1.2(b) - The Synthesis and Transport of Proteins Flashcards
What is the endoplasmic reticulum (ER) ?
It forms a network of membrane tubules continuous with the nuclear membrane
What is the golgi apparatus?
A series of membrane flattened discs
What are lysosomes?
Membrane-bound organelles containing a variety of hydrolases that digest proteins, lipids, nucleic acids and carbohydrates
Difference between RER and SER
RER has ribosomes while SER lacks ribosomes
Describe the synthesis of cytosolic proteins
The proteins are synthesised in cytosolic ribosomes and once the synthesis is complete remain in the cytosol
Describe the synthesis of transmembrane proteins
Synthesis is started in cytosolic ribosomes however a signal sequence halts translation and directs the ribosome to dock with the ER forming RER. Translation continues after docking and the protein is inserted into the membrane of the ER
What is a signal sequence?
A short stretch of amino acids at one end of a polypeptide that determines the eventual location of a protein in a cell
How do proteins move between membranes?
Once the protein is in the ER, they are transported by vesicles that bud off the ER and fuse the Golgi apparatus. The protein then moves through the Golgi apparatus and undergoing post-translational modifications at the same time. Vesicles then transport proteins to the plasma membrane of lysosomes by moving along microtubules and fusing with other membranes
What major modification occurs in the Golgi apparatus?
Addition of carbohydrate groups formed by the addition of various sugars catalysed by enzymes over multiple steps
Examples of secreted proteins
Peptide hormones and digestive enzymes
How are secreted proteins synthesised?
They start synthesis in cytosolic ribosomes and complete their translation in ribosomes on the RER before entering it lumen. The proteins move through the Golgi apparatus and are then packaged into secretory vesicles which move to and fuse with the plasma membrane, releasing proteins out of the cell.
What is proteolytic cleavage?
A type of post-translational modification required to make some secreted proteins active if they have been synthesised as inactive precursors
