lecture 2- membranes

Question 1

membranes and their environment

Answer 1

membranes can act independently of their environment, but also with it- needs to receive and interpret signals from outside environment to know what to do, has to communicate with its surroundings

Question 2

structural components of cell membranes

Answer 2

phospholipid & protein

• lipids cannot dissolve in water, so lipids make a good barrier (water can’t pass through), cell membrane made up of only lipids cannot exist in aqueous environment –> so convert lipid to phospholipid, substitute phosphate group for fatty acid (add some polarity)

Question 3

describe structural changes made to phospholipid in membrane

Answer 3

add choline to the phosphate group, since nitrogen has 4 bonds, it is positive (have put a phosphocholine group on one glycerol carbon), so have added a polar component to the lipid

• phosphocholine is the hydrophilic head (allows this area to be dissolved in water, can exist in water)
• the twin fatty acid tails are the hydrophobic tails, water insoluble, form the barrier of the membrane of the inside and outside

Question 4

phospholipids are ___, meaning they are both __ and __, part likes __ and part likes __

Answer 4

amphipathic
hydrophilic and hydrophobic
water and lipid

Question 5

which sides of the membrane do the hydrophilic heads face and hydrophobic tails face

Answer 5

hydrophilic layer faces outside the cell and inside the cell

hydrophobic tails buried inside membrane and forms water insoluble layer

membrane is fluid (always in motion), lipids form lipid bilayer

Question 6

membranes made only of lipids have disadvantages-

Answer 6

fairly rigid, impermeable

Question 7

proteins can exist in different configurations in the lipid bilayer, name 2 kinds of proteins and the one subset

Answer 7

1- peripheral proteins
2- integral proteins
3- transmembrane proteins

Question 8

describe peripheral proteins as a part of the membrane

Answer 8

on the surface, peripheral proteins exist on the inside or outside of membrane

• usually small & loosely connected (no strong covalent bonding)
• act as surfactants (surface-acting proteins)- they lower the rigidity of the membrane and make it more flexible

Question 9

peripheral proteins act as ___, they lower the rigidity of the membrane and make it more ___

Answer 9

surfactants
flexible

Question 10

describe integral proteins as a part of the membrane

Answer 10

permanent anchored part of the membrane, have many functions- function as receptors, or cell-surface recognition proteins, or as enzymes

Question 11

describe transmembrane proteins as a part of the membrane

Answer 11

subset of integral proteins that span the whole width of the membrane

• can act as receptors and can form a channel in the membrane, can act as transport proteins to bind substances and carry across membrane , can also act as pumps (transport substance across membrane against a gradient)

Question 12

proteins are arranged in such a way in the membrane to be referred to as…

Answer 12

fluid mosaic model (arranged in mosaic tile fashion)

Question 13

describe the distribution of amino acids of the proteins in a membrane

Answer 13

within membrane: nonpolar amino acids (9 nonpolar aa’s, aromatic aa’s), hydrophobic- no charge at all
- anchors protein into membrane/lipid bilayer

on outer surfaces of membrane in fluid- polar amino acids, hydrophilic
- extend into extracellular fluid & cytosol

Question 14

describe the distribution of amino acids as a part of the pores that span the membrane

Answer 14

amino acids on the outside of the pore are nonpolar, interact with fatty acid tails

• the inside of the pore has polar aa’s, allows water and soluble substances to pass through

Question 15

describe transport proteins as simple channels & aquaporins

Answer 15

some transport proteins form simple channels, just a hole in the membrane

water channel (aquaporin)- allows free passage of water in and out of cell (always open) –> the cell regulates the number of these channels inserted into membrane in order to regulate the amount of water in/out

Question 16

describe an example of a very large & complex transmembrane protein

Answer 16

acetylcholine receptor- large transmembrane protein made of 15 subunits, arranged in a circular ring, middle of ring creates a channel in the membrane

• when acetylcholine binds to receptor, opens the channel

Question 17

some transmembrane proteins form pumps…

Answer 17

• pumping goes on against the gradient

sodium pump: splits ATP (energy), pumps 3 sodiums outward and 2 potassium inward

Question 18

some transmembrane proteins span the membrane between 2 adjacent cells…

Answer 18

form pores that allow communication between one cell to the other through movement of small substances like ions
- these are called gap junctions

Question 19

name 4 mechanisms of membrane transport

Answer 19

1- diffusion
2- facilitated diffusion
3- endocytosis/exocytosis
4- active transport

Question 20

define diffusion

Answer 20

the passive, random movement of a substance through a membrane from an area of high to an area of low concentration (downhill)
- passive- does not require energy on the part of the cell

Question 21

describe the process of diffusion in terms of ions

Answer 21

ions always going to be moving due to thermal energy, as they move, going to collide with one another – every once in a while, big enough collision that it will push an ion through the membrane

• the energy is from collisions due to thermal energy, more collisions there are –> greater probability it will push one through the membrane from high to low conc.
• also collisions going on on the inside b/c some ions on inside –> every once in a while, can push an ion back out (so theoretically can move in both directions)

Question 22

in the process of diffusion, the energy is from ___ due to ___

net movement will always be from __ to __ concentration

Answer 22

collisions
thermal energy
high to low

Question 23

what can diffuse through the lipid bilayer without a protein?

Answer 23

anything that is nonpolar:
respiratory gases- O2, CO2
small uncharged molecules (not polar)

anything water soluble, amino acids, ions, glucose, urea cannot diffuse- needs protein carrier

Question 24

what is more permeable- Na or K?

Answer 24

K is 30x more permeable than Na

Question 25

diffusion can be explained with this equation

Answer 25

Fick’s law of diffusion ds/dt = Ds x As x dc/dx

rate of diffusion = conc grad x SA x diffusion coeff / membrane thickness

ds/dt = the rate of diffusion
the change in concentration of substance / change in
time
Ds = diffusion coefficient (measure of the permeability of a substance across that membrane) – nonpolar substance has higher permeability –> higher rate of diffusion

As - stands for surface area of membrane that’s available for the substance to diffuse across (nonpolar like oxygen diffuse faster than Na+ b/c surface area)

dc/dx
dc = the larger the conc gradient, faster the diffusion
dx = area that has to be crossed (thicker the membrane, slower the diffusion)

Question 26

describe facilitated diffusion

Answer 26

diffusion with the aid of a carrier molecule (still passive, still goes from high to low conc), just needs some type of protein carrying molecule that can bind a substance and carry across

• usually speeds things up, increases rate of diffusion
• ex: myoglobin (transport of oxygen across membrane from cell into mitochondria by carrying it)

Question 27

which is faster- simple or facilitated diffusion

Answer 27

facilitated is faster

• simple diffusion is like everyone walking thru a door, have to open and close the door each time (slower)- ion diffusion through a channel is simple diffusion

Question 28

example of facilitated diffusion & simple

Answer 28

facilitated- myoglobin transports oxygen
simple- ion diffusion through a channel

Question 29

define exocytosis

Answer 29

the quantitative release of a substance from a vesicle; vesicle opens and releases all the substance at once, takes energy to do this but not necessarily against the concentration gradient

Question 30

define endocytosis

Answer 30

membrane invaginates and buds off and forms vesicle and engulfs something from outside to bring inside

Question 31

do exocytosis and endocytosis require energy?

Answer 31

yes

Question 32

define active transport

Answer 32

the net movement of a substance against the concentration gradient (low to high conc) requiring the expenditure of energy on the part of the cell (energy usually in the form of splitting ATP)

• protein pump has ability to split ATP and use that energy to transport substances across membrane against gradient

Question 33

the major lipids of the plasma membrane are ___ and ___

Answer 33

phospholipids and phosphoglycerides

Question 34

the sterol molecule ___ is is a critical component of lipid bilayer, found in both leaflets and serves to stabilize the membrane at normal body temp

Answer 34

cholesterol

Question 35

as temperature increases, fluidity of the membrane ___

Answer 35

increases

Question 36

a minor lipid component of the plasma membrane is ___, an example of this called ___ plays an impt role in anchoring proteins to the outer leaflet of the membrane

Answer 36

glycolipid
GPI

Question 37

both cholesterol & glycolipids are ___, just like phospholipids, oriented with their polar groups on the outer surface and their hydrophobic portion on the interior of the membrane

Answer 37

amphipathic

Question 38

transmembrane proteins have both hydrophilic and hydrophobic regions…the hydrophobic region is often in the form of ___ and spans the membrane; hydrophilic amino acid residues are then exposed to the aqueous environment on either side of the membrane

Answer 38

alpha helix

Question 39

the superfamily of membrane proteins that serve as receptors for many hormones, neurotransmitters, and numerous drugs , these receptors are coupled to ___ and are called ___

these proteins span the membrane with _____ domains

Answer 39

heterotrimic G proteins
G protein- coupled receptors

7 alpha-helical domains

Question 40

membrane transporters are divided into 3 groups based on their mode of transport:

Answer 40

1- uniporter (facilitated transporter)- transports a single molecule across the membrane

2- symporter (cotransporter)- couples the movement of 2 or more molecules/ions across the membrane

3- antiporter (exchange transporter)- couples the movement of 2 or more molecules/ions across the membrane, but in opposite directions

Question 41

the ATPase ion transporters are are subdivided into…

Answer 41

1- P-type ATPases
2- V-type ATPases

Question 42

describe P-type ATPases

Answer 42

they are phosphorylated during the transport cycle

ex: Na+/K+ ATPase
- with the hydrolysis of each ATP molecule, it transports 3 Na+ out of cell and 2 K+ into cell

Question 43

describe V-type ATPases

Answer 43

found in the membranes of several intracellular organelles (aka vacuolar H+ ATPases)

• the H-ATPase in the plasma membrane plays impt role in urinary acidification

Question 44

a specific type of endocytosis is ___, which consists on the nonspecific uptake of small molecules and water into the cell

Answer 44

pinocytosis

Question 45

the movement of water across cell membranes occurs by the process of ___

the movement of water is passive with the driving force for this movement being the ___ difference across the cell membrane

Answer 45

osmosis

osmotic pressure

Question 46

osmotic pressure is determined by the number of ___ dissolved in ___

it is not dependent on factors such as…

Answer 46

solute molecules
the solution

size of molecules, their mass, or chemical nature

Question 47

what is the misspelled word on p.5 of textbook

Answer 47

imbedded/embedded