Membrane transport

Question 1

what are the two main classes of membrane proteins that mediate molecular traffic? how are they different?

Answer 1

transporters - have moving parts (some are called carriers, some are “pumps”); channels - form pores; weaker interaction

Question 2

what kind of molecules are allowed to easily diffuse across membrane?

Answer 2

water and nonpolar molecules to diffuse across

Question 3

what molecules must always use some kind of transport protein? give some examples

Answer 3

Polar molecules (sugars, nucleotides, ions, etc.)

Question 4

what are 3 characteristics of membrane transport proteins?

Answer 4

1. almost always multi-pass transmembrane proteins 2. allow hydrophilic solutes to cross without touching the hydrophobes in the lipid bilayer 3. Each membrane protein is specialized as to what it transports (specific)

Question 5

what does passive transport handle? what does it depend on?

Answer 5

transport of uncharged molecules is dependent on a downhill process called a concentration gradient (high to low)

Question 6

what type of gradient is used for substances that have a charge? what does this depend on?

Answer 6

transport is dependent on the membrane potential (electrical potential difference across the cell membrane) AND concentration gradient together is called electrochemical gradient

Question 7

how is passive trans different from active transport?

Answer 7

Active transport is an uphill process, requiring transporters to be linked to energy

Question 8

what are transporters also known as? what are they responsible for? how does this work? are these specific?

Answer 8

carrier proteins or permeases; Responsible for both active (pumps) and passive transport; Bind solutes then undergo conformational changes to transfer the bound solute across the membrane; yes specific for solute

Question 9

what is the basis of transport kinetics? what is this similar to? what circumstance makes transport at Vmax?

Answer 9

Binding of transported molecule affects rate of transport (diffusion kinetics); Similar to Michaelis-Menton for enzymes; If all solute sites on the transport protein are bound, rate of transport is maximal

Question 10

what does Vmax measure? what does Km mean? what does a lower Km indicate?

Answer 10

rate at which the carrier can “flip” its conformation; solute concentration at which transport rate is half maximum value; higher affinity of transporter to solute

Question 10

what is coupled transport? what are two examples of this? what do they do?

Answer 10

transport of one solute to transport another; symporters - simultaneous transfer of second solute in same direction as first solute antiporters - exchangers that move one substance in and another out

Question 11

what are the three types of active transport?

Answer 11

Coupled transport, ATP-driven pumps, Light- or redox-driven pumps

Question 12

what is a common co-transported solute? why is that? what is this specific transport driven by?

Answer 12

sodium; because Na is so high extracellularly, it “drags” other solutes into cells with it; ion gradient

Question 13

what is the function of antiporters? what are two examples?

Answer 13

Function to maintain cellular pH; Na-H exchanger that couples influx of Na to efflux of H+; Na and HCO3- (bicarbonate) are exchanged for Cl- and H+

Question 14

why are ATP-driven pumps given their name?

Answer 14

Called transport ATPases as they involve hydrolysis of ATP to ADP and phosphate

Question 15

what are the 3 classes of ATPase pumps?

Answer 15

P-type pumps, ABC transporters, V-type pumps

Question 16

what do P-type pumps do?

Answer 16

(self-phosphorylating): multipass proteins that maintain ionic gradients

Question 17

what are ABC transporters?

Answer 17

largest family of membrane proteins; ATP binding cassettes

Question 18

what are V-type pumps?

Answer 18

large turbine-like protein machines; puts hydrogen into organelles

Question 19

what is an example of a P-type pump? How is it used?

Answer 19

Ca pump in sarcoplasmic reticulum (SR) of skeletal muscle cells; SR serves as intracellular store of Ca in muscle; Ca pump moves Ca from the cytosol back into the SR

Question 20

name example of P-type pump found in all animal cell membranes; how does it work? where is this important?

Answer 20

Na-K pump; Operates an antiporter that pumps Na out of cell and K in, both moved against gradients, uses ATP hydrolysis; Important for action potential maintenance in neurons

Question 21

where are ABC transporters of clinical importance?

Answer 21

can be used as Multidrug resistance protein (MDR) transporter; can be overexpressed in cancer cells which causes cellular resistance to chemo drugs (pumps out chemo drugs so it does no harm to cell) OR in CFTR (cystic fibrosis) where it causes chloride accumulation

Question 22

where are ABC transporters typically found? what do they do here?

Answer 22

Present in ER membranes to transport degraded peptides from cytosol into ER, then out to plasma membrane for review by cytotoxic T-cells

Question 23

where does an ABC transporter act as a channel as well? what does it do and where? how is it different than most ABCs?

Answer 23

acts as a chloride channel in plasma membrane of epithelial cells; here it regulates ions in most exocrine tissues (pancreas, lung, sweat glands); transport process is driven by opening/closing chloride channel

Question 24

How do channels work? how do the solutes move here?

Answer 24

All channels allow solutes to gain access to cells via passive transport (facilitated diffusion); Solutes move based on concentration gradient

Question 25

what do charged solutes form?

Answer 25

form a membrane potential (difference in charge) across membrane

Question 26

channels can also be what? what are 3 examples? how does this compare to transporting? when does this occur and how is it facilitated?

Answer 26

pores; gap junctions, porins, ion channels; faster rate than transporting; Involved only in passive movement and facilitate this by forming hydrophilic transmembrane channels

Question 27

how are ion channels different than aquaporins? (3 reasons) where are ion channels present?

Answer 27

They are selective for the ions that can pass; They are gated, opening briefly and then closing; sometimes they can become refractory (quiet) (opened by different stimuli); present in all animal cells

Question 28

what are the 3 types of ion channels?

Answer 28

voltage-gated (electrical charge stimuli), ligand-gated (intra or extra ligand), mechanically gated (physical stimulus)