MCB 2050 - Endomembrane System Exam Questions with Correct Answers Latest Update
2025/2026
Endomembrane system - Answers Dynamic, coordinated network of all the cell's organelles and
related structures (except peroxisomes, mitochondria and chloroplasts because transport is not
via vesicles).
What occurs in the endomembrane system? - Answers Large amounts of material (e.g. lipids,
proteins, etc.) are exchanged (trafficked) between each organelle/structure via small,
membrane-bound transport vesicles.
Distinct structure and function of the organelles in the endomembrane system are
characterized by: - Answers -Contain a particular set of proteins.
-Perform a unique set of activities.
-Provides compartmentalization and functional diversity.
-Conserved in eukaryotes.
-Dynamic structures.
Vesicle transport step 1: - Answers Cargo-containing vesicles buds off the donor membrane
compartment.
What is the role of vesicle coat proteins? - Answers -Select which donor membrane and lumens
cargo proteins can enter or not enter the nascent transport vesicle.
-Regulate vesicle formation and budding.
Vesicle transport step 2: - Answers Nascent vesicle is transported through the cytosol to the
recipient membrane compartment.
-Involves molecular motors and cytoskeleton highways (i.e. motor proteins direct vesicle
movement within the cell by linking to the vesicle surface and to the cytoskeleton element).
What is the role of vesicle receptor proteins? - Answers -Regulate the intracellular trafficking of
the vesicle to the proper recipient membrane.
-Regulate vesicle-recipient membrane fusion.
Vesicle transport step 3: - Answers Vesicles fuses with the proper recipient membrane
compartment.
-Vesicles (donor) membrane and lumenal cargo proteins are incorporated into the recipient
compartment.
Vesicle transport step 4: - Answers Entire process of budding and fusion is repeated and can
,occur in the reverse direction.
-Other receptor proteins regulate the recycling of any proteins that escaped to the recipient
compartment back to the donor membrane compartment.
Biosynthetic pathway - Answers Materials are transported from the ER to the Golgi to
endosomes and then to either lysosomes (referred to as vacuoles in plants and fungi/yeasts) or
in some instances, to the plasma membrane (PM).
-Proteins that end up in the lysosome are for lysosome function, not garbage.
Constitutive secretion (secretory pathway) - Answers ER-derived materials are continually
transported from the Golgi to the PM and/or released via exocytosis outside of the cell (i.e.
extracellular space).
-Secretory transport vesicles components are incorporated into the PM and vesicle lumens
cargo is released into the extracellular space.
Regulated secretion (secretory pathway) - Answers -Occurs only in specialized cells.
-ER-derived materials from the Golgi are stored in secretory granules.
-In response to a cellular stimuli, secretory granules fuse with the PM release their cargo into
the extracellular space via exocytosis.
Give an example of regulated secretion: - Answers Regulated release (secretion) of
neurotransmitters by nerve cells into the synaptic cleft.
Endocytic pathway - Answers -Operates in the opposite direction of the secretory pathway
(materials move into the cell).
-Materials from the PM (e.g. receptor proteins destined for degradation) and/or extracellular
space are incorporated into the cell via endocytosis and then transported to endosomes and to
lysosomes (vacuoles).
Three pathways of the endomembrane system: - Answers 1. Biosynthetic
2. Secretory (constitutive or regulated)
3. Endocytic
Endoplasmic reticulum structure: - Answers -Highly complex network of membrane-enclosed,
rod-like tubules and sheet-like cisternae (i.e. flattened sacs).
-Lumen = aqueous space inside of ER tubules and cisternae.
-Highly dynamic network in constant flux.
, Reticulons structure and function: - Answers -Mediate the shape of tubules and cisternae.
-Unique ER integral membrane proteins that possess a 'hair-pin' (V-shaped) secondary structure
and regulate ER membrane curvature ('bending').
ER subdomains: - Answers Distinct regions of the ER network that possess unique
morphologies and/or functions.
>20 ER subdomains each with a unique complement of proteins and membrane lipids that
mediate its distinct function(s).
Rough ER (subdomain) - Answers Mostly cisternae with bound ribosomes, involved in protein
and membrane phospholipid synthesis.
Smooth ER (subdomain) - Answers Mostly curved tubules lacking ribosomes, involved in Ca2+
storage and hormone synthesis.
Examples of ER subdomains: - Answers -Outer nuclear membrane: continuous with RER,
contains NUPs and attached ribosomes.
-Mitochondrial and Plasma membrane Associated Membranes (MAM & PAM): regions of the ER
that make direct contact with mitochondria or PM, involved in membrane lipid exchange.
-ER Exit Sites (ERES): regions where transport vesicles bud off from the ER en route to the Golgi.
Two sites for protein synthesis (translation in the cell): - Answers 1. Free ribosomes in the
cytosol
2. ER membrane-bound ribosomes
Fate of proteins synthesized at free ribosomes: - Answers Fate of the nascent protein in the
cytosol remains in the cytosol (e.g. glycolytic enzyme) OR targets (post-translationally) to the
proper intracellular destination (e.g. mitochondria, nucleus, chloroplasts, etc.).
Fate of proteins synthesized at the ER membrane-bound ribosomes: - Answers Remains in the
RER or localizes (moves laterally in the ER membrane) to another ER subdomain OR targets
from the ER to another post-ER compartment in the endomembrane system via transport
vesicles.
Co-translational translocation of a soluble protein into the RER lumen - step 1: - Answers -
Translation of mRNA begins on a free ribosome in the cytosol.
-N-terminus of the nascent, growing polypeptides emerges from the ribosome and contains a
signal sequence.
-Exposed signal sequence is recognized by the signal recognition particle (SRP).
2025/2026
Endomembrane system - Answers Dynamic, coordinated network of all the cell's organelles and
related structures (except peroxisomes, mitochondria and chloroplasts because transport is not
via vesicles).
What occurs in the endomembrane system? - Answers Large amounts of material (e.g. lipids,
proteins, etc.) are exchanged (trafficked) between each organelle/structure via small,
membrane-bound transport vesicles.
Distinct structure and function of the organelles in the endomembrane system are
characterized by: - Answers -Contain a particular set of proteins.
-Perform a unique set of activities.
-Provides compartmentalization and functional diversity.
-Conserved in eukaryotes.
-Dynamic structures.
Vesicle transport step 1: - Answers Cargo-containing vesicles buds off the donor membrane
compartment.
What is the role of vesicle coat proteins? - Answers -Select which donor membrane and lumens
cargo proteins can enter or not enter the nascent transport vesicle.
-Regulate vesicle formation and budding.
Vesicle transport step 2: - Answers Nascent vesicle is transported through the cytosol to the
recipient membrane compartment.
-Involves molecular motors and cytoskeleton highways (i.e. motor proteins direct vesicle
movement within the cell by linking to the vesicle surface and to the cytoskeleton element).
What is the role of vesicle receptor proteins? - Answers -Regulate the intracellular trafficking of
the vesicle to the proper recipient membrane.
-Regulate vesicle-recipient membrane fusion.
Vesicle transport step 3: - Answers Vesicles fuses with the proper recipient membrane
compartment.
-Vesicles (donor) membrane and lumenal cargo proteins are incorporated into the recipient
compartment.
Vesicle transport step 4: - Answers Entire process of budding and fusion is repeated and can
,occur in the reverse direction.
-Other receptor proteins regulate the recycling of any proteins that escaped to the recipient
compartment back to the donor membrane compartment.
Biosynthetic pathway - Answers Materials are transported from the ER to the Golgi to
endosomes and then to either lysosomes (referred to as vacuoles in plants and fungi/yeasts) or
in some instances, to the plasma membrane (PM).
-Proteins that end up in the lysosome are for lysosome function, not garbage.
Constitutive secretion (secretory pathway) - Answers ER-derived materials are continually
transported from the Golgi to the PM and/or released via exocytosis outside of the cell (i.e.
extracellular space).
-Secretory transport vesicles components are incorporated into the PM and vesicle lumens
cargo is released into the extracellular space.
Regulated secretion (secretory pathway) - Answers -Occurs only in specialized cells.
-ER-derived materials from the Golgi are stored in secretory granules.
-In response to a cellular stimuli, secretory granules fuse with the PM release their cargo into
the extracellular space via exocytosis.
Give an example of regulated secretion: - Answers Regulated release (secretion) of
neurotransmitters by nerve cells into the synaptic cleft.
Endocytic pathway - Answers -Operates in the opposite direction of the secretory pathway
(materials move into the cell).
-Materials from the PM (e.g. receptor proteins destined for degradation) and/or extracellular
space are incorporated into the cell via endocytosis and then transported to endosomes and to
lysosomes (vacuoles).
Three pathways of the endomembrane system: - Answers 1. Biosynthetic
2. Secretory (constitutive or regulated)
3. Endocytic
Endoplasmic reticulum structure: - Answers -Highly complex network of membrane-enclosed,
rod-like tubules and sheet-like cisternae (i.e. flattened sacs).
-Lumen = aqueous space inside of ER tubules and cisternae.
-Highly dynamic network in constant flux.
, Reticulons structure and function: - Answers -Mediate the shape of tubules and cisternae.
-Unique ER integral membrane proteins that possess a 'hair-pin' (V-shaped) secondary structure
and regulate ER membrane curvature ('bending').
ER subdomains: - Answers Distinct regions of the ER network that possess unique
morphologies and/or functions.
>20 ER subdomains each with a unique complement of proteins and membrane lipids that
mediate its distinct function(s).
Rough ER (subdomain) - Answers Mostly cisternae with bound ribosomes, involved in protein
and membrane phospholipid synthesis.
Smooth ER (subdomain) - Answers Mostly curved tubules lacking ribosomes, involved in Ca2+
storage and hormone synthesis.
Examples of ER subdomains: - Answers -Outer nuclear membrane: continuous with RER,
contains NUPs and attached ribosomes.
-Mitochondrial and Plasma membrane Associated Membranes (MAM & PAM): regions of the ER
that make direct contact with mitochondria or PM, involved in membrane lipid exchange.
-ER Exit Sites (ERES): regions where transport vesicles bud off from the ER en route to the Golgi.
Two sites for protein synthesis (translation in the cell): - Answers 1. Free ribosomes in the
cytosol
2. ER membrane-bound ribosomes
Fate of proteins synthesized at free ribosomes: - Answers Fate of the nascent protein in the
cytosol remains in the cytosol (e.g. glycolytic enzyme) OR targets (post-translationally) to the
proper intracellular destination (e.g. mitochondria, nucleus, chloroplasts, etc.).
Fate of proteins synthesized at the ER membrane-bound ribosomes: - Answers Remains in the
RER or localizes (moves laterally in the ER membrane) to another ER subdomain OR targets
from the ER to another post-ER compartment in the endomembrane system via transport
vesicles.
Co-translational translocation of a soluble protein into the RER lumen - step 1: - Answers -
Translation of mRNA begins on a free ribosome in the cytosol.
-N-terminus of the nascent, growing polypeptides emerges from the ribosome and contains a
signal sequence.
-Exposed signal sequence is recognized by the signal recognition particle (SRP).
