Unit 9 - Secretion & Endocytosis

Readings -

Understand the processes involved in the Secretory Pathway
Understand the processed involved in the Endocytotic Pathway
Be able to compare and contrast the Secretory and Endocytotic Pathways
Be able to explain with examples the role of receptor proteins in secretion and endocytosis
Be able to explain with examples the recycling of receptor proteins in the endocytotic pathway.
To be able to trace a molecule through each of the secretory and endocytotic pathways describing all of the structures and process through which they pass.
Understand how intracellular digestion occurs.

There are two major transport pathways, both of which involve processing and modification of proteins and lipids, the

There are two secretory pathways, the
- constitutive secretory pathway that continually moves vesicles with membrane and their associated proteins to the plasma membrane and the
- regulated secretory pathway in which vesicles containing proteins to be secreted on demand are stored until a signal is received that initiates exocytosis.
There is also a special pathway for lysosomal proteins that have a special receptor and targeting process involving mannose-6-phosphate.
Cells ingest fluids, macromolecules and small particles by endocytosis. This occurs mainly through receptor mediated endocytosis.
The resulting vesicles are delivered to early endosomes where the receptors typically detach from the cargo and are recycled to the cell surface from which the originate.
Most of remaining contents of the early endosome is delivered to late endosomes and then to lysosomes where is degraded by hydrolytic enzymes.
Late endosomes are centres for recycling of mannose-6-phosphate receptors that carry lysosomal proteins.
Lysosomes contain a wide variety of hydrolyases that degrade macromolecules.
The acid pH of endosomes and lysosomes is generated by proton pumps in their membranes.

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Compare and contrast the regulated and constitutive secretory pathways with regard to the types of material carried and the regulation of secretion. (2).
What determines whether a protein enters the regulated or the constitutive secretory pathway?(1)
What are dense core secretory granules? (1) How do the form and what is their fate? (2)
Explain proteolytic processing of proteins with an example. (2)
In many cell types secretory vesicles are continually fusing with the plasma membrane. The membrane surface of the vesicles is added to the plasma membrane. Despite the addition of this extra area of membrane to the plasma membrane, the surface area of the cell remains essentially stable. Explain. (3)
Trace a molecule of a lysosomal protein from its point of synthesis on a ribosome in the cytosol, to its final destination in a lysosome (2).
Explain recycling of mannose-6 phosphate receptors.(2)

What is receptor mediated endocytosis? (1) How does it occur? (2)
What are lysosomes? What are they good for, and how are they made? (1)
Explain receptor recycling. (1)
The sorting or proteins of different sorts into vesicles destined for different targets occurs in both the secretory and in the endocytotic pathways. Where does sorting occur in each pathway. What are the similarities and the differences.(3)
What is the difference between early and late endosomes (2).
What is transcytosis and what is its significance (2).
Trace a molecule of a cargo receptor in the plasma membrane through the endocytotic pathway and back to the plasma membrane.(2)
Trace a molecule of mannose-6-phosphate receptor from its origin on a ribosome to the cell surface and then back to the trans Golgi network.(3+)
Compare and contrast protein sorting in the trans Golgi network with that occuring in early endosomes and in late endosomes (3).