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Cholesterol
...location?
...how much
- does not span the bilayer; it is located in 1/2 of the bilayer
- how much depends on the animal membrane (there is none present in fungi, plants, and bacteria)
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Cholesterol
- OH part?
- hydrophilic (nestled up against the -OH in the membrane)
- the rest of the molecule is hydrophobic
- forms more than one hydrogen bonds with lipid portion
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Chains of cholesterol are __.
What does cholesterol do to a membrane?
- rigid
- adds rigidity to membrane
- - at higher temps it makes the membrane more rigid
- decreases permeability of membrane (lipid bilayer) to ions, small polar molecules, etc. [if they do get through, it is with the help of proteins]
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What does it do to transition temperature?
lowers transition temp by decreasing fluidity (it'll be harder to pack the lipid molecule together)
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Explain cholesterol in terms of temperature.
- at higher temperatures, because of rigidity, it decreases fluidity
- at lower temperature, it adds fluidity
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Due to the __ of rings, it gives the membrane __.
- rigidity
- mechanical strength
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Plants have no __, but may hve __ that may do the same thing.
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Plant membranes must do what?
stay fluid just as animal membranes must
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What can limit mobility of membrane? Think of proteins and lipids.
- - proteins will slow down lipid movement due to interactions with other proteins. It will either stop them or slow them down.
- - temperature (if no regulation, temperature of membrane is same as environment)
- - fatty acid chains and the distance between them
- - not much surface area--> slows them down
- - proteins move way slower (their size affects fluidity)
- - cytoskeleton and attachment will stop it by anchoring the molecule in place
- - extracellular matrix will slow down or stop if the proteins are attached
- - protein complex--> slow movement
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Why is the membrane moving anyway?
their own thermal energy
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Why is membrane fluidity important to a cell?
- permeability facilitation (even though there can be a leak in the lipid bilayer between the bilayers)
- exocytosis, endocytosis
- flexibility + order (membrane is the midpoint), mitosis, meiosis, growth
- regulation of interaction (you don't want proteins to ALWAYS interact; fluidity allows them to come together)
- - lipid composition change based on temperature
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Asymmetry of membranes
- which membranes?
most, if not all, membranes
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What can vary on the membranes?
- carbs (usually stick out only in EM)
- protiens are asymmetrical, as well as lipids
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Explain the lipid variations
All three are made on __
- phosphatydylinositol: found inside facing cytosol
- phosphatydylserine: found inside facing cytosol
- phosphatydylcholine: outer part of the bilayer facing the EM
- the cytosolic surace
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True or False: movement across memrbanes is easy.
False: not easy
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Explain in depth PC?
- made inside ER, goes to vesicles, and fuses with plasma membranes, going to the outer leaflet by flip flopping.
- Flip flopping is very rare; PC is helped by proteins called flipases
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Which movement is more rapid?
lateral movement is more rapid than flip flop movement
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__ can move in and out of membrane. __ exhibits this. You are more apt to find cholesterol in what?
- lipids
- cholesterol
- within a cell that has a ton of it as opposed to one that does not
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Asymmetry o membranes allows __ of proteins.
functioning
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What are the functions of the membrane?
- barrier and compartmentalization
- regulation of movement in and out of compartment
- mechanical strength
- access to info and interaction with envi.
- fusion/ splitting of cells and vesicles
- interaction of cells and organelles with one another
- allows cell to organize (site of some biochem reactions)
- site of energy conversions
- electrical insulation
- cell movement
- cell shape
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Explain barrier and compartmentalizaton
- lipid bilayer does this; whaevter is inside usually stays in; out stays out unless you have help
- benefit: have internal environment different from outside
- compartmentalizes cytoplasm
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Explain regulation of movement in and out of compartment.
- proteins do this
- if something does not readily move and has no protein to help, it will not get through; allows control
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The function of the membrane relies on __. __ are the most important.
- lipids nad proteins
- proteins
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Molecules on the surface of animal cells, especially __, allow __ and __.
How?
- glycoproteins
- recognition
- interaction
- receptor-mediated endocytosis
- cell-cell recognition and adhesion
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Receptor-mediated endocytosis
- There are three types of endocytosis. What are the other two?
- phagocytois: changes shape and moves out around cell; pushes inward; unspecific
- pinocytosis: no receptors; membrane pushes inward; unspecific
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All three endocytosis end up with what?
material inside a depression, which splits from membrane
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Receptor-mediated endocytosis
-explain it.
- Receptor proteins on surface that will specifically bind those components to the receptor
- Allows uptake of things available in low concentration in environment
- - will result in increased concentration in cell
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How is cholesterol circulated in the bloodstream?
in packets of about 1500 cholesterol with some protein
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WHere is cholesterol produced?
by liver's hepatocytes and secreted by liver
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Why would other cells need cholesterol?
make membranes; they would take in the packets
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What are the two types?
- LDL
- HDL: more proteins; take cholesterol back to the liver where it gets destroyed
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What do animal cells do when they need cholestero?
make receptors
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Basic system for taking up these particuels?
- receptors associate in forming pits before anything
- receptors get stuck in pits and bond to LDL
- If the pit is deep enough, it breaks off--> becomes a coated vesicle because clathrin is always there on the outside
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After the vesicle is formed, what does it want to do? Why?
- vesicle wants to get rid of clathrin; now able to fuse with lysosome or endosome
- if not removed, the clathrin gets in the way of the binding
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Once in the lysosome, what happens?
the food is broken down to substituents--> able to be used
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What causes pits to form?
without Clathrin, there is no Receptor mediated endocytosis; it is needed to form pits
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Once there's enough free cholesterol, what happens?
- its a feedback loop
- it turns off the LDL receptor synthesis and cholesterol synthesis
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What happens to receptors in receptor-mediated endocytosis?
receptors are recycled back to membrane (some systems don't do that)
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Anyy genetic defect in LDL receptor proteins lead to __.
- oversupply of cholesterol in bloodstream--> plaque in arteries (usually at site of injury, etc.)
- - reduces elasticity and diameter--> strain on heart
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How could receptor proteins be defective?
- mutations that affect binding site (bind something it isn't supposed to_
- cell just dones't make them
- shape change so it cant be inserted into membrane
- cant bind clathrin and get caught/ accumulate in pits
- can't be recycled
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