Test 4: Chapter 11

Question 1

*plasma membrane

Answer 1

• 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
• nutrients pass in and waste out
  
  • to do so, membrane is penetrated by highly selective channels and transporters(proteins)

Question 2

*Cell membrane roles

Answer 2

1. Cell communication
   
   1. receptor proteins let it receive signals
2. Import and export of molecules
   
   1. transport proteins
3. Cell growth and motility
   
   1. flexibility and capacity for expansion
   2. enlarges in area by adding new membrane without ever losing its continuity, and it can deform without tearing

Question 3

*lipid bilayer

Answer 3

• 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

Question 4

*phospholipids

Answer 4

• 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

Question 5

*lipid bilayer flexibility

Answer 5

• 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

Question 6

*types of lipid bilayer movement

Answer 6

• 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

Question 7

*fluidity of lipid bilayer depends on its composition

Answer 7

• 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

Question 8

*beginning of membrane assembly

Answer 8

• 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

Question 9

*some phospholipids are confined to one side of membrane

Answer 9

• 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

Question 10

*orientation of cell membraanes

Answer 10

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

Question 11

glycolipids

Answer 11

• 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

Question 12

inositol phospholipids

Answer 12

• small component of plasma membrane
• relay signals from cell surface to cell interior
  
  • found in cytosolic half

Question 13

membrane proteins

Answer 13

• 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

Question 14

how membrane proteins associate with lipid bilayred

Answer 14

1. Many membrane proteins extend through the bilayer, with part of their mass on either side(transmembrane proteins~amphipathic)
   
   1. detergents remove these integral membrane proteins
      
      1. ​peripheral membrane proteins can be removed more easily
2. 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
3. lie entirely outside the bilayer, on one side or the other, attached to the membrane only by one or more covalently attached lipid groups
4. bound indirectly to one or the other face of the membrane, held in place only by their interactions with other membrane proteins

Question 15

polypeptides cross lipid bilayer as an alpha helix

Answer 15

• 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

Question 16

Beta sheets

Answer 16

• some polypeps cross lipid bilayer as B sheet rolled into a cylinder shaped B barrel
  
  • philic face inside and phobic face outside and contact phobic core of lipid bilayer
• EX: porin proteins: form large, water-filled pores in mitochondrial and bacterial outer membrane
  
  • mitochondria and bacteria surrounded by double membranes: porins allow passage of only small nutrients across outer membranes

Question 17

detergents

Answer 17

• small, amphipathic, lipidlike molecules differ from membrane phospholipids in that they have only a single hydrophobic tail
• one tail = detergent molecs are shaped like cones
• in water, they aggregate in small clusters called micelles, rather than forming a bilayer as do the phospholipids, which—with their two tails—are more cylindrical in shape
• When mixed in great excess with membranes, the hydrophobic ends of detergent molecules interact with the membrane-spanning hydrophobic regions of the transmembrane proteins, as well as with the hydrophobic tail of phospholipid molecs, disrupts bilayer and separates proteins from phospholipids
• Because the other end of the detergent molecule is hydrophilic, these interactions bring the membrane proteins into solution as protein–detergent complexes; at the same time, the detergent solubilizes the phospholipids
• The protein–detergent complexes can then be separated from one another and from the lipid–detergent complexes for further analysis.

Question 18

Bacteriorhodopsin

Answer 18

• small protein(~250aa) found in plasma membrane of an archaean
• acts as a membrane transport protein that pumps H+ (protons) out of the cell using sunlight as energy
  
  • each contains a single light absorbing nonprotein
    
    • retinal(it’s purple)
    • retinal is attached to an a helix
    • when it absorbs a photon of light, changes shape and causes proteins to change, making one H+ transfer from retinal to outside bacterium
• concentration gradient of H+ inside to outside

Question 19

plasma membrane is reinforced by underlying cell cortex

Answer 19

• plants, yeasts, bacteria: cell wall
• animals: cell cortex(meshwork of fibrous proteins-rich in actin)
  
  • red blood cells: protein spectrin(long, thin, flexible)
    
    • spectrin abnormalities = anemia
  • actin and motor protein myosin are more complex
• cell cortex functions:
  
  • provide mechanical strength
  • selectively take up materials from environment
  • change shape actively
  • move
  • restrain diffusion of proteins w/in plasma membrane

Question 20

membrane domains

Answer 20

• functionally specialized regions on cell/organelle surface to confine to particular localized areas in bilayer membrane
  
  • plasma membrance proteins can be tethered to structures outside the cell(extracellular matrix or an adjacent cell)
  • cells can create barriers
• some membrane proteins move freely
• others have highly controlled mobility
  
  • confined to membrane domains in
    
    • cell cortex
    • matrix outside
    • other cells
    • diffusion barriers
• tight junctions act as diffusion barrier

Question 21

cell surface is coated with carbohydrate

Answer 21

• some lipids in outer later of plasma membrane have sugar covalently attached to them
• most are oligosaccharides(glycoproteins)
  
  • proteoglycans: contain 1+ long poly sacch chain
• glycocalyx/carbohydrate layer: sugar coating formed by all of the carbohydrate on glycoproteins, proteoglycans, and glycolipids on outside of plasma membrane
  
  • protects cell from mechanical damage
• As the oligosaccharides and polysaccharides adsorb water, they also give the cell a slimy surface, which helps motile cells such as white blood cells squeeze through narrow spaces and prevents blood cells from sticking to one another or to the walls of blood vessels
  
  • also facilitate cell-cell recognition and adhesion
  • lectins bind to particular oligosaccharide side chains
• sugars can be joined to form elaborate branch structure
• the carb layer on surface of cells is like a distinctive uniform, making each cell type distinct and recognizable

Question 22

GlycocalyxAids In Cell-Cell Recognition and Adhesion

Answer 22

• Eg. Response to inflammation
• Leukocytes: white blood cells
• Lectins: proteins that bind oligosaccharide carbohydrate
  
  • E.g. selectin

Question 23

4 classes of membrane proteins

Answer 23

1. transporters and channels
2. anchors
3. receptors
4. enzymes

Question 24

transporters and channels

Answer 24

• transporters(Na+ pump)
  
  • actively pumps Na+ out of cells and K+ in
• ion channels(K+ leak channels)
  
  • allows K+ ions to leave cells, thereby having a major influence on cell excitability

Question 25

anchors

Answer 25

* EX: integrins * link intracellular actin filaments to extracellular matrix protein

Question 26

receptors

Answer 26

* EX: platelet-derived growth factor (pDGF) receptor * binds extracellular pDGF and, as a consequence, generates intracellular signals that cause the cell to grow and divide

Question 27

integral vs peripheral membrane proteins

Answer 27

* integral membrane proteins: proteins that are directly attached to the lipid bilayed and can only be removed with detergents * peripheral membrane proteins: everything else * gentle extraction procedures can remove(interffere with protein-protein interactions