Membrane structure

Question 1

what is the cell membrane composed of? What holds it together?

Answer 1

Thin film of lipid and protein molecules held together by noncovalent interactions

Question 2

what is an important characteristic of the cell membrane?

Answer 2

Fluid and dynamic (needs to move)

Question 3

what is the cell membrane called? what function does it serve the cell?

Answer 3

lipid bilayer; impermeable barrier to water soluble molecules

Question 4

since the membrane is an impermeable barrier, how is the cell able to receive substances? what are some general examples?

Answer 4

Transmembrane proteins mediate all other functions of membrane; transport, catalysis, structure links, receptor action

Question 5

what makes up animal cell membranes? (percent)

Answer 5

50% of animal cell membranes are lipid, most of rest is protein

Question 6

what term describes lipid molecules? what does it mean?

Answer 6

amphiphilic - hydrophilic polar and hydrophobic nonpolar ends - this forms bilayer

Question 7

what are the most abundant membrane lipids? what do they look like? what are the main membrane lipids called?

Answer 7

phospholipids with two hydrophobic hydrocarbon tails (fatty acids); phosphoglycerides

Question 8

in the three most common phospholipids found in mammalian membranes, what are their backbones made of?

Answer 8

backbone is a 3-carbon glycerol

Question 9

what are the 4 common phospholipids called?

Answer 9

phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, sphingomyelin

Question 10

one of the 4 common phospholipids is not like the rest, which one is it and what makes it different?

Answer 10

sphingomyelin; these are built from sphingosine, not glycerol

Question 11

what serves as supportive region in the bilayer? how does it do this?

Answer 11

cholesterol and glycolipids; Cholesterol serves to increase impermeability of the lipid bilayer and prevents hydrocarbon tails from crystallizing (stabilizing)

Question 12

how is cholesterol oriented within the phospholipids?

Answer 12

sterol is oriented with its hydroxyl polar head close to polar heads of phospholipids

Question 13

how much lipid is the cell membrane composed of?

Answer 13

cell membranes have diff percent of lipids depending on their function

Question 14

how does the membrane maintain its flexibility?

Answer 14

Lipid molecules in membranes are disordered(making it flexible); the spacing between them is irregular and flexible

Question 15

why is fluidity of the membrane important? How does it maintain this?

Answer 15

for transport and enzymes to work; Lipid bilayer has bent tails to make the chains harder to pack together and the bilayer more difficult to freeze thus maintaining functionality at lower temps

Question 16

what are lipid rafts? why do they form? what are they made of?

Answer 16

specialized domains; form transiently only when needed for protein transport or signal conversion; Made of specific lipids and proteins

Question 17

what are the organelles lipid droplets used for? what do they contain? where are they formed?

Answer 17

Used to store excess cellular lipids, surrounded only by a phospholipid monolayer; contain neutral lipids like triglycerides and cholesterol esters; formed in the ER membrane

Question 18

what is an adipocyte?

Answer 18

carry excess lipids; one big fat droplet

Question 19

what does the lipid composition cause within the bilayer? what does this cause? what two reasons make it important?

Answer 19

asymmetry; Makes charges between the two halves of the bilayer different (providing electrochemical gradient for transport); crucial for converting extracellular signals into intracellular ones; also for marking dying cells

Question 20

what are glycolipids? where are they found? what are their 3 functions?

Answer 20

sugar-containing lipid molecules found exclusively in noncytosolic side of lipid bilayer; help cell interact with its surroundings, protect from harsh surroundings, affect electrical field of membrane and ion concentration

Question 21

where are sugars added to the glycolipids? why here?

Answer 21

Sugars are added in lumen of Golgi apparatus (environment similar to extracellular)

Question 22

what are gangliosides? where are they found?

Answer 22

complex glycolipids found mostly in nerve cells

Question 23

where can membrane proteins be located on the bilayer?

Answer 23

transmembrane- goes thru bilayer; intracellular/cytosolic- cytosol side of the bilayer; extracellular/noncytosolic - on cell surface

Question 24

what role does a glycosylphosphatidylinositol (GPI) anchor play in transport? Describe the 3 steps the protein must take to reach its destination.

Answer 24

1. Special membrane proteins that are extracellular are first made in the ER of the cell as single-pass initially
2. Transmembrane segment is cleaved (part that helped it go thru membrane) and the GPI anchor is added instead (only hold protein has)
3. Protein is delivered via transport vesicle to the cell membrane

Question 25

how do peripheral membrane proteins attach?

Answer 25

bound via some noncovalent interaction with other membrane proteins

Question 26

how are transmembrane proteins attached? How do they function?

Answer 26

held in the hydrophobic core of the lipid bilayer and cannot be released easily; function on both sides of the bilayer or transport molecules across it

Question 27

how does the membrane-spanning domain (transmembrane domain) TMD work within the lipid bilayer?

Answer 27

outside of the bilayer is hydrophilic and inside of it is hydrophobic, so TMD are made of non-polar amino acids; the amino acids form hydrogen bonds with each other, so peptide chain curls into an alpha-helix as it crosses bilayer

Question 28

what are single and multi-pass proteins?

Answer 28

Single-pass membrane proteins cross the lipid bilayer only once; multi-pass cross multiple times

Question 29

what can multi-pass proteins form? what can this turn into? why is it done?

Answer 29

beta sheets; which form into a barrel; which is a protein to satisfy hydrogen bonding requirements

Question 30

what are beta barrels also called? where are they more common?

Answer 30

porin proteins; bacterial and mitochondrial membranes

Question 31

what are multi-pass proteins constructed from? what 3 functions do the constituents have?

Answer 31

constructed from alpha helices that can slide, open and shut, transport or transduce

Question 32

how else can a beta barrel be used? what does it look like? what other function do they provide?

Answer 32

can form channels; may have amino acid loops that project into the center, filling the hole; may function as receptors or enzymes

Question 33

what other structure can be used to form a channel? how is it done?

Answer 33

alpha helices; Multiple transmembrane helices can arrange to create channels through the membrane; function similar to porin proteins

Question 34

what are hydropathy plots used for?

Answer 34

Illustrate the number of amino acids of a transmembrane protein (~20-30) and how many of these TMD are present

Question 35

what does it mean to by glycosylated? where are these effected proteins located? are they always found here? where does glycosylation occur?

Answer 35

contain sugar; most cell membrane proteins; yes, sugars are always present on the noncytosolic side of membrane; sugars are added in the ER and Golgi to proteins destined for the membrane

Question 36

where are carbs found on the cell? what protein can assist? what does this protein do?

Answer 36

coat the surface of all eukaryotic cells; lectins: carbohydrate-binding proteins; Mediate cell-cell adhesion processes and other recognition routes

Question 37

what are detergents? what can they do? how can this be used?

Answer 37

Small amphiphilic molecules of various structure; Hydrophobic ends of detergents bind to hydrophobic regions of membrane proteins and pull membrane proteins into solution; Good for studying membrane protein characteristics and activities

Question 38

what does it mean to have domains for proteins? what 2 variables make this possible? why is it done?

Answer 38

specialized regions where membrane proteins are segregated into specific areas of cell surfaces (each their own domain); 1. Proteins don’t swim through the lipids 2. Held in place by tight junctions, cytoskeleton, or protein-protein interactions; Keeps proteins AND lipids in appropriate places for specific functionality

Question 39

what are the 3 types of protein-protein interaction used with the special protein domain?

Answer 39

◦ Self-aggregation ◦ Tethering ◦ Interaction with other cells

Question 40

in red blood cells what protein tethers to the membrane proteins? what is the purpose of this protein?

Answer 40

spectrin protein; forms cell cytoskeleton; interacts with membrane proteins to produce a flexible membrane

Question 41

what disorder does a mutation in spectrin protein cause? what effect does it have? what can it lead to?

Answer 41

Hereditary spherocytosis; produces RBCs that have unusual cell membrane properties (sphere shape) inflexible, destroyed quickly; leads to anemia

Question 42

what do nucleated cells have that is analogous to spectrin's function? where is it found? what is its purpose?

Answer 42

"cortical cytoskeletal network" more complex network that makes up the cortical region of the cytoplasm; Cytoskeletal filaments can be attached to cytosolic membrane and form barriers to avoid protein movement

Question 43

what is the purpose of corralling proteins?

Answer 43

helps increase signal by concentrating activated signaling complexes