Test 4: Chapter 11 Flashcards
1
Q
*plasma membrane
A
- a protein-studded, fatty film
- so thin that it cannot be seen directly in a light microscope
- separates and protects chemical components from the outside environment AND molecs from switching sides
- Structure:
- two-ply sheet of lipid molecs about 5nm(50 atoms) thick that are studded with proteins
- bacteria only have one-ply
- two-ply sheet of lipid molecs about 5nm(50 atoms) thick that are studded with proteins
- nutrients pass in and waste out
- to do so, membrane is penetrated by highly selective channels and transporters(proteins)
2
Q
*Cell membrane roles
A
- Cell communication
- receptor proteins let it receive signals
- Import and export of molecules
- transport proteins
- Cell growth and motility
- flexibility and capacity for expansion
- enlarges in area by adding new membrane without ever losing its continuity, and it can deform without tearing
3
Q
*lipid bilayer
A
- all cell membranes are made of lipids and proteins and are generally the same structure
- lipds arrange in 2 closely apposed sheets(lipid bilayer)
- permeability barrier to most water-soluble molecs
4
Q
*phospholipids
A
- most abundant lips in cell membranes
- phosphatidylcholine(most common in cell membranes) has choline attached to phosphate group as its hydrophilic head
- amphipathic
- hydrophilic heads face water on both surfaces of bilayer, but hydrophobic tails are shielded from water on interior
- self-sealing: any tear makes free edge exposed to water which is energetically unfavorable, so molecs of bilayer spontaneously fix small tears
- large tears: sheet folds in on itself and breaks up into separate closed vesicles
5
Q
*lipid bilayer flexibility
A
- membrain is a 2dimensional fluid: keeps molecs from changing places w/in molec –> integrity
- flexible: sets limit of 25nm to size of vesicle that cell membranes form
- synthetic bilayers allow movements to b measured
6
Q
*types of lipid bilayer movement
A
-
Flip-flop:
- phospholipid molecs tumble from one half of bi/monolayer to other
- occurs
-
lateral diffusion(lipids exchanging place with neighbor lipids in same monolayer(contiuous-all the time))
- leads to rapid lateral diffusion of lipied molecs, so lipid in artificial bilayer can diffuse a length equal to that of an entire bacterial cell(~2nm) in one second
-
flexion(flexing hydrocarbon tails) and rotation(rotating rapidly about their axis)
- up to speeds of 500 revolutions/sec
7
Q
*fluidity of lipid bilayer depends on its composition
A
- the closer and more regular the packing of the tails, the more viscous and less fluid the bilayer will be
- length and the number of double bonds affect compactness
- short chain = more fluid
- double bonds = unsaturated bc not all Hs are attached to carbon backbone
- double bond = kink in tail = more fluid
- no double bonds = saturated
- Bacteria+yeast: higher temp = long tail and less double bonds
- cholesterol
- 20% of lipids in membrane are cholesterol
- makes bilayer less fluid
- fluidity allows membrane proteins to diffuse rapidly, ensures even distribution of membrane molecs btwn daughter cells, and allows membrane fusion
8
Q
*beginning of membrane assembly
A
- enzymes bound to cytosolic surface of ER produce new phospholipids which are deposited ONLY in cytosolic half of bilayer
- free fatty acids act as substrates
- they are then catalyzed by scramblases which remove randomly selected phospholipids from one half of the lipid bilayer and insert them in the other
- ensures even distribution of phospholipids
- some remain in ER, others refill membrane
- Bits of membrane are continually pinching off the ER to form small, spherical vesicles that then fuse with other membranes, such as those of the Golgi apparatus
9
Q
*some phospholipids are confined to one side of membrane
A
- Golgi apparatus contains flippases which remove specific phospholipids from the side of the bilayer facing the exterior space and flip them into the monolayer that faces the cytosol
- initiates and maintains assymetric arrangement of phospholipids
10
Q
*orientation of cell membraanes
A
cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either the cell exterior—in the case of the plasma membrane—or to the interior space (lumen) of an organelle
11
Q
glycolipids
A
- found mostly in plasma membrane and ONLY in noncytosolic half of the bilayer
- Their sugar groups face the cell exterior, where they form part of a continuous coat of carbohydrate that surrounds and protects animal cells
- glycolipids acquire their sugar groups in Golgi apparatus where there are no filppases to transfer them to cytosolic
12
Q
inositol phospholipids
A
- small component of plasma membrane
- relay signals from cell surface to cell interior
- found in cytosolic half
13
Q
membrane proteins
A
- animals: constitute 50% of membrane
- but, lipids are much smaller than proteins, so there are 50 times more lipids
- functions:
- transport nutrients across bilayer
- anchor membrane to macromolecs
- receptors that detect chemical signals
- each type has diff proteins and diff functions
14
Q
how membrane proteins associate with lipid bilayred
A
- Many membrane proteins extend through the bilayer, with part of their mass on either side(transmembrane proteins~amphipathic)
- detergents remove these integral membrane proteins
- peripheral membrane proteins can be removed more easily
- detergents remove these integral membrane proteins
- located almost entirely in the cytosol and are associated with the cytosolic half of the lipid bilayer by an amphipathic α helix exposed on the surface of the protein
- lie entirely outside the bilayer, on one side or the other, attached to the membrane only by one or more covalently attached lipid groups
- bound indirectly to one or the other face of the membrane, held in place only by their interactions with other membrane proteins
15
Q
polypeptides cross lipid bilayer as an alpha helix
A
- portions of a transmembrane protein located on either side of the lipid bilayer are connected by specialized membrane-spanning segments of the polypeptide chain
- run through the hydrophobic environment of the interior of the lipid bilayer, are composed largely of amino acids with hydrophobic side chains.
- interact w/ hydrophobic tails of lipids
- polypeptide backbone is hydrophilic, so atoms from backbone H bond w/ one another forming alpha helix
- hydrophobic side chain is on outside of helix and touches phydrophobic lipids tails
- polypep backbone forms H bonds on inside
- most polypep chains cross membrane only once
- multipass polypeps have 1+ amphipathic region coming from alpha helix that had philic and phobic side chains