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