Secretion

It is the release of useful products from a cell

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.

• 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)

Through 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

Bloodstream > endoplasmic reticulum > Golgi apparatus > secretory vesicles

It is a membrane bound cisternae (broad and flat sheets) studded with ribosomes

It is formed from interconnecting tubules with no ribosomes It is connected to 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

• important in lipid synthesis e.g. making the phospholipids present in membranes • important in lipid metabolism e.g. steroid hormones • important in detoxification reactions

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

It is a patch of RER which has no ribosomes

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

• 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

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

The cytoskeleton directs the vesicles towards the cell surface. The vesicle then fuses with the membrane and the products are released. A new patch of membrane is formed.

• concentration of contents • targeting and transport to cell surface or lysosomes • membrane fusion and release of products • delivery of new membrane lipids and proteins to cell surface

Target information is encoded in the gene sequence of secretory and membrane proteins and if a mutation occurs in this region then this will lead to mistargeting It can be very serious e.g. cause cystic fibrosis

• Regulated pathway - secretory vesicles are stored in the cell until the cell receives external signals to secrete them • Lysosomal pathway - the Golgi sends vesicles to lysosomes • Constitutive pathway - vesicles are secreted all the time