Topic 5 - chapter 11 Membrane structure

Question 1

The lipid bilayer is a …-dimensional fluid

Answer 1

two-dimensional

Question 2

The fluidity of a lipid bilayer depends on its …? Which 3 factors?

Answer 2

composition

• phospholipid tail length
• phospholipid saturation level
• presence of cholesterol

Question 3

T or F - the lipid bilayer is symmetrical

Answer 3

false - asymmetrical

Question 4

What are the most abundant lipids in cell membranes? Whatis the most common membrane lipid?

Answer 4

phospholipids (head linked to rest of lipid via phosphate group)
phosphatidylcholine

Question 5

In the membrane lipid phosphatidylcholine, what makes up the hydrophilic & hydrophobic regions?

Answer 5

hydrophilic (polar head) - phosphate & choline

hydrophobic (non-polar tail) - hydrocarbon tail

Question 6

What are 3 different types of membrane lipids? egs…

Answer 6

phospholipid - phosphatidylserine
sterol - cholesterol
glycolipid - galactoserebroside

Question 7

Which polar molecule can dissolve in water?

Answer 7

acetone

Question 8

Which polar molecule can not dissolve in water?

Answer 8

2-methylpropane

Question 9

What is the difference between amphipathic and hydrophobic?

Answer 9

amphipathic molecules will always form a bilayer in water and spontaneously form sealed compartments

Question 10

Lipid asymmetry is preserved during …?

Answer 10

membrane transport

Question 11

Why is the lipid bilayer considered two-dimensional?

Answer 11

• phospholipids form spherical liposomes in water
• membrane behaves like a fluid
• temperature-dependant
• not much ‘flip-flopping’

Question 12

Approximate thickness of lipid membrane?

Answer 12

5nm

Question 13

3 main functions of plasma membranes…

Answer 13

• receiving info from from environment via receptor proteins
• import & export of small molecules via transport proteins
• cell growth & movement due to flexibility of membrane

Question 14

What is the hydrophilic/hydrophobic part of phosphatidylserine?

Answer 14

hydrophilic - serinephosphate

hydrophobic - hydrocarbon chain

Question 15

What is the hydrophilic part of cholesterol?

Answer 15

OH group

Question 16

What is the hydrophilic part of galactocerebroside?

Answer 16

sugar (galactose) & -OH group

Question 17

Size range of cell vesicles membranes can form?

Answer 17

25nm - 1mm

Question 18

Which ways can phospholipids move within the plane of the membrane?

Answer 18

lateral diffusion
flexion
rotation
‘flip-flopping’ (rarely)

Question 19

What is meant by fluidity of a cell?

Answer 19

the ease with which lipid molecules move within the plane of the bilayer

Question 20

Which structure(s) mainly affect membrane bilayer fluidity? Which 2 factors of those structures?

Answer 20

nature of the hydrocarbon tails:

• length (the shorter, the more fluid)
• number of double bonds (the more, the more fluid)

Question 21

What is saturated and unsaturated?

Answer 21

based on number of hydrogen bonds attached to the carbon tail
saturated - fatty acid tail with no double bonds and full complement of hydrogen atoms
unsaturated - has at least one double bond thus does not have maximal hydrogen atoms

Question 22

Usual hydrocarbon tail length?

Answer 22

18 - 20 carbon atoms

Question 23

How does cholesterol fit into the lipid bilayer?

Answer 23

in animal cells, cholesterol fits in between the kinked gaps left by double bonds thus stiffening the bilayer and making it less permeable

Question 24

Why is membrane fluidity so important?

Answer 24

• enables rapid diffusion of proteins
• enables protein interactions with one another
• enables membrane lipids & proteins to diffuse to other regions of the cell
• allows membranes to fuse with one another & mix their molecules
• ensures membrane molecules are distributed evenly between daughter cells during cell division

Question 25

In eukaryotic cells, what are phospholipids made from?

Answer 25

enzymes bound to ER membrane

Question 26

What enables the transfer of phospholipid molecules from one monolayer to another ('flip-flopping')?

Answer 26

flippases (enzymes)

Question 27

Glycolipids are mainly located where? Their sugar groups?

Answer 27

in the plasma membrane and are found ONLY in the noncytosolic half of the bilayer their sugar groups are exposed to the exterior of the cell forming a protective carb coating surrounding most animal cells

Question 28

List 4 functions that membrane proteins serve

Answer 28

transporters anchors receptors enzymes

Question 29

Transporters - protein example & function...?

Answer 29

transporters - Na+ pump: actively pumps Na+ out of cells and K+ into cells

Question 30

Anchors - protein example & function...?

Answer 30

anchors - integrins: link intracellular actin filaments with extracellular matrix proteins

Question 31

Receptors - protein example & function...?

Answer 31

receptors - platelet-derived growth factor (PDGF) receptor: binds extracellular PGDF -> intracellular signals for cell growth & division

Question 32

Enzymes - protein example & function...?

Answer 32

enzymes - adenylyl cyclase: catalyses production of intracellular signalling molecule cyclic AMP in response to extracellular signals

Question 33

What are integral membrane proteins?

Answer 33

can be transmembrane proteins, monolayer-associated proteins or lipid-linked proteins that can only be removed by disrupting the bilayer with detergents

Question 34

What are peripheral membrane proteins?

Answer 34

all other remaining membrane proteins such as protein-attached proteins

Question 35

Which membrane proteins are located entirely in the cytosol?

Answer 35

monolayer-associated via amphipathic alpha helix

Question 36

What are the hydrophobic & hydrophilic regions of alpha helixes?

Answer 36

hydrophobic amino acid side chains | hydrophilic (polar) peptide bonds (forming hydrogen bonds creating alpha helixes)

Question 37

Most common form in which polypeptide chain crosses lipid bilayer?

Answer 37

alpha helix

Question 38

What is the most 'striking' beta barrel structure?

Answer 38

porin protein in mitochondrial & bacterial membranes - allows passage of nutrients & small ions & blocks larger molecules (AB's & toxins)

Question 39

A bit about detergents...

Answer 39

amphipathic lipidlike molecules that solubilise membranes by destroying the lipid bilayer

Question 40

2 common detergents used in research?

Answer 40

sodium dodecyl sulphate (SDS) | Triton X-100

Question 41

How do detergents seperate transmembrane proteins from the bilayer membrane?

Answer 41

By using their hydrophobic regions that bind to membrane-spanning hydrophobic regions of transmembrane proteins and hydrophobic tails of phospholipid molecules

Question 42

Name 2 known transmembrane proteins whose structures are known...

Answer 42

- bacteriorhodopsin (proton pump) made up of 7 alpha helices - photosynthetic reaction centre made up of 4 subunits; L, M, H and cytochrome (L & M have 5 alpha helices and form the core while H has only 1 alpha helix) cytochrome is connected only to the transmembrane proteins and is outside of the cell

Question 43

Plasma membranes are reinforced by ...? Describe

Answer 43

the cell cortex - a series of meshwork of fibrous proteins mainly comprised of spectrin (100nm long flexible rods) in dimers connected to membrane via attachment proteins linking it to transmembrane proteins

Question 44

What are 4 ways in which movement of membrane proteins can be restricted?

Answer 44

- tethered to cell cortex inside the cell - tethered to extracellular matrix molecules outside the cell - tethered to surface proteins of another cell - tight junctions confine membrane proteins to certain areas

Question 45

What forms the carbohydrate layer?

Answer 45

oligosaccharides - glycolipids - glycoproteins - proteoglycans (contain long polysaccharide chains)

Question 46

Functions of the carbohydrate layer...?

Answer 46

- protect cell surface from mechanical & chemical damage - allows easy passage for WBCs - prevents blood cells sticking together or to blood vessel walls - can make many different arrangements for cell-cell recognition via lectins (eg. infection fighting and egg & sperm)