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True or False:
the nuclear envelope and the ER are continuous.
How are the ribosomes arranged on the ER (think of the diagram)?
Where are they located?
- in chains held together by mRNA, which they are translating
- -called polyribosomes, or polysomes
- ribosomes are on the outside
Explain the steps of cell fractionation.
- 1) remove tissue. mince with blade into small pieces. suspend in cold isotonic homogenization medium.
- 2) homogenize the tissue fragment
- 3) Isolate subcellular component from homogenate by differential centrifugation.
What is homogenate?
suspension of material from broken cells
In centrifugation, what are you going to use to crush it?
At the first centrifugation, what are you going to have?
very large particles, such as nuclei adn unbroken cells
What will you take after the first centrifugation?
What will you get?
- the postnuclear supernatant
- pellet (with large particles like mitochondria)
At the last centrifugation, what will you get?
- pellet containing microsomes (small fragments of membrane as well as free ribosomes)
- - some microsomes will be from the SER and RER
What are microsomes?
How will you tell the microsomes of the SER from those of the RER?
- fragments of the ER
- they look like little vesicles because membranes have no free edges
- use density gradient because RER is heavier
Homogenization and initial fractionation steps can be done by ? Further separation can be done by __.
- differential centrifugation
- density gradient centrifugation
Explain density gradient centrifugation.
developed in the mid-1950s to separate DNA strands stained with different heavy metals
layer less dense layers one after the other
they will sink until they find their proper densities
a fairly fine gradient is needed to separate it
In the sedimentation steps of differential centrifugation, the particles will continue to do what as long as what and why?
- subjected to centrifugation because the medium is less dense than the particles
In density gradient centrifugation, the medium is composed of a __ and the particles move until they what?
- density gradient
- they reach a place in the tube equal to their own density, where they form bands
What are the proteins synthesized by ribosomes bound to the RER destined for?
- Secretory proteins (ex. peptide hormones)
- other proteins released into the eR lume (Rough ER enzymes)
- Integral membrane proteins (glycoproteins)
- glycolipid- anchored plasma membrane proteins
- peripheral proteinis of the plasma membrane
What are the proteins synthesized by ribosomes in the cytosol destined for?
- soluble cytosol proteins (enzymes of glycolysis)
- lipid-anchored membrane proteins (cytoplasmic face)
- peripheral proteins of the plasma membrane (cytosolic face)
- mito and chloroplasts proteins
- nuclear proitens
How does a cell know where its destined to be?
surface receptors on the the ER that respond to a particular amino acid sequence