Membrane transport Flashcards
what are the two main classes of membrane proteins that mediate molecular traffic? how are they different?
transporters - have moving parts (some are called carriers, some are “pumps”); channels - form pores; weaker interaction
what kind of molecules are allowed to easily diffuse across membrane?
water and nonpolar molecules to diffuse across
what molecules must always use some kind of transport protein? give some examples
Polar molecules (sugars, nucleotides, ions, etc.)
what are 3 characteristics of membrane transport proteins?
- 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)
what does passive transport handle? what does it depend on?
transport of uncharged molecules is dependent on a downhill process called a concentration gradient (high to low)
what type of gradient is used for substances that have a charge? what does this depend on?
transport is dependent on the membrane potential (electrical potential difference across the cell membrane) AND concentration gradient together is called electrochemical gradient
how is passive trans different from active transport?
Active transport is an uphill process, requiring transporters to be linked to energy
what are transporters also known as? what are they responsible for? how does this work? are these specific?
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
what is the basis of transport kinetics? what is this similar to? what circumstance makes transport at Vmax?
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
what does Vmax measure? what does Km mean? what does a lower Km indicate?
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
what is coupled transport? what are two examples of this? what do they do?
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
what are the three types of active transport?
Coupled transport, ATP-driven pumps, Light- or redox-driven pumps
what is a common co-transported solute? why is that? what is this specific transport driven by?
sodium; because Na is so high extracellularly, it “drags” other solutes into cells with it; ion gradient
what is the function of antiporters? what are two examples?
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+
why are ATP-driven pumps given their name?
Called transport ATPases as they involve hydrolysis of ATP to ADP and phosphate
what are the 3 classes of ATPase pumps?
P-type pumps, ABC transporters, V-type pumps
what do P-type pumps do?
(self-phosphorylating): multipass proteins that maintain ionic gradients
what are ABC transporters?
largest family of membrane proteins; ATP binding cassettes
what are V-type pumps?
large turbine-like protein machines; puts hydrogen into organelles
what is an example of a P-type pump? How is it used?
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
name example of P-type pump found in all animal cell membranes; how does it work? where is this important?
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
where are ABC transporters of clinical importance?
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
where are ABC transporters typically found? what do they do here?
Present in ER membranes to transport degraded peptides from cytosol into ER, then out to plasma membrane for review by cytotoxic T-cells
where does an ABC transporter act as a channel as well? what does it do and where? how is it different than most ABCs?
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
How do channels work? how do the solutes move here?
All channels allow solutes to gain access to cells via passive transport (facilitated diffusion); Solutes move based on concentration gradient
what do charged solutes form?
form a membrane potential (difference in charge) across membrane
channels can also be what? what are 3 examples? how does this compare to transporting? when does this occur and how is it facilitated?
pores; gap junctions, porins, ion channels; faster rate than transporting; Involved only in passive movement and facilitate this by forming hydrophilic transmembrane channels
how are ion channels different than aquaporins? (3 reasons) where are ion channels present?
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
what are the 3 types of ion channels?
voltage-gated (electrical charge stimuli), ligand-gated (intra or extra ligand), mechanically gated (physical stimulus)
