Endomembrane system Flashcards
Endoplasmic Reticulum
-Continuous network of flattened scas, tubules and associated vesicles
-The membrane-bound sacs are called ER cisternae and the space inside them is the ER lumen
Main functions of ER
-Biosynthesis of proteins for:
* plasma membrane or into the organelles of the endomembrane system
*Export from the cell
-Lipid synthesis
-Membrane synthesis
Rough ER
-Contains ribisomes
-Morphology= flattened sheets
-Function: Biosynthesis and processing of proteins
Smooth ER
-No ribisomes
-Morphology= tubular
-Function: Steroid biosynthesis, drug detoxification, carbohydrate metabolism, calcium storage
Protein synthesis for EMS overview
Signal recognisiton particle (SRP) recognise and binds the ER signal sequence on the peptide and attaches the ribisome polypeptide complex to the ER membrane
signal sequence
-If first amino acids are positively charged, followed by hydrophobic amino acids and then polar aa, it designates that protein to EMS
-signal sequence is 15-30aa
protein synthesis mechanism
BLAST to find signal sequence
Does not work because signal sequence is never part of finsished protein but is cleaved
-Must use genome
Drug detoxification
-Often involves hydroxylation catalysed by cytochrome P450 enzymes
-drugs are usually non-polar and undergo hydroxylation by CYP that add hydroxyl groups to drugs, making them polar and more soluble, allowing the drug to be secreted by urine
Carbohydrate metabolism
-The live stores glucose as glycogen in granules associated with smooth ER
-expresses glucose-6-phosphatase to catalyse:
glucose-6-phosphate + H2O-> glucose and P
Calcium storage
Sarcoplasmic reticulum is specialised smooth ER in muscle cells
-stores and releases Ca
glogi apparatus
-series of flattened membrane-bounded cisternae
-A series of cisternae, usually 3-8, is called a Golgi stack
-Some cells have one large stack and others, especially secretory cells, have hundreds or thousands of stacks
Movement models for golgi
-Stationary cisternae mode
-Cisternal maturation model
Stationary cisternae model
-Each cisterna in the Golgi stack is a stable structure
-Transport of materials from one cisternae to another is mediated by shuttle vesicles
-These bud off from one cisterna and fuse with the cisterna in a cis-to-trans sequence
Cisternal maturation model
-Golgi cisternae are transient compartments
-These gradually change from CGN through medial cisternae to TGN
-Enzymes not needed in later compartments are returned to earlier compartments in vesicles
Glycosilation
The addition of carbohydrate side chains (moiety) to proteins to form glycoproteins
N-linked glycosilation
Involves the addition of an oligosaccharide to the nitrogen atom of certain asparagine residues
O-linked glycosilation
It involves the addition of the oligosaccharide to the oxygen atom on the hydroxyl group of certain serine, threonine and rarely tyrosine residuals
Initial carbohydrate units of golgi
All carbohydrate side chains initially have a common core oligosaccharide consisting of two units of N-acetyl glucosamine, 9 mannose units and 3 glucose units
Migration through golgi
Golgi processing
-Dolichol phosphate n the ER membrane acts as the carrier of oligosaccharide units for protein glycosilation. In mammals, n=18
-Core oligosaccharide synthesis begins in the cytoplasm as N-acetyl-glucosamine (N) and mannose (M) units are added to the dolichol phosphate
-Translocation of the oligosaccharide from cytosol to ER lumen is catalysed by flippase
-Compeltion of the core oligosaccharide occurs in ER lumen as more M and glucose units are added
Transfer of the completed core oligosaccharide to an asparagine residue of the recipient protein is catalysed by an oligosaccharyl transferase
-Final processing involves the removal of certain glucose and mannose units in ER before transfer of glycoprotein to Golgi
golgi processsing overview
-Dolichol phosphate
-Core oligosaccharide
-Translocation
-Completion
-Transfer
-Final processing
Targeting of soluble lysomal proteins
-soluble lysosomal enymes in ER and early Golgi compartments undergo N-glycosylation followed by removal of glucose and mannose units
-Mannose residues on the side chains are phosphorylated within the Golgi complex, forming an oligosaccharide containing mannose-6-phosphate
-This tag ensures delivery of lysosomal proteins to the lysosomes (since they are digestive enzymes, they have to be transported to lysosome specifically)
lysosomal targeting mechanism
Exocytosis
The process by which secretory vesicles release their contents outside the cell
Endocytosis
Process by which cells internalise external materials
Secretory pathways
Allow proteins to move from the ER through the Golgi to secretory vesicles and secretory granules
types of secretion
-Constitutive
-Regulated
-Polarised
Exocytosis mechanism
endocytosis mechanism
Phagocytosis
taking up large solid particles (like unicellular organisms)
Pinocytosis
Taking up large quantities of liquid containg soluble suspended material
Phagocytosis mechanism
-Bacterium attached to plasma membrane. The plasma membrane forms a pseudopod around the bacterium. The phagocytic vacuole fuses with an early endosome
-The phagocytic vacoule makes transient contact with early and late endosomes during maturation
-Mature lysosome cause bacterium to be broken down by lysosomal hydrolases
Rececptor mediated endocytosis
-used for specific uptake of nutrients
1.Binding
2.Lateral diffusion
3.Invagination
4.Vesicle formation
5.Uncoating
6.Fusion with early endosome and release of ligand
7a.Transport to late endosome and then to lysosome for digestion
7b. Transport of receptors to cell surface for recycling
7c.Transcytosis
Transcytosis use
Polarised secretion by moving a substance from one side of cell to other
Lysosomes
-Contain acid phosphatase and several other hydrolytic enzymes (acid hydrolases)
-Lumenal side of the membrane is coated with glycoproteins to protect the membrane from degradation
-Have an internal acidic environment of (pH 4.0-5.0)
-ATP-dependant proton pumps in the membrane are responsible for an acidic environment
Formation of lysomes
Heterophagic lysome
extracellular substances
Autophagic substances
intracellular substances
Autophagy
Digestion and recycling of intracellular organelles
Peroxisomes
Bound by single membranes
-considered part of endomembrane system (+ is they have mebrane from ER and - is they dont have double membrane)
-Defining characteristic is presence of catalase for degrading H2O2
essential role of peroxisomes
-Hydrogen peroxide metabolism
-Detoxification of harmful compounds
-Oxidation of fatty acids
-Metabolism of nitrogen-containing compounds
-Catabolism of unusual substances (like xenobiotics)
E.Constitutive secretion
-small intestine cells release mucus as part of their function
E.Regulated secretion
B cells produce insulin
E.Polarised secretion
Secretion occurs on specific side of cells like neurotransmitters
