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- 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)
- 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
Chains of cholesterol are __.
What does cholesterol do to a membrane?
- 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]
What does it do to transition temperature?
lowers transition temp by decreasing fluidity (it'll be harder to pack the lipid molecule together)
Explain cholesterol in terms of temperature.
- at higher temperatures, because of rigidity, it decreases fluidity
- at lower temperature, it adds fluidity
Due to the __ of rings, it gives the membrane __.
- mechanical strength
Plants have no __, but may hve __ that may do the same thing.
Plant membranes must do what?
stay fluid just as animal membranes must
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
Why is the membrane moving anyway?
their own thermal energy
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
Asymmetry of membranes
- which membranes?
most, if not all, membranes
What can vary on the membranes?
- carbs (usually stick out only in EM)
- protiens are asymmetrical, as well as lipids
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
True or False: movement across memrbanes is easy.
False: not easy
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
Which movement is more rapid?
lateral movement is more rapid than flip flop movement
__ can move in and out of membrane. __ exhibits this. You are more apt to find cholesterol in what?
- within a cell that has a ton of it as opposed to one that does not
Asymmetry o membranes allows __ of proteins.
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
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
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
The function of the membrane relies on __. __ are the most important.
- lipids nad proteins
Molecules on the surface of animal cells, especially __, allow __ and __.
- receptor-mediated endocytosis
- cell-cell recognition and adhesion
- 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
All three endocytosis end up with what?
material inside a depression, which splits from membrane
- 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
How is cholesterol circulated in the bloodstream?
in packets of about 1500 cholesterol with some protein
WHere is cholesterol produced?
by liver's hepatocytes and secreted by liver
Why would other cells need cholesterol?
make membranes; they would take in the packets
What are the two types?
- HDL: more proteins; take cholesterol back to the liver where it gets destroyed
What do animal cells do when they need cholestero?
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
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
Once in the lysosome, what happens?
the food is broken down to substituents--> able to be used
What causes pits to form?
without Clathrin, there is no Receptor mediated endocytosis; it is needed to form pits
Once there's enough free cholesterol, what happens?
- its a feedback loop
- it turns off the LDL receptor synthesis and cholesterol synthesis
What happens to receptors in receptor-mediated endocytosis?
receptors are recycled back to membrane (some systems don't do that)
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
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