Endoplasmic Reticulum and Golgi Apparatus Flashcards
What is the endoplasmic reticulum?
The Endoplasmic Reticulum is a continuous network of membrane tubules that is continuous with the outer layer of the nuclear membrane.
What is the perinuclear space?
The space between the double bilayer membrane of the nucleus.
It runs continuously with the endoplasmic reticulum.
What are the proteins made in the cytosol used for?
It is used to make proteins that will be used within the cell (in the nucleus, mitochondria, peroxisomes, cytosol etc.)
What are proteins made in the ER used for?
The protein create will either be used outside the cell, become embedded within the plasma membrane, or go to any of the organelles use within the secretory pathway (i.e. golgi apparatus, ER, lysosomes).
What are the steps involved in the secretory pathway?
- mRNA will get translated via a ribosome that is found on the endoplasmic reticulum (specifically the rough endoplasmic reticulum).
- The protein is formed and it will bud off into a vesicle to then go and join the golgi apparatus where it undergoes a maturation process (i.e sugar chains are added; processed and modified).
- Then it buds off into another vesicle for it to be transported outside the cell or embedded to the cell’s membrane via exocytosis.
What are the two types of protein that can be formed on the ER?
- Soluble proteins: proteins that are soluable inside of the lumen of the ER, eg. Insulin.
- Proteins containing transmembrane domains: proteins that have a transmembrane helices that are embedded in the plasma membrane- they use their transmembrane domains to act as signals to enter the ER.
Name the different functions that the ER has
- Protein synthesis.
- Glycosylation: attaching sugar chains to proteins.
- Folding and assembling newly synthesised proteins into their correct 3D structure.
- Sit of lipid synthesis (SER) (cholesterol, phospholipids, steroids).
- Ca 2+ imounding: storing calcium ions with the lumen of the ER and maintaining it at low levels in the cytoplasm, increasing it when required for cell signalling.
- Also contains a large number of cytochrome p450 enzymes which are used in detoxification (if toxic molecules are eaten).
What cell is used to view the secretory pathway using electron microscopy and why?
Pancreatic Acinar cells are used as they contain lots of digestive enzymes which are stored by secretory granules and released after fusing with the enzyme when needed.
What is the first step of protein synthesis in regards to the ER?
A ribosome will attach to the mRNA strand and will start scanning untill it find the AUG codon- only after it has found the start codon it will start initiating translation.
What is the second step of protein synthesis in regards to the ER?
Proteins that are meant to be synthesised in the ER will have a hydrophobic chain of amino acids at the N-terminal, which is also known as a signal peptide as it will be used to insert the the protein within the ER.
What is the third step of protein synthesis in regards to the ER?
The signal peptide will emerge from the ribosome after translation has started, but translation is stalled untill the signal recognition particle (SRP) recognises and binds to the signal peptide.
What is the fourth step of protein synthesis in regards to the ER?
On the surface of the endoplasmic reticulum there will be a ribosome receptor, peptide translocation complex and a SRP receptor. The ribosome will bind to the ribosome receptor, the SRP will bind to the SRP receptor and the signal peptide will bind to the peptide translocation complex so the protein that is being translated can be feed through inside the lumen of the ER. Then after this is done, translation can continue.
What is the fifth step of protein synthesis in regards to the ER?
The signal peptide is no longer needed (used only to direct the mRNA to the ER) so it will be cleaved off using the enzyme signal peptidase as well as GTP being hydrolised into GDP and Pi.
What is the sixth step of protein synthesis in regards to the ER?
Translation will continue and the rest of the protein will be created within the lumen of the ER, for it to be then folded and processed into its final, functioning 3D structure using helper proteins- the protein may also then bind with other proteins to form multi-protein complexes.
Where does insulin synthesis occur? (soluble proteins)
Within beta-cells that can be found within the pancreas.
What is the first step of insulin synthesis?
A ribosome present in the cytoplasm will translate the mRNA molecule for insulin into preproinsulin, which will be then transported to the ER via a vesicle. It also has a signal sequence attached to it.
What is the second step of insulin synthesis?
In the ER, the preproinsulin will get modified and have its signal sequence cleaved off at the N-terminal, turning it into proinsulin. In addition it will also have disulfide bonds formed within it as part of it being folded into its final 3D structure.
What is the third step of insulin synthesis?
The proinsulin will then bud off into a vesicle where it will travel to the golgi apparatus to be cleaved into three segments: a chain, b chain and the c peptide- where the a chain and b chain still connected by the disulfide bonds, becoming mature insulin.
NOTE: the c peptide is used in the cell membrane for g-protein signalling.
What is the fourth step of insulin synthesis?
The mature insulin is now folded into its proper and functional 3D structure and buds off into a vesicle where it is take and fused to the cell membrane, releasing the insulin into the bloodstream.
What is the name of the enzyme used to cleave the signal peptide during insulin synthesis?
Signal peptidase.
What are the lengths of the different proteins in insulin synthesis?
Preproinsulin – 110 amino acids long
Signal sequence – 24 amino acid long
Mature insulin – A chain: 21 amino acids long, B chain 30 amino acids long.
How many modifications can a protein undergo when processed via the endoplasmic reticulum?
It can undergo five principle modifications
