6.2 principles of receptor mediated endocytosis

Question 1

Describe the uptake of occupied insulin receptors

Answer 1

receptors only congregate over coated pits when insulin bound (2.5) insulin bound –> coated pit –> receptors bind

1. insulin binding induces conformational change in receptor
2. allows insulin to be recognised by coated pits
3. insulin bound to receptor fuses with endosome (still bound to R)
4. degraded in lysosome
5. allows for reduction in number of insulin receptors on membrane (down-regulate)
6. desensitises cell to continued presence of high circulating insulin concentration

Question 2

What is pinocytosis?

Answer 2

invagination of plasma membrane to form a lipid vesicle
allows uptake of IMPERMEABLE extracellular solutes & retrieval of plasma membrane
there are 2 forms: fluid phase and receptor mediated endocytosis

Question 3

What is receptor mediated endocytosis (RME)?

Answer 3

specific BINDING of molecules to cell surface receptors permitting the SELECTIVE uptake of substances into cell

Question 4

What is phagocytosis?

Answer 4

in macrophages & neutrophils
in response to binding of a particle to receptor in plasma membrane
cell extends pseudopods allowing further receptor interaction and membrane evagination and particle internalisation
fuse with lysosome –> phagolysosome (particle material degraded)
clearance of damaged cellular materials / invading organisms for destruction

Question 5

What is transcytosis? give an example?

Answer 5

ligands remain bound to receptors transported across cell
e.g. maternal immunoglobulins to foetus via placenta
transfer IgA from circulation –> bile in liver
IgA receptor cleaved during transport –> release of immunoglobulin with a bound ‘secretory component’ derived from receptor

Question 6

viruses and toxins entering the cell

Answer 6

membrane-enveloped viruses and some toxins, use endocytic pathways to enter cell, by binding to receptor on membrane
in ENDOSOME (acid pH favourable) - viral membrane fuse with endosomal membrane - releasing viral RNA to cell - translated and replicated to form new viral particles

Question 7

Describe the uptake of ferric (Fe3+) ions

Answer 7

1. 2xFe3+ bind to apotransferrin, turning into transferrin in circulation
2. transferrin binds to transferrin receptor @ neutral pH (internalised)
3. transferred to endosome, Fe3+ released from transferrin, but apotransferrin still bound to transferrin receptor
4. complex sorted through CURL, returned back to plasma membrane (pH 7.4) where apotransferrin dissociates from transferrin