Intracellular Membrane Compartments Flashcards
Q1: What are intracellular membrane compartments and why are they important?
✅ Answer: Intracellular membrane compartments are membrane-bound organelles (e.g., ER, Golgi, lysosomes) that allow for spatial and temporal separation of cellular functions. They enable regulated protein sorting, trafficking, and signalling across different environments with specific pH, ion concentrations, and proteins.
Q2: What distinguishes prokaryotic from eukaryotic cells regarding membrane compartments?
✅ Answer:
Prokaryotes lack internal membrane-bound organelles.
Eukaryotes have complex internal membranes (ER, Golgi, lysosomes, etc.), allowing compartmentalized protein processing and signalling.
Q3: Distinguish between rough ER and smooth ER in structure and function.
✅ Answer:
Rough ER: Studded with ribosomes → synthesizes membrane-bound or secreted proteins.
Smooth ER: No ribosomes → involved in lipid and steroid synthesis (e.g., cholesterol)
What determines whether a protein is synthesized on free vs. bound ribosomes?
✅ Answer:
Proteins with an ER signal peptide are synthesized on ribosomes bound to rough ER.
Cytosolic proteins are synthesized on free-floating ribosomes in the cytoplasm.
What are the three key components for protein translocation into the ER?
✅ Answer:
ER signal sequence (on the growing protein)
Signal recognition particle (SRP)
SRP receptor on ER membrane
These components ensure proteins are properly translocated into the ER lumen or embedded in the membrane.
What are the two types of proteins translocated into the ER and their fates?
✅ Answer:
Water-soluble proteins: Fully enter the ER lumen → secreted or organelle-targeted.
Transmembrane proteins: Partly inserted into the ER membrane → become membrane proteins.
What happens to misfolded proteins in the ER?
✅ Answer: They are retrotranslocated back into the cytosol, deglycosylated, ubiquitylated, and degraded in proteasomes — ensuring quality control.
What are the roles of the Golgi apparatus in protein processing?
✅ Answer: The Golgi:
Modifies proteins (e.g., glycosylation, phosphorylation)
Sorts proteins at the trans-Golgi network (TGN) to one of three destinations:
Lysosome
Plasma membrane (constitutive secretion)
Regulated secretion vesicles
What is the difference between constitutive and regulated secretion pathways?
✅ Answer:
Constitutive: Continuous secretion (e.g., ECM proteins)
Regulated: Proteins are stored in vesicles and secreted only upon stimulus (e.g., insulin)
What sorting signal directs proteins to lysosomes?
✅ Answer: Mannose-6-phosphate (M6P) is added in the Golgi to lysosomal hydrolases.
Recognized by M6P receptors in the TGN → vesicles bud and fuse with lysosomes.
What are the key components of vesicle transport systems?
✅ Answer:
Exit signal
Destination signal
Cargo receptor
Adaptor proteins
Coat proteins (e.g., COPI, COPII, clathrin)
Compare COPI, COPII, and clathrin vesicle coats.
COPII: ER → Golgi (anterograde): Forward trafficking
COPI: Golgi → ER (retrograde): Retrieval of ER-resident proteins
Clathrin: TGN → Endosomes/PM: Sorting at late stages of secretion
Outline the steps of exocytosis.
✅ Answer:
Vesicle tethering to plasma membrane
Docking via SNARE proteins (v-SNARE & t-SNARE)
Fusion of membranes
Release of contents
Recycling of vesicle membrane
How are SNARE proteins involved in vesicle fusion?
✅ Answer:
v-SNAREs (on vesicle) pair with t-SNAREs (on target membrane)
Facilitate membrane fusion through tight pairing, bringing membranes close enough to merge.
What is the function of the KDEL sequence?
✅ Answer: KDEL is a retrieval signal on ER-resident proteins that ensures they are returned from the Golgi back to the ER via COPI vesicles, maintaining ER function.
Why is trafficking considered a highly regulated process?
✅ Answer: Because:
Each protein has specific signals
Requires correct folding and modification
Uses energy-dependent mechanisms
Involves dozens of proteins for accuracy (e.g., GTPases, SNAREs, adaptors)
