Endomembrane system Flashcards
Compartmentalisation
- increases the efficiency of all the sub cellular processes by concentrating the required components in a confined space within the cell
- achieved by means of organelles
Components of the endomembrane system
- ER
- Golgi complex
- lysosomes
- vacuoles
Define ER
- Probably evolved from the invagination of the plasma membrane.
- Network of membranes that penetrates much of the cytoplasm.
- Enclosed within the ER is an extensive space/lumen which is separated from the surrounding cytosol by the ER membrane
About smooth ER
- SER lacks ribosomes
- Membrane of the SER are highly curved and tubular forming an interconnected system of pipelines
- Present in skeletal muscle, kidney tubules and steroid-producing endocrine glands.
What are the functions of SER?
- Synthesis of steroid hormone in the endocrine cells
- Detoxification in the liver (of drugs such as ethanol) etc. by enzymes of cytochrome p450 family- oxygenases.
toxins -> p450 enzyme -> intermediate (reactive metabolite) [antioxidant protection] -> conjugation -> Waste (water soluble)
eliminated via bowel or kidney
- Isolating the calcium ions within the cytoplasm of cells.
- Sarcoplasmic Reticulum
What is sarcoplasmic reticulum
The regulated release of calcium ions from the SER of skeletal and cardiac muscle cells.
It triggers contractions
What is RER
- Ribosomes are present and they are bound to the cytosolic surface
- composed of a network of flattened sacs (cisternae)
- continuous with the outer membrane of nuclear envelope
- site of synthesis of protein, carbohydrate chains, and phospholipids present in cell membranes
- lumen of the ER cistern provides a specialised local environment that favours the modification, folding and assembly of subset of cell’s proteins
Functions of RER/ Explain protein synthesis
-Protein synthesis is carried out in cytoplasm by free ribosomes or at RER by membrane bound ribosomes
Types of protein synthesised by RER
-Secreted proteins
-integral membrane proteins
-soluble proteins of endo-membrane system
How do cell know where the protein will go?
- Signal Hypothesis
- Site of protein is determined by sequence of amino acids in the N- terminal portion of the polypeptide
it is the first part that emerges from the ribosome during protein synthesis - Signal sequence is 6-15 amino acids long, which targets the nascent polypeptide inside the ER lumen by co translational translocation
Enzymes in protein processing
- Signal peptidase: cleaves the signal peptide once inside the ER
- Molecular Chaperones: promotes correct protein folding and prevent misfolding
-Protein disulphide Isomerase: ue to oxidising condition, they can catalyse disulphide bond formation
What is membrane biosynthesis
- Membrane lipids are synthesised entirely within the ER
- Newly synthesised lipids and proteins are inserted into existing membranes in the ER
What are glycoproteins
Proteins produced on membrane bound ribosomes
oligosaccharyltransferases / glycosyltransferases - addition of sugers to oligosaccharide chain is catalyzed by large family of membrane bound enzymes
What is N linked glycosylation
In RER, a premade scaffold of sugars attached to the protein at Asparagine (which has N)
eg) N-Acetylglucosamine
Why is glycosylation important for proteins?
- imp cell recognition and adhesion
- gives specificity to proteins
- retain stability on some secreted glycoproteins
- crucial to dictate proper conformation of many membrane proteins
- eg) A, B, O RBC antige
What is ERGIC?
- The region has been named (endoplasmic reticulum Golgi intermediate compartment)
- After protein bud from ER membrane , transport vesicles fuses to form large vesicle and interconnected tubules in the region btw ER and Golgi complex
movement of vesicles occurs along tracks of microtubule
What is anterograde and retrograde transport?
What is Golgi complex
- Flattened, disc like, membranous curved cistern with dilated rims and associated vesicles and tubules
- Usually present near the nucleus
- A Golgi stack comprises of less than 8 cisternae
- Vesicles bud from peripheral domains of cistern
- Functions include protein sorting and processing.
Explain the functional compartmentalisation of Golgi complex.
The cis/entry face:
- closest to the ER
- interconnected network of tubules referred to as cis Golgi network (CGN)
- Sorts which protein should be transported to Golgi or back to er
The bulk of the Golgi complex:
consist of large flattened cistern which are divided into tubules and vesicles called divided into cis, medial and trans cistern.
The trans or exist face:
- contains a distinct network of tubules and vesicles called trans Golgi network.
- Sorting station where proteins are segregated into diff. types of vesicles heading either to plasma membrane or to various intracellular destination
What are the two models of cargo transport
1) cistern maturation model
2) Vesicular transport model
Explain Cisternae maturation model
- each cisterna physically moves from the cis to the trans end of stack, changing the composition in porgress
- Transport vesicles carry resident Golgi enzymes in a retrograde direction
-Dynamic model.
Explain Vesicular transport model
- The cistern of a Golgi stack remains in place as stable compartment
- Cargo is shuttled in vesicles that bud from one compartment and fuse with the next compartment along the stack.
- Accounts for a steady Golgi model.
Explain glycosylation in Golgi
- assembly of carbohyderate component in glycoprotein and glycolipid.
- in RER, the sequence in which sugars are incorporated into oligosaccharide is determined by the arrangement of specific glycosyltransferase that comes in contact of newly synthesised protein as it moves through the golgi.
- eg) Sialyltransferase is localised in the trans face of Golgi stack
- Golgi complex also process N-linked glycosylated proteins coming from RER
Difference between glycosylation in RER and Golgi
- In RER, the glycosylation assembles a single core oligosaccharide
while, the glycosylation steps in Golgi are varied, producing carbohyderate domain of remarkable sequence diversity. - RER- N linked glycosylation
Golgi complex- O linked glycosylation is completed. - Golgi is the site of synthesis of most of the cell’s complex polysaccharide
eg) Pectin, hemicellulose and comp. of ECM - some types of glycolipids and phospholipids are synthesised by Golgi apparatus
- Golgi complex and vesicles are more prominent in secretory cells
eg) goblet cells of intestine secrete polysaccharide rich mucus into the gut.
