Membrane Transport Flashcards
- characteristics that affect permeability of molecules - membrane proteins & topology - examples of common membrane proteins - important differences btwn ion channels and transport proteins (carriers) - channels? how are they activated? - facilitated diffusion across plasma membrane - primary, secondary, tertiary active transport
1
Q
membrane permeability and transport
A
- interior of lipid bilayer is hydrophobic -> passage of most polar molecules is restricted
- 15-30% of all membrane proteins are transport proteins
2
Q
transport proteins
A
- transfer specific water/soluble molecules across plasma membrane
- the smaller the molecule and the less strongly associated with water, the more rapidly it diffuses across the membrane:
(most easily diffused to most difficult)
hydrophobic molecules: O2, CO2, N2, steroid hormones
small uncharged polar molecules: H2O, urea, glycerol
large uncharged polar molecules: glucose, sucrose
ions: H+, Na+, HCO-3, K+, Ca2+, Cl-, Mg2+
3
Q
membrane potential
A
- resting membrane potential: membrane potential of unstimulated cell
- difference in electrical charge on the 2 sides of the membrane is due to a slight excess of positive ions over negative ones on one side and slight deficit on other
4
Q
resting membrane potential of cells
A
- result of an active transport (electrogenic) and a passive diffusion:
- Na,K ATPase pumps Na+ out of cell and draws K+ ions into cell
- K+ tends to diffuse out of cell through potassium channels to reach an equilibrium whereas negative charged ions (phosphates and proteins) stay inside the cell
- interior of cell will turn more negative (-70 to -90 mV)
5
Q
electrochemical gradient
A
- combination of membrane potential and concentration gradient of solute
- electrochemical gradient of a charged solute affects its transport
6
Q
proteins can associate with plasma membrane by different ways:
A
- single alpha helix
- multiple alpha helices
- rolled-up beta sheet (beta barrel)
- attached only to one layer (with one hydrophobic face)
- attached to membrane by a covalently bound lipid chain
- via an oligosaccharide
- /8. attached to other proteins
7
Q
transporters share common structural features:
A
- typically built from 10 or more alpha helices that span the membrane (transmembrane domains)
- substrate binding sites are located midway through the membrane
- show 2 different states: 1) inward-open, and 2) outward-open conformation
- binding sites are accessible by passageways from only one side of membrane at one time
- they would be able to work in the reverse direction if ion and solute gradients are adjusted
8
Q
most membrane proteins cross lipid bilayer in an alpha-helical conformation:
A
- Na/glucose contransporter SGLT
- glucose transporter GLUT
- Na/Ca exchanger (NCX)
9
Q
2 main classes of membrane transport proteins are:
A
- channels
2. transporters (carriers)
10
Q
channels
A
- form pores for specific solutes (ions, water, ammonia)
- they interact with solute much more weakly compared to transporters
11
Q
transporters
A
- bind specific substrate (solute) to be transported and undergo a series of conformational changes that alternately expose solute-binding sites on one side of membrane and then to other to transfer solute across it
12
Q
ion channels
A
- have a region that forms a gate and a region that forms a pore for one specific solute
- pore narrows to atomic dimensions in one region -> selectivity filter
13
Q
vestibule and selectivity filter in K+ channel
A
- in vestibule: ions are hydrated
- in selectivity filter: ions have lost their water and oxygens of the carbonyl groups of the channel to accommodate dehydrated solutes
- since Na+ is smaller than potassium, it can not be successfully accommodated and will not be recognized in the filter
14
Q
Ion channel types
A
- voltage-gated
- ligand-gated (extracellular ligand)
- ligand-gated (intracellular ligand)
- mechanically gated
15
Q
aquaporins
A
- specific WATER channels
- cells that secrete or absorb high amounts of water express aquaporins on their plasma membrane making water movement more efficient
- ex: cells lining ducts of exocrine glands and cells in the kidney
- some aquaporins are hormone-responsive and play an important role in formation of a concentrated urine
16
Q
Anti-diuretic hormone (ADH)
A
- stimulates aquaporins in the collecting ducts
- water deficit increases extracellular osmolarity which activates osmoreceptors in the hypothalamus. This in turn, causes ADH secretion in the posterior pituitary and increases water permeability in the collecting ducts.
17
Q
passive transport
A
- occurs spontaneously down a gradient via diffusion (through plasma membrane or channels or passive transporters)
18
Q
active transport
A
- requires energy as it moves solutes against their concentration gradients (always mediated by transporters)
19
Q
each transporter can have one or more specific binding sites for its solute (substrate). T/F?
A
True
20
Q
outward-open state
A
- binding site for solutes is exposed to outside
21
Q
occluded state
A
- binding sites are not accessible
22
Q
inward-open state
A
- binding sites exposed to the inside
23
Q
simple diffusion
A
- no membrane proteins are involved
- direction of transport is determined simply by the relative concentrations of molecules inside and outside of cells
- net flow of molecules is always down their concentration gradient (from compartment of high concentration to one with a lower concentration)
24
Q
facilitated diffusion
A
- movement of molecules in direction determined by their relative concentrations inside and outside of cell
- no external source of energy is provided so molecules travel across membrane in direction determined by their concentration gradients
- in case of charged molecules, by their electric potential across the membrane
- passage is mediated by proteins that enable transported molecules to cross membrane without directly interacting with its hydrophobic interior (allows passage of polar molecules)
25
carrier-mediated transport
- saturable
26
active transport
- uses energy or a gradient generated by another active transporter
- classified according to direction of transport as well as the use of energy
27
active transport - direction
- uniport: moving 1 molecule
- symport: moving 2 molecules in 1 or same direction (co-transport)
- antiport: moving 2 molecules in counter directions
28
active transport - energy
- primary active
- secondary active
- tertiary active
29
primary active
- Na+, K+ ATPase
30
secondary active
- Na, proton exchanger, NHE
| - driven by gradient that was generated by a primary active transporter
31
tertiary active
- proton/peptides co-transporter, PEPT
| - driven by a gradient that was generated by a secondary active transporter
