Module 4- Membrane bound organelles
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what 2 contents in the cell make up most of the cell?
- intracellular fluid
- makes up about 54% of the cell volume and is composed of water, salts, and organic molecule, protein filaments that make up the cytoskeleton, soluble proteins, ribosomes and proteasomes
sub-cellular structures outside the nucleus such as the Golgi, ER, lysosomes etc.
what are part of the endomembrane system?
endoplasmic reticulum, Golgi, endosomes, and lysosomes
endoplasmic reticulum (3)
- is a continuous network of flattened sacs, tubules and vesicles through the cytoplasm of a eukaryotic cell
- ER cisternae: membrane-bound sacs
- ER lumen: space inside them
compare rough ER and smooth ER.
- Rough ER: characterized by ribosomes on the cytosolic side of the membrane
- Smooth ER: lacks ribosomes and has a membranous structure with more space in the lumen to accommodate special molecules
what is the function of transitional elements (TEs)
- subdomain of rough ER
- play a role in the formation of transition vesicles that shuttle lipids and proteins from the ER to the Golgi complex
list the 6 functions of the rough ER.
- involved in the biosynthesis and processing of proteins
- is the site for initial step of addition of carbohydrates to glycoproteins
- folding of polypeptides
- recognition and removal of mis-folded proteins
- assembly of multi-meric protein complexes
- role in quality control
describe one of the functions of rough ER: rough ER is involved in biosynthesis and processing of proteins
- ribosomes on the cytosolic side of the rough ER membrane synthesize both membrane bound and soluble proteins for the endomembrane system
- newly synthesized proteins are inserted into the endomembrane system through a pore complex as they are synthesized
describe one of the functions of rough ER: rough ER is involved in quality control
In ER-associated degradation (ERAD), proteins that are incorrectly folded, modified, or assembled are exported for degradation
list the 4 functions of the rough ER
- drug detoxification
- carbohydrate metabolism
- calcium storage
- steroid biosynthesis
describe one of the functions of smooth ER: Drug detoxification
- ex. hydroxylation
- adding hydroxyl groups to hydrophobic drugs increase their solubility, making them easier to excrete from the body
- hydroxylation catalyzed by a member of the cytochrome p-450 family of proteins called monooxygenases
what happens to the smooth ER when there is a continuous exposure to a drug?
continuous exposure to a drug can cause rapid increases in the detoxifying enzymes and a proliferation of smooth ER
describe one of the functions of smooth ER: carbohydrate metabolism (2)
- is involved in breakdown of stored glycogen
- contains glucose-6-phosphatase, an enzyme unique to smooth ER
- glucose-6-phosphatase hydrolyzes the phosphate from glucose-6-phosphate to form free glucose
describe one of the functions of smooth ER: calcium storage (2)
- calcium ions are pumped into the ER by ATP-dependent calcium ATPases and are released when needed for muscle contraction
- ER lumen contains high concentrations of calcium-binding proteins
describe one of the functions of smooth ER: Steroid biosynthesis
- smooth ER in some cells is the site of cholesterol and steroid hormone synthesis
- large amounts of smooth ER are found in cells that synthesize these
- smooth ER has also been found associated with plastids in some plants, maybe involved in phytohormone synthesis
describe the process of membrane biosynthesis. (3)
- fatty acids for membrane phospholipids are synthesized in the cytoplasm and incorporated into the ER membrane on the cytosolic side
- transferred to the luminal side of the bilayer by enzymes called phospholipid translocators (flippases)
- phospholipid exchange proteins convey specific phospholipids to mitochondria, chloroplasts or peroxisomes
- is a series of flattened membrane-bounded cisternae
- the Golgi complex lumen (intracisternal space) is part of the endomembrane system
- series of cisternae usually 3-8 is called a Golgi stack
describe the 2 faces of the Golgi stack.
- cis face: oriented toward the ER, the Golgi compartment= cis-Golgi network (CGN)
- medial cisternae: between the cis face and trans face
- trans face: opposite side, the compartment= trans-Golgi network (TGN)
where do transport vesicles form and are taken in?
- incoming transport vesicles come to cis face
- newly forming vesicles form in the trans face
what are the 5 functions of the Golgi complex?
- modifying, sorting, and packaging proteins for cell secretion (exocytosis) or use within the cell
- transport of lipids
- creation of lysosomes
- production of carbohydrates and proteoglycans
- possible role in apoptosis
the structure of the Golgi, is it fixed or fluid? What are the 2 models that depict the flow of lipids and proteins through the Golgi complex?
- The stationary cisternae model:
- -each cisternae in the Golgi stack is stable structure
- -transport of materials from one cisternae to another is mediated by shuttle vesicles
- -these bud off from one cisternae and fuse with the next cisternae in the cis to trans sequence
- The cisternae maturation model:
- -the Golgi cisternae are transient compartments
- -these gradually change from from CGN through medial cisternae to TGN
- -enzymes not needed in later compartments are returned to earlier compartments in vesicles
anterograde transport (2)
- movement of material towards the plasma membrane
- as a secretory granule fuses with the plasma membrane and discharges its content, a bit of membrane from the ER becomes part of the PM
retrograde transport (3)
- flow of vesicles from Golgi cisternae back to the ER
- allows the cell to balance the flow of lipids toward the PM
- also ensures a supply of materials for forming new vesicles
list the three ways proteins can move from one compartment to another.
- gates transport
- transmembrane transport
- vesicular transport
describe the process of vesicular transport. (4)
- a membrane enclosed transport intermediate transfers proteins from one compartment to another
- transport vesicles are loaded with cargo in one compartment (donor)
- they pinch off
- they discharge the cargo in another compartment (target)
what are coat proteins?
what are it's 3 principle functions?
- coat proteins: proteins on the vesicle surface
- 1. it gathers membrane proteins in a specialized membrane patch, selecting appropriate molecules for transport
- 2. assembles into a curved basket like a lattice, molding the formation of the vesicle
- 3. may be involved in interactions with microtubules that help move the vesicle throughout the cell
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