Secretion Flashcards
What is secretion ?
It is the release of useful products from a cell
What happens in secretion ?
Cells take small molecules from the bloodstream and transform them by intracellular mechanisms into complex functional molecules. They are then released into the cells’ external environment.
Give some examples of typical secretory products
- collagen etc (from the extra cellular matrix)
- saliva, pancreatic juice, sweat and mucus (from the exocrine glands)
- hormones (examples of endocrine products)
- albumens, blood clotting proteins and antibodies (examples of plasma proteins)
How was the secretory pathway discovered ?
Through autoradiography
Summarise the process of autoradiography
1) Radioactive amino acids were injected into a rat
2) The pancreas was removed at regular intervals and a sample was prepared to use in an electron microscope
3) The sample was coated with a photographic emulsion and left in a dark box for a few weeks
4) Radioactive decay of the specimen exposed the emulsion
5) The photographic film was developed and silver granules formed above the radioactive amino acids
This allowed for the secretory pathway to be discovered because the silver granules were present in different structures at different times
What is the secretory pathway ?
Bloodstream > endoplasmic reticulum > Golgi apparatus > secretory vesicles
Summarise the structure of the RER
It is a membrane bound cisternae (broad and flat sheets) studded with ribosomes
Summarise the structure of the SER
It is formed from interconnecting tubules with no ribosomes
It is connected to the RER
What is the function of the RER
- allows for entry of proteins into the endoplasmic reticulum
- allows for the insertion of membrane proteins into the lipid bilayer
- allows simple glycosilation (attaching sugars to proteins) reactions to take place
- important for quality control to ensure correct protein folding
What is the function of the SER ?
- important in lipid synthesis e.g. making the phospholipids present in membranes
- important in lipid metabolism e.g. steroid hormones
- important in detoxification reactions
How do proteins get into the RER ?
1) mRNA leaves the nucleus through a nuclear pore and encoded in the mRNA is a signal sequence
2) the ribosome attaches to the signal sequence
3) the signal sequence is translated and this makes a signal peptide (it is hanging off the ribosome and mRNA)
4) the RER has receptors for the signal peptide and so the signal peptide binds to the RER and takes the ribosome and mRNA with it
5) once bound, the ribosome continues to translate the mRNA and feeds the signal peptide through a pore into the RER
What is transitional endoplasmic reticulum ?
It is a patch of RER which has no ribosomes
Summarise the structure of the Golgi apparatus
It consists of a stack of flattened cisternae and has 3 main faces - the cis face (forming face), medial face and trans face
Each face can do its own set of chemical reactions
Summarise the main functions of the Golgi apparatus
- receives vesicles containing proteins, membrane and membrane proteins from the endoplasmic reticulum
- carries out further quality control and sends faulty materials back to the endoplasmic reticulum
- carries out complex glycosilation reactions
- sorts and packages contents
- targets products to their final destination
How does the Golgi sort and package products ?
The Golgi receives a great jumble of material from the RER and so it needs to sort out what goes to each pathway
- lysosomal proteins contain a number of amino acids that get glycosilated with mannose - 6 - phosphate and so these proteins bind to mannose - 6 - phosphate receptors on the Golgi and bud off in vesicles
- there are also specific receptors for specific secretory products in the regulated pathway so the same thing happens
- anything not sorted ends up in the constitutive pathway
