1.2 The synthesis and transport of proteins Flashcards
Internal membranes
Eukaryotic cells have system of internal membranes, which increases the total area of the membrane.
Because of their size eukaryotic cells have a relatively small surface area to volume ratio. The plasma membrane of the eukaryotic cells is therefore to small an area to carry out all the vital functions carried out by membranes.
Endoplasmic reticulum
(ER) forms a network of membrane tubules continuous with the nuclear membrane.
Golgi apparatus
The golgi apparatus is a series of flattened membrane discs
Lysosomes
Membrane bound organelles containing a variety of hydrolases that digest proteins, lipids, nucleic acids and carbohydrates.
Vesicles
They transport materials between membrane compartments.
Synthesis of lipids and proteins
Lipids and proteins are synthesised in the ER, (rough ER (RER) has ribosomes on its cytosolic face while smooth ER (SER) lacks ribosomes).
Lipids are synthesised in the smooth endoplasmic reticulum (SER) and inserted into its membrane.
The synthesis of all proteins begins in the cytosolic ribosomes and the synthesis of cytosolic proteins is completed there, and these proteins remain in the cytosol.
Transmembrane proteins carry a signal sequence, which halts translation and directs the ribosome synthesising the protein to dock with the ER, forming RER.
Translation continues after docking, and the protein is inserted into the membrane of the ER.
Signal sequence
A signal sequence is a short stretch of amino acids at one end of the polypeptide that determines the eventual location of a protein in a cell.
Movement of proteins between membranes
Once the proteins are in the ER , they are transported by vesicles that bud off from the ER and fuse with the Golgi apparatus.
As proteins move through the golgi apparatus they under go post-transtional modification.
Molecules moving through the Golgi apparatus
Molecules move through the golgi discs in vesicles that bud off from one disc and fuse to the next one in the stack. Enzymes catalyse the addition of various sugars in multiple steps to form the carbohydrates.
Addition of carbohydrate groups is an major modification.
How do vesicles move outside of the golgi apparatus?
Vesicles that leave the Golgi apparatus take proteins to the plasma membrane and lysosomes.
Vesicles move along the microtubules to other membranes and fuse with them within the cell.
The secretory pathway
Secreted proteins are translated in ribosomes on the RER and enter its lumen.
(peptide hormones and digestive enzymes are examples of secreted proteins)
The proteins move through the golgi apparatus and are then packaged into secretory vesicles. These vesicles move to and fuse with the plasma membrane, releasing the proteins out of the cell.
Many secreted proteins are synthesised as inactive precursors and require proteolytic cleavage to produce active proteins.
The secretory pathway - Proteolytic cleavage
Proteolytic cleavage is another type of post-transitional modification. Digestive enzymes are one example of secreted proteins that require proteolytic cleavage to become active. Specific names of digestive enzymes are not required.
