11.1 The Composition and Architecture of Membranes

Question 1

the lipid bilayer is

Answer 1

stable in water

Question 2

glycerophospholipids, sphingolipids, and sterols:

Answer 2

-virtually insoluble in water
-spontaneously form microscopic lipid aggregates when mixed with water

Question 3

hydrophobic interactions

Answer 3

the clustering of hydrophobic molecule surfaces in an aqueous environment to find the lowest-energy environment by reducing the hydrophobic surface area exposed to water

Question 4

micelles

Answer 4

spherical structures containing amphipathic molecules arranged with hydrophobic regions in the interior and hydrophilic head groups on the exterior

favored when the cross-sectional area of the head group is greater than that of the acyl side chain(s)

wedge-shaped

Question 5

bilayer

Answer 5

lipid aggregate in which two lipid monolayers (leaflets) form a 2-dimensional sheet

favored when the cross-sectional areas of the head group and acyl side chain(s) are similar

cylindrical

Question 6

vesicle (liposome)

Answer 6

forms spontaneously when a bilayer sheet folds back on itself to form a hollow sphere

Question 7

fluid mosaic

Answer 7

pattern formed by individual lipid and protein units in a membrane
-pattern can change while maintaining the membrane permeability

Question 8

functions of biological membranes

Answer 8

flexible
self-repair
selectively permeable

Question 9

flexible

Answer 9

permit shape changes that accompany cell growth and movement

Question 10

self-repair

Answer 10

permit exocytosis, endocytosis, and cell division

Question 11

selectively permeable

Answer 11

serve as molecular gatekeepers

Question 12

proteins and enzymes in and on membranes

Answer 12

transporters
receptors
ion channels
adhesion molecules

Question 13

transporters

Answer 13

move specific organic solutes and inorganic ions across the membrane

Question 14

receptors

Answer 14

sense extracellular signals and trigger molecular changes in the cell

-movement of a signal, not a molecule!

Question 15

ion channels

Answer 15

mediate electrical signaling between cells

Question 16

adhesion molecules

Answer 16

hold neighboring cells together

Question 17

endomembrane system is dynamic and functionally differentiated

Answer 17

single membrane
double membrane

Question 18

single membrane surrounds

Answer 18

endoplasmic reticulum (make protein)
golgi apparatus (post translational modification sorting of that protein)
lysosomes
various small vesicles

Question 19

double membrane surrounds

Answer 19

nucleus
mitochondrion
chloroplasts (in plants)

Question 20

general sequence of membrane trafficking for an extracellular protein

Answer 20

endoplasmic reticulum
cis Golgi
trans Golgi
secretory vesicle
secretion from the cell

Question 21

the functional specialization of each membrane type is reflected in

Answer 21

its unique lipid composition

Question 22

membrane trafficking

Answer 22

process by which membrane lipids and proteins that are synthesized in the ER move to their destination organelles or to the plasma membrane

Question 23

where do lipids and proteins undergo covalent modifications?

Answer 23

in the Golgi apparatus- dictates the eventual location of the mature protein

Question 24

changes in lipid composition during membrane trafficking

Answer 24

sphingolipids and cholesterol largely replace phosphatidylcholine

Question 25

how are plasma membrane lipids distributed

Answer 25

plasma membrane lipids are asymmetrically distributed between the two leaflets of the bilayer

Question 26

lipid transfer proteins (LTPs)

Answer 26

soluble proteins that contain a hydrophobic lipid-binding pocket to carry a lipid from one membrane to another can be bispecific move lipids basically sometimes ATP dependent

Question 27

groups of membrane proteins

Answer 27

-receptors for extracellular signals -transporters to carry specific polar or charged compounds across the plasma membrane or between organelles -enzymes

Question 28

posttranslational modification of membrane proteins

Answer 28

glycosylation attachment of 1+ lipids

Question 29

glycosylation

Answer 29

attachment of oligosaccharides to proteins -typically on the outer face of the plasma membrane

Question 30

attachment of 1+ lipids

Answer 30

serve as hydrophobic anchors or targeting tags

Question 31

membrane proteins differ in the nature of their association with the membrane bilayer

Answer 31

integral membrane proteins peripheral membrane proteins amphitropic proteins

Question 32

integral membrane proteins

Answer 32

firmly embedded within the lipid bilayer

Question 33

peripheral membrane proteins

Answer 33

associate with the membrane through electrostatic interactions and hydrogen bonding with hydrophilic domains of integral proteins and charged head groups of membrane lipids more loose

Question 34

amphitropic proteins

Answer 34

associate reversibly with membranes -found in both membranes and the cytosol

Question 35

monotopic proteins

Answer 35

have small hydrophobic domains that interact with only a single leaflet of the membrane

Question 36

bitopic proteins

Answer 36

span the bilayer once, extending on either surface -have a single hydrophobic sequence somewhere in the molecule

Question 37

polytopic

Answer 37

cross the membrane several times have multiple hydrophobic sequences of ~20 residues that each cross the membrane when in the alpha-helical conformation

Question 38

phospholipids lie

Answer 38

-on the protein surface -at interfaces between monomers of multisubunit proteins, forming a "grease seal"

Question 39

the topology of an integral membrane protein can often be predicted

Answer 39

from it sequence

Question 40

integral membrane protein: an alpha-helical sequence of

Answer 40

20-25 residues (each 1.5A) is just long enough to span the thickness (30A) of the lipid bilayer stabilized by intrachain hydrogen bonding and the hydrophobic effect

Question 41

hydropathy index

Answer 41

expresses the free-energy change associated with the movement of an amino acid side chain from a hydrophobic environment to water -ranges from highly exergonic to highly endergonic

Question 42

hydropathy plots

Answer 42

average hydropathy index plotted against residue number easy way for questions to be asked about membrane proteins

Question 43

window

Answer 43

segment of given length

Question 44

hydropathy index (y-axis)

Answer 44

average hydropathy for a window

Question 45

residue number (x-axis)

Answer 45

the residue in the middle of the window

Question 46

beta barrel

Answer 46

structural motif in which 20+ transmembrane segments form beta sheets that line a cylinder -stabilized by intrachain hydrogen bonds can't assume all beta barrels are porins

Question 47

porins

Answer 47

proteins that allow certain polar solutes to cross the outer membrane of gram-negative bacteria -have beta barrels lining the transmembrane passage

Question 48

in beta conformation:

Answer 48

-7 to 9 residues are needed to span a membrane -alternating side chains project above and below the sheet *alternating: hydropathy index does not work very well

Question 49

in beta strands of membrane proteins:

Answer 49

-every second residue in the membrane-spanning segment is hydrophobic and interacts with the lipid bilayer -aromatic side chains are commonly found at the lipid-protein interface

Question 50

which amino acid side chains serve as membrane interface anchors

Answer 50

Tyr and Trp

Question 51

positive-inside rule

Answer 51

positively charged Lys and Arg residues in the extramembrane loop of membrane proteins occur more commonly on the cytoplasmic face

Question 52

GPI-anchored protein

Answer 52

exclusively on the outer face and are clustered in certain regions extracellular spot of PM