Vesicular Transport Flashcards
Where do membrane bound vesicles carry cargo?
In their lumen
Vesicles carry cargo from a _______ to a _______ compartment.
Donor
Recipient
Where do vesicles bud from?
Specialised coated regions of the donor compartment
Where are vesicles coated in COPII travelling from and to?
ER to golgi
Where are vesicles coated in COPI travelling from and to?
Golgi to ER
Where are vesicles coated in Clathrin travelling between?
Golgi, lysosomes and the plasma membrane
What is the structure of a clathrin subunit?
3 large and 3 small polypeptide chains which form a 3 legged triskelion structure
How do the individual subunits interact to form clathrin?
triskelions create a basket like framework of pentagons and hexagons
Which side of the membrane is clathrin found on?
The cystolic side
What is adaptin?
A protein complex which attaches clathrin to the membrane as well as interacting with transmembranous proteins
What is dynamin?
A protein which binds in a ring to the neck of a vesicle allowing it to pinch off
What do vesicles have on their surface which identifies the cargo and origin of the vesicle?
V-snare proteins
Where are complementary receptors (of vesicle surface proteins) found?
On the target membrane
What are v-snare proteins complementary receptors called?
T-snare proteins
What happens when v and t snares meet?
They wrap around each other forming a stable trans-snare complex which locks the two membranes together
How many different pairs of snares are there?
20
What is exocytosis?
Secretion from the cell
Where do the molecules that are secreted go?
Into the extracellular matrix
To the plasma membrane
Why would a protein that has entered the golgi not be secreted?
If it is returned to the golgi
If it is retained in the golgi
If it is directed to another organelle
What are the different pathways in which a molecule can be excreted from the cell?
Constitutive pathway
Regulated secretory pathway
Which is the default pathway for exocytosis?
Constitutive pathway
Describe the process of a protein being exocytosed in the constitutive pathway.
- The vesicle buds from the golgi
- The vesicle is headed to the plasma membrane
- The vesicle fuses with the plasma membrane immediately
- The vesicle contents is exocytosed
Describe the process of a protein being exocytosed in the regulated secretory pathway.
- The vesicle buds from the golgi
- The vesicle is headed to the plasma membrane
- The vesicle only fuses with the plasma membrane once a signal is received
- The vesicle contents is exocytosed
What type of proteins are exocytosed in the secretory pathway?
Secretory proteins
What does the regulated secretory pathway allow for?
Storage of secretory proteins
Further processing of the protein
Release of the protein in response to a trigger
Why is storage of secretory proteins important in the regulated secretory pathway?
Because it protects the cells from substances that may be harmful to it - hydrolytic enzymes
Why is further processing of secretory proteins important in the regulated secretory pathway?
Because some proteins are synthesised in inactive forms and therefore can be activated before being exocytosed
What is proteolysis?
The modification of proteins by breaking them down into smaller polypeptides or amino acids
What might vesicles do whilst waiting for a signal to exocytose their contents?
‘Dock’ with the plasma membrane but not fuse
Give three examples of substances that might be exocytosed from a cell in response to a signal.
Digestive enzymes in response to food in the gut
Insulin in response to high blood glucose levels
Histamine in response to the presence of antigens
What is endocytosis?
Uptake into a cell
When must the extra components found in the plasma membrane by removed and recycled?
After exocytosis because the surface area is increased
What does the endocytotic-exocytotic cycle do?
Keep a balance between secretion and internalisation
What percentage of the plasma membrane is clathrin coated pits?
2%
Explain the process of pinocytosis.
- Clathrin coated pits rapidly invaginate and form vesicles
- Vesicles lose their coat and fuse with early endosomes
- Any extracellular constituents that have been taken up can be delivered to lysosomes for digestion
What substances are absorbed during receptor-mediated endocytosis?
Only receptor-specific substances
Proteins
Hormones
Metabolites
Explain the process of receptor-mediated endocytosis.
The plasma membrane invaginates forming a vesicle containing the absorbed substance which then buds off
Explain the process of phagocytosis.
- Specialised white blood cells take up large particles (microorganisms, dead cells, cell debris)
- The particle binds to the surface of the phagocyte and is recognised by cell surface receptors
- The cell extends pseudopods which engulf the particle forming a large endocytotic vesicle = phagosome
- The phagosome fuses with a lysosome and its contents is digested
