Membrane Transport

Question 1

membrane permeability and transport

Answer 1

• interior of lipid bilayer is hydrophobic -> passage of most polar molecules is restricted
• 15-30% of all membrane proteins are transport proteins

Question 2

transport proteins

Answer 2

• 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+

Question 3

membrane potential

Answer 3

• 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

Question 4

resting membrane potential of cells

Answer 4

• 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)

Question 5

electrochemical gradient

Answer 5

• combination of membrane potential and concentration gradient of solute
• electrochemical gradient of a charged solute affects its transport

Question 6

proteins can associate with plasma membrane by different ways:

Answer 6

1. single alpha helix
2. multiple alpha helices
3. rolled-up beta sheet (beta barrel)
4. attached only to one layer (with one hydrophobic face)
5. attached to membrane by a covalently bound lipid chain
6. via an oligosaccharide
7. /8. attached to other proteins

Question 7

transporters share common structural features:

Answer 7

• 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

Question 8

most membrane proteins cross lipid bilayer in an alpha-helical conformation:

Answer 8

• Na/glucose contransporter SGLT
• glucose transporter GLUT
• Na/Ca exchanger (NCX)

Question 9

2 main classes of membrane transport proteins are:

Answer 9

1. channels

2. transporters (carriers)

Question 10

channels

Answer 10

• form pores for specific solutes (ions, water, ammonia)

- they interact with solute much more weakly compared to transporters

Question 11

transporters

Answer 11

• 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

Question 12

ion channels

Answer 12

• 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

Question 13

vestibule and selectivity filter in K+ channel

Answer 13

• 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

Question 14

Ion channel types

Answer 14

• voltage-gated
• ligand-gated (extracellular ligand)
• ligand-gated (intracellular ligand)
• mechanically gated

Question 15

aquaporins

Answer 15

• 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

Question 16

Anti-diuretic hormone (ADH)

Answer 16

• 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.

Question 17

passive transport

Answer 17

• occurs spontaneously down a gradient via diffusion (through plasma membrane or channels or passive transporters)

Question 18

active transport

Answer 18

• requires energy as it moves solutes against their concentration gradients (always mediated by transporters)

Question 19

each transporter can have one or more specific binding sites for its solute (substrate). T/F?

Answer 19

True

Question 20

outward-open state

Answer 20

• binding site for solutes is exposed to outside

Question 21

occluded state

Answer 21

• binding sites are not accessible

Question 22

inward-open state

Answer 22

• binding sites exposed to the inside

Question 23

simple diffusion

Answer 23

• 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)

Question 24

facilitated diffusion

Answer 24

• 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)

Question 25

carrier-mediated transport

Answer 25

• saturable

Question 26

active transport

Answer 26

• uses energy or a gradient generated by another active transporter
• classified according to direction of transport as well as the use of energy

Question 27

active transport - direction

Answer 27

• uniport: moving 1 molecule
• symport: moving 2 molecules in 1 or same direction (co-transport)
• antiport: moving 2 molecules in counter directions

Question 28

active transport - energy

Answer 28

• primary active
• secondary active
• tertiary active

Question 29

primary active

Answer 29

• Na+, K+ ATPase

Question 30

secondary active

Answer 30

• Na, proton exchanger, NHE

- driven by gradient that was generated by a primary active transporter

Question 31

tertiary active

Answer 31

• proton/peptides co-transporter, PEPT

- driven by a gradient that was generated by a secondary active transporter