15.1 The Structure of the Plasma Membrane

Question 1

what are the main functions of a cell’s plasma membrane?

Answer 1

1. defining the cell boundary → separates it from the environment
2. serves as a selective barrier → determines the composition of the cytoplasm; allows only certain molecules through to the cytoplasm
3. mediating the interactions between the cell and its environment (mostly through embedded proteins)

Question 2

the fundamental structure of the plasma membrane

Answer 2

phospholipid bilayer

Question 3

describe the electron micrograph of the phospholipid bilayer in the plasma membrane

Answer 3

• darkly stained electron-dense polar head groups
• bands of lightly stained hydrophobic fatty acids alternate with polar head groups

Question 4

proteins embedded in the bilayer carry out functions including (1) and (2)

Answer 4

1. selective transport
2. cell-cell recognition

Question 5

what are the 5 major phospholipids that make up the animal plasma membrane

Answer 5

Outer leaflet
1. phosphatidylcholine
2. sphingomyelin

Inner leaflet
3. phosphatidylethanolamine
4. phosphatidylserine
5. phosphatidylinositol

Question 6

which PM phospholipids have an overall negative charge? neutra/no charge?

Answer 6

Negative: phosphatidylserine, phosphatidylinositol
Neutral: phosphatidylcholine, sphingomyelin, phosphatidylethanolamine

Question 7

the overall negative charge of the inner leaflet of the plasma membrane is largely due to the predominance of ()

Answer 7

phosphatidylserine

Question 8

() is the only major PM phospholipid that is synthesized in the Golgi

Answer 8

sphingomyelin

Question 9

localization of phosphatidylserine in the inner leaflet plays an important role in ()

Answer 9

programmed cell death

Question 10

In addition to the phospholipids, animal cell plasma membranes also contain:

Answer 10

1. glycolipids (minor component)
2. cholesterol (major component)

Question 11

glycolipids are found exclusively on the () of the PM

Answer 11

outer leaflet

Question 12

cholesterol exists as a major component of the PM having a () molar amount ratio as phospholipids

Answer 12

1:1

Question 13

cholesterol is more concentrated on the (1) leaflet of the PM due to high affinity for (2)

Answer 13

1. outer
2. sphingomyelin

Question 14

The two general features of lipid bilayers are:

Answer 14

1. water-insoluble hydrophobic fatty acid chains → bilayer serves as a barrier between 2 aqueous environments by being impermeable to water-soluble molecules (ions and most biological molecules)
2. viscosity (as a fluid) → lateral diffusion of lipids and proteins is possible because the membrane is soft and flexible

Question 15

states that membranes are 2D fluids with proteins inserted into bilayers; proteins and lipids are able to diffuse laterally through the membrane

Answer 15

fluid mosaic model

Question 16

the fluid mosaic model was proposed by () in 1972

Answer 16

Sanger and Nicolson

Question 17

how was lateral movement of embedded proteins in the PM first shown?

Answer 17

• fusion of human and mouse cells to produce human-mouse cell hybrids
• antibodies of each cell were labeled with different fluorescent dyes to easily distinguish them
• after incubation, the differently labeled antibodies were shown to have mixed and were no longer separated from each other

Question 18

what are the 2 classes of membrane-associated proteins (MAPs)?

Answer 18

1. peripheral membrane proteins
2. integral membrane proteins

Question 19

() are MAPs that are indirectly associated to the plasma membrane through protein-protein interactions

Answer 19

peripheral membrane proteins

Question 20

the bonds associating peripheral membrane proteins are usually ()

Answer 20

ionic

Question 21

bonds associating the peripheral MAPs are easily disrupted by (1), allowing the proteins to be soluble in (2)

Answer 21

1. polar reagents
2. aqueous environments

Question 22

examples of peripheral MAPs

Answer 22

proteins of the cortical cytoskeleton (spectrin, actin, band 4.1)

Question 23

MAPs with hydrophobic portions that are directly inserted into the lipid bilayer

Answer 23

integral membrane proteins

Question 24

integral MAPs can be dissociated from the PM only by reagents that disrupt ()

Answer 24

hydrophobic interactions

Question 25

what characteristic of detergents allow them to disrupt the association of integral MAPs from the PM?

Answer 25

detergents have hydrophobic and hydrophilic portions → amphiphatic molecules that can solubilize integral proteins

Question 26

integral membrane proteins are either:

Answer 26

1. transmembrane proteins 2. anchored to the plasma membrane by covalently attached lipids

Question 27

these integral MAPs span the lipid bilayer with portions exposed on both sides of the membrane

Answer 27

transmembrane proteins

Question 28

most transmembrane proteins of PM are (1) with their (2) exposed on the surface of the cell

Answer 28

1. glycoproteins 2. oligosaccharides

Question 29

give 2 major examples of transmembrane protein structure (found in red blood cells):

Answer 29

1. glycophorin 2. band 3

Question 30

glycophorin is a ()-pass transmembrane protein

Answer 30

single

Question 31

band 3 is a ()-pass transmembrane protein

Answer 31

multiple

Question 32

what are the mechanisms of proteins anchored to the PM by covalently attached lipids or glycolipids

Answer 32

1. glycosylphosphatidylinositol (GPI) anchors (outer leaflet) 2. N-terminal myristoyl group (inner leaflet) 3. C-terminal (cysteine residues) prenyl group or palmitoyl group (inner leaflet)

Question 33

describe the mechanism of how proteins are associated to the outer leaflet of the PM via GPI anchors

Answer 33

- glycosylated proteins from the ER are initially associated with the plasma membrane through their C-terminus - C-terminus is cleaved and exchanged for GPI anchor → protein is associated to PM only through glycolipid anchor

Question 34

describe the mechanism of how proteins are associated to the inner leaflet of the PM via the addition of an N-terminal myristoyl group

Answer 34

- addition of myristic acid (14 C fatty acid) to N terminus of polypeptides synthesized by free ribosomes

Question 35

describe the mechanism of how proteins are associated to the inner leaflet of the PM via the addition of a C terminal prenyl/palmitoyl group

Answer 35

- addition of prenyl groups (15 or 20 C fatty acid) or palmitic acid (16 Cs) to C terminus of polypeptides synthesized by free ribosomes

Question 35

carbohydrate coat formed by oligosaccharides of glycolipids and glycoproteins → covers cell surface

Answer 35

glycocalyx

Question 36

the glycocalyx is formed from:

Answer 36

1. heavy glycosylation of the extracellular portions of transmembrane proteins in PM 2. exposure of the carbohydrate portions of glycolipids in PM on the cell surface

Question 37

functions of the glycocalyx

Answer 37

1. protects cell surface from ionic and mechanical stress 2. forms a barrier to invading microorganisms 3. oligosaccharides participate in cell-cell interactions

Question 38

regions in which the mobility of MAPs are restricted

Answer 38

plasma membrane domains

Question 39

what are the 2 distinct plasma domains of epithelial cells?

Answer 39

1. apical domain 2. basolateral domain

Question 40

epithelial cell PM domain that faces intestinal lumen; covered by microvilli to increase surface area → efficiently facilitates nutrient absorption

Answer 40

apical domain

Question 41

epithelial cell PM domain that faces underlying connective tissue; specialized to mediate transfer of absorbed nutrients into the circulatory system

Answer 41

basolateral domain

Question 42

restrict PM proteins to the appropriate domains of the epithelial cell surface

Answer 42

tight junctions

Question 43

motility of PM proteins can also be restricted as a result of association with the (1) or with (2) (location of lipid-protein interactions

Answer 43

1. cytoskeleton 2. specialized lipid domains

Question 44

() are specialized lipid domains formed by clusters of sphingomyelin, glycolipids, and cholesterol

Answer 44

lipid rafts

Question 45

the rigidity of lipid rafts is mostly due to () and its interaction with (2) to form small semisolid patches

Answer 45

1. cholesterol 2. sphingomyelin

Question 46

lipid rafts are enriched in (1) and (2) that are involved in various functions

Answer 46

1. GPI-anchored proteins 2. transmembrane proteins

Question 47

() are a subset of lipid rafts that start as invaginations in the PM; formation required high content of membrane cholesterol

Answer 47

caveolae