The synthesis and transport of Proteins

• Protein synthesis occurs in two stages, transcription (DNA to mRNA) and translation (mRNA to polypeptide).
• Intracellular membranes


  increase the total area of the membrane in eukaryotic cells
• Endoplasmic reticulum


  Rough-has ribosomes attached

  Smooth- does not have ribosomes attached

  Folds proteins and transports complete proteins to the golgi apparatus

  Forms a network of membrane tubules which branch off from the nuclear membrane
• Golgi apparatus


  A series of flattened discs. Molecules move through the discs in vesicles that bud from one disc and fuse to the next one in the stack.
• Main modification to proteins in the golgi apparatus?


  the addition of carbohydrates
• job of the golgi apparatus?


  to process and package proteins to be used inside and outside of the cell.
• Lysosomes


  membrane-bound organelles containing a variety of hydrolases. These are enzymes which digest proteins, lipids, nucleic acids and carbohydrates.
• Secretatory Vesicles


  Vesicles are fluid-filled sacs which transport materials between different membrane compartments
• Where are lipids made?

  smooth endoplasmic reticulum, they are then inserted into the membrane of the SER
• Where are all proteins made (initially)?

  Ribosomes in the cytosol
• Cytosolic proteins


  Proteins which are synthesised and remain in the cytosol.
• How is the RER formed?

  At the end of translation, the ribosome synthesising protein will move to the endoplasmic reticulum and dock on to it, this forms the rough endoplasmic reticulum. This is because of a signal sequence in the protein, which is a short-stretch of amino acids at one end of the polypeptide. It tells the protein where to go with the protein.
• How are proteins transported?


  Once proteins are in the ER they are transported around by vesicles which bud off from the ER membrane and move across the cell to fuse with the golgi apparatus.
• Does translation continue or halt after docking with the ER?


  Translation continues after docking and the protein is inserted into the membrane of the endoplasmic reticulum.
• When and where does post-translational modification occur?


  as the protein moves through the golgi apparatus, it goes through post-translational modification.
• carbohydrates are formed from...?


  Enzymes catalyse the additions of various sugars in multiple steps to form the carbohydrates.
• How do vesicles deliver proteins from the golgi apparatus>}?


  When vesicles leave the golgi apparatus to take proteins to the main cell membrane and sometimes to lysosomes. They do this by moving through the cell and fusing with them. They move through microtubules