthesized by the ribosome

Question 42

how are transmembrane proteins sorted to the ER

Answer 42

- stop transfer sequences halt the translocation of proteins resulting in it being stuck through the lipid bilayer - once the stop sequence enters the translocator, it discharged the sequence into the bilayer - the rest of the protein is translated in the cytosol

Question 43

how are multi-pass transmembrane proteins sorted to the ER

Answer 43

- they have internal ER signal sequences - they contain multiple start and stop sequences - these sequences help anchor the protein across the membrane

Question 44

what affects localization of protein sorting to the ER

Answer 44

the presence and location of start and stop transfer sequences