secretory pathway + endocytosis (L19+20)

how is lipid asymmetry achieved/maintained

scramblase (ER membrane) : phospholipids added to cytosolic half of bilayer, scramblase flips phospholipid molecules. → symmetric growth of both halves of bilayer + equilibrates lipids

flippase (plasma membrane): new membrane delivered by exocytosis, flippase catalyses flipping of specific phospholipids to cytoplasmic monolayer →ensures asymmetry maintained. flippase is ATP dependent

how is asymmetry maintained in RBCs

negatively charged phospholipids (phosphatidylserine) mainly in cytosolic leaflet

phosphatidylserine transferred from EC leaflet to cytosolic leaflet by translocase.

scramblase abolishes asymmetry → net equilibrium favours translocase under normal conditions

what is the secretory pathway

process by which cells produce, modify, and transport proteins and lipids that are either secreted outside the cell or delivered to various membrane-bound organelles, such as the plasma membrane or lysosomes

transport from ER → golgi → final destination (in vesicles)

what is the structure of the ER and what are its functions

connected to the nuclear envelope, ER forms hollow tubes + flattened sacs- chambers are cisternae. two domains (SER+RER)

functions:

1. quality control

2. synthesis

3. storage

4. detoxification

how are newly made membrane + secreted proteins translocated into the ER

chaperones help newly synthesised linear sequences of AA to fold correctly into tertiary + quaternary structures. ER lumen is full of chaperone proteins to prevent proteins getting stuck. can occur either during translation or after

signal sequence on growing PP chain is cleaved by signal peptidase

what is the SER responsible for

storage + detoxification

• phospholipid + cholesterol synthesis

• steroid hormone production

• synthesis + storage of glycerides

• synthesis + storage of glycogen

• important role as calcium store

how does transport occur between the ER and golgi

in the form of vesicles + tubules- vesicles bud off the ER and are received by the golgi

what are the 3 types of vesicle coating and what is the purpose of a vesicle coating

clathrin (endocytosis + transport between golgi and endosome),

COPI (back and forth through golgi complex + golgi to ER) ,

COPII (ER to golgi complex)

coat aids formation of the vesicle but must be discarded before vesicle can fuse with the target component

how do vesicles reach the correct target

two types of SNAREs: v-SNAREs (in vesicle membrane) + t-SNAREs (target membrane)

SNARE complex is a helical bundle consisting of 3 compartments, v+t SNAREs form tight interactions, bringing membranes close + promoting fusion

what is the structure of the golgi apparatus + what are its functions

composed of flattened discs (cisternae)

cisternae communicate with the ER + cell membrane through vesicles + tubules

functions: (3 primary)

• modification + packaging of secreted proteins

• renewal + modification of plasma membrane

• delivery of material to other organelles (especially in endocytic pathway)

many modification processes take place, each in a specific cisternae

what are the 3 pathways of golgi → plasma membrane transport

1. signal mediated diversion to lysozomes

2. signal mediated diversion to secretory vesicles (for regulated secretion)

3. constitutive secretory pathway

what 3 things can happen to endocytosed material

1. recycled across basolateral membrane

2. transcytosis across apical membrane

3. degradation in lysosomes

what is phagocytosis

uptake of large particles (bacteria + apoptotic cells)

ligand coated particles bind to phagocyte surface receptors which causes pseudopods to form + engulf particle

what was the first hint that membrane recycling occurs

macrophages phagocytosed 1.1 micrometre beads, by counting the beads it was possible to estimate how much membrane was internalised:

30% of the total surface area was internalised per hour but there was no change in cell size therefore cell membrane must be recycled

what is macropinocytosis

cells form actin driven ruffles which sometimes fuse to form macropinosomes- mechanistically similar to phagocytosis

non selective uptake of extracellular material

used by cancer cells to take up nutrients

how does clathrin mediated endocytosis occur

• LDL synthesises cholesterol, LDL receptors are synthesised when a cell needs cholesterol

• clathrin coated pit curves to become invaginated, becomes a coated vesicle

• coat dissolves + vesicle fuses with early endosome- delivers LDL + receptor

• receptor + LDL dissociate, LDL is targeted to lysosome where cholesterol is released

• receptor buds off in transport vesicles + recycles at the plasma membrane

what do mutations in LDL receptors lead to

defective binding of LDL or defective internalisation

familial hypercholestrolemia

what is required to pinch off clathrin coated vesicles

dynamin

what happens to cargo for degradation

incorporated into intraluminal vesicles (ILV)