Transport across Biological Membranes Flashcards
Where does phospholipid synthesis occur?
in the outer cytosolic leaflet of the ER membrane
How are phospholipids distributed equally between the outer and inner membranes of the ER membrane?
The ER enzyme scramblase catalyses a flip-flop movement (e.g trans-bilayer movement)
What happens to newly synthesised membranes?
Transported to plasma membrane and to other organelles
What does the inner luminal leaflet of the ER become on the plasma membrane?
the extracellular leaflet of the plasma membrane
How does the inner luminal leaflet of the ER become the extracellular leaflet of the plasma membrane?
A vesicle of membrane buds off the ER.
The vesicle fuses with the membrane in a ay that the inner luminal leaflet is now on the outside
Explain how the asymmetric distribution glycolipids occurs in the plasma membrane?
- Enzymes that add sugar are only found in the inner luminal leaflet of the ER
- So when the vesicles fuse with the plasma membrane it ends up on the outside of the cell
Where does glycosylation occur?
In the lumen of the ER/golgi
Where are the glycolipids located?
On the extracellular leaflet of the plasma membrane
Based on sphingosine
Where are integral membrane proteins found?
Embedded into the membrane
Where are peripheral membrane proteins located?
On the cytoplasmic side or extracellular side when in association with integral membrane proteins
What are the three types of transmembrane protein?
Single pass transmembrane protein (alpha helix)
Multi-pass transmembrane protein (multiple α-helices)
Multi-pass transmembrane protein (B-sheets rolled up into a B-barrel)
What do soluble proteins for export contain at their N-terminal end?
A 15-20 amino acid hydrophobic signal peptide
How are integral transmembrane proteins inserted into the membrane?
- Signal peptide directs the growing polypeptide chain as it emerges from the ribosome to a translocator in the ER membrane
- The growing polypeptide chain is threaded through the membrane (translocation)
- Signal peptide is cleaved by signal peptidase and newly synthesised protein is released into ER lumen
How does the integration of a single pass transmembrane protein occur?
- Some proteins contain both a signal (start-transfer) peptide and a stop-transfer sequence
- Signal peptide directs the growing polypeptide chain to the translocator and the growing chain is threaded through membrane
- When translocator interacts with stop-transfer sequence translocation ceases and the translocator discharges the protein laterally into the bilayer
- Signal peptide is cleaved
How does the integration of a double pass transmembrane protein occur?
- Double pass membrane spanning proteins have an internal start-transfer peptide and a stop-transfer sequence
- Polypeptide synthesis is initiated in the cytoplasm, signal peptide directs the growing chain to the translocator and chain is threaded through
- When the translocator interacts with the stop-transfer sequence translocation stops and translocator discharges protein into bilayer laterally
- Signal peptide is not cleaved
