Vesicular Transport Flashcards
Endocytosis and exocytosis
Endocytosis is the capture of molecules from outside.
Exocytosis allows the secretion of molecules from the inside.
What are carbohydrates in the membrane for
What are the proteins for
Cell protection and tagging.
Transport functions
What does the lipid bilayer contain
And what is the structure of these like
Rich in amphipathic molecules.
Two fatty acids- phospholipids and sphingomyelin.
Phospholipids have glycerol and one oxygen ion on the phosphate
Sphingomyelin does not have a glycerol but it still has a phosphate.
What are three types of phospholipids
What are their charges
Phosphatidylethanolamine
Phosphatidylserine
Phosphatidylcholine
Serine is the only one with a negative charge and the rest are neutral.
How are the membranes flexible
unsaturated fatty acids provide some disorder and flexibility to the membrane
They have a double bond so cause a kink in the fatty acid tail
What is the structure of omega 3 and 6 and where are they from
Omega 3 has three double bonds. The first double bond is three carbons into the chain.
Omega 6 has two double bonds and the first is six carbons into the chain.
Omega 3 is made by sea plants and then fish eat it. Omega six is made by land plants and animals eat it.
They help to keep our membranes flexible.
What does cholesterol do
Which lipids have the most of it
It has a rigid structure and helps to seal the plasma membrane preserving internal molecules.
It fills the gaps between the phospholipids to stabilise the bilayer.
Phosphatidylcholine has the most.
Then sphingomyelin
Then phosphatidylserine has the least because of its negative charge.
What format are the double bonds normally in
What is exposed to water and what is in the middle
Cis.
Polar heads are exposed to water. Hydrophobic tails are in the middle.
Cholesterol has this structure too.
What is the charge on the inside of the membrane and why
Negative because many things inside the cell such as vesicles are also negative so they are repelled from the membrane.
What part of the membrane is phosphatidylserine found in and how much of it is there
When will it change its position in the membrane.
It is only found on the inner layer of the bilayer.
Because it makes up 4% of the whole membrane it makes up 8% of the inner membrane.
It flips to the outer surface only during apoptosis.
What is a peripheral protein
Tethered to the membrane by other things.
How does cholesterol enter cells
Low density lipoprotein LDL carries cholesterol in the blood and it will bind to its receptor which is found is coated pits.
The pit invaginates and forms a coated vesicle carrying the LDL and cholesterol into the cell.
The coated vesicle uncoats and fuses with the early endosome which is acidic so LDL is released from its receptor.
The LDL receptor is taken back up to the membrane. The LDL is taken to the lysosome and digested. The cholesterol is released into the cell.
Why are the lysosome and endosome so acidic
Proton pumps
What does adaptin do
When LDL binds to the receptor adaptin binds to the intracellular tail of the LDL receptor.
It will recruit clathrin molecules that start coating the inside of the membrane. This causes the membrane to bend and invaginate and form a vesicle.
What can defective endocytosis cause
Atherosclerosis
Mutations in the LDL receptor cause accumulation of lipoproteins in the blood and the formation of plaques blocking blood arteries.
Clathrin structure
What does dynamin do and what happens if it’s mutated
Tri legged curved structure.
Pinches the vesicle off the cell membrane by hydrolysing GTP into GDP.
All the vesicles line up along the membrane but are unable to pinch off without dynamin
COPII
COPI
Clathrin
Helps to form vesicles from the ER which travel to the Golgi
Helps to form vesicles from the Golgi
Helps to form vesicles from the membrane that go to the endosome
Phagocytosis
How do vesicles form
Vesicle formation without clathrin and actin derived membrane invagination.
Microbe adheres to a phagocyte and the phagocyte forms pseudopods that engulf the particle.
Phagocyte vesicle fuses with lysosome and the microbe is digested.
The residue is removed by exocytosis
What is autophagy
Helps to eliminate malfunctioning cell elements.
A membrane forms around the diseased organelle creating an autophagosome.
This will fuse with a lysosome and be digested.
Why is it a problem for internal membrane compartments to fuse and what helps it to happen
They are all negatively charged so won’t want to fuse together.
SNARE proteins help this to happen.
How are newly made proteins exocytosed
The newly made proteins move to exit sites which are randomly formed in the membrane network of the ER.
The proteins are packaged into vesicles which fuse to form transport intermediates.
These move along microtubule tracks to the Golgi.
They then exit the Golgi and move in transport vesicles pulled outwards by microtubules to the plasma membrane.
The vesicles fuse with the membrane and release the contents.
Constitutive exocytosis vs regulated exocytosis
Constitutive means there is continuous secretion
Regulated means only secretion in response to a stimulus.
How do SNARE proteins work and what do they do
Which one has a transmembrane region
They help fusion by overcoming the repulsion of two negatively charged membranes.
Three SNARE proteins coil around eachother to force membranes together.
Vesicular SNARE (VAMP or synaptobreuin)
Two target membrane SNAREs called syntaxin and SNAP25.
Syntaxin has a transmembrane region.
They form a tight four helical coiled coil on initial contact. SNAP25 contributes two helixes and syntaxin and VAMP give one each.
What happens in a coiled coil and what is one helix like
Two alpha helixes wrap around eachother.
One side of a helix will have aliphatic amino acids and the other side will have polar residues which makes it amphipathic.
Two amphipathic helixes are aligned with their hydrophobic sides together.
