Membranes and cellular transport

Question 1

Membrane functions

Answer 1

-Boundary and permeability barrier
-Organisation and localisation of function (specific reaction occurs in specific organelle)
-Transport Processes
-Signak Detection
-Cell-to-cell interactions

Question 2

Passive transport

Answer 2

Exergonic movement down the concentration gradient
-Simple diffusion
-Facillitated diffusion

Question 3

Active Transport

Answer 3

Endergonic movment up concentration gradient away from equilibrium
-Primary active transport

Question 4

Electrochemical Potential

Answer 4

Movement of ion is determined by electrochemical potential
-This is the combined effect of its concentration gradient and the charge gradient across the membrane

Question 5

Membrane potential

Answer 5

Charge gradient across the membrane
-Created by active transport of ions across the membrane

Question 6

Simple diffusion

Answer 6

-Unassisted movement
-High to low
-Movement towards equilibrium

Question 7

E. simple diffusion

Answer 7

Typically gases, nonpolar molecules, small polar molecules (H2O, ethanol and glycerol)
-Includes: O2 and CO2 in and out of RBC

Question 8

Facilitated diffusion

Answer 8

-Protein-mediated movement down gradient
-Limited number of transport proteins => can become saturated

Question 9

E. facillitated diffusion

Answer 9

Polar or charged molecules like glucose

Question 10

Transport proteins

Answer 10

Large, integral membrane proteins with multiple transmembrane segments

Question 11

Carrier proteins

Answer 11

Bind solute molecules on one side of a membrane, undergo a conformational change and release the solute on other side of membrane
-Include transporters and permeases

Question 12

Channel proteins

Answer 12

Hydrophilic channels through membrane to provide passage route for solutes

Question 13

Alternating conformational model of carrier proteins

Answer 13

-Facilitated diffusion involves binding a substrate on a specific solute-binding site
-The carrier protein and solute form an intermediate
-After conformational change, the “product” is released (transport solute)
-Carrier proteins are regulated by external factors

Question 14

Carrier protein characteristics

Answer 14

-High specificty
-Can become saturated
-Competitive inhibition is possible (because they can become saturated; useful for drug design)

Question 15

Uniporter

Answer 15

-A carrier protein that transports a single solute across the membrane

Question 16

Coupled transport

Answer 16

Two solutes are simultaneously transported

Question 17

Symport/cotransport

Answer 17

2 solutes are moved across a membrane in the same direction

Question 18

Antiport

Answer 18

Solutes are moved in opposite directions

Question 19

E. uniport

Answer 19

Glucose transporter (GLUT1) in erythrocytes

Question 20

GLUT1 mechanism

Answer 20

-Glucose binds to GLUT1 transporter protein that has a binding site open to the outside of the cell (T1 conformation)
-Glucose binding causes the GLUT1 transporter to shift to its T2 conformation with the binding site open to the inside of the cell
-Glucose is released to the interior of the cell, initiating a second conformational change in GLUT1
-Loss of bound glucose causes GLUT1 to return to its original (T1) conformation, ready for a further transport cycle

Question 21

Anion exchange protein example

Answer 21

Chloride bicarbonate exchanger
-Antiport carrier
-Reciprocal exchange of Cl and HCO3- ions in 1:1 ratio
-Follows a ping pong model

Question 22

Chloride bicarboante exchanger mechanism

Answer 22

-Cl binds to the protein on one side of membrane
-Binding causes a conformational change in the protein allowing the release on other side
-Cl release facilitates HCO3 binding, triggering another conformational change, releasing HCO3 on the other side

Question 23

Channel proteins

Answer 23

Forms hydrophilic channels through membranes
-Allows direct movement
Highly selective
-Includes ion channels, porins and aquaporins

Question 24

Types of channel proteins

Answer 24

-Ligand-gated
-Mechanically-gated
-Always open
-Voltage-gated

Question 25

where is mechanical gated found

Answer 25

Mainly found in plant cells where high water pressure causes channels to open or close

Question 26

Active transport use

Answer 26

-Uptake of essential nutrients
-Waste removal
-Maintenance of nonequilibrium of certain ions

Question 27

Direct active transport

Answer 27

Involves a transport system coupled to an exergonic chemical reaction
-most commonly hydrolysis of ATP

Question 28

Most common type of direct active transport

Answer 28

Hydrolysis of ATP drives the outward transport of protons which creates an electrochemical potential across the membrane

Question 29

Indirect active transport

Answer 29

Involves the coupled transport of a solute and ions
-The exergonic inward movement of protons provides the energy to move a transported solute against its concentration or electrochemicl gradient

Question 30

Na+/K+ ATPase pump overview

Answer 30

Direct active transport
-Most cells have high K+ and low Na+ outward
-Pumps K+ inward and Na+ outward
-Allosteric protein with 2 alternate conformational states: E1 and E2

Question 31

Na+/K+ ATPase mechanism

Answer 31

-3 Na+ from inside the cell bind to E1
-Na+ binding triggers autophosphorylation of the alpha subunit using ATP and ADP is released
-A conformational change to E2 expels 3 Na+ to the outside of the cell
-2 K+ from outside bind to E2
-K+ binding triggers dephosphorylation causing a conformational change back to E1
-2 K+ expelled to inside as ATP binds and the pump returns to initial stae

Question 32

E1 conformation

Answer 32

Pump open to inside
initial state

Question 33

E2

Answer 33

Pump open to outside
ready to start second half of cycle

Question 34

Na+/glucose symporter mechanism

Answer 34

-2 Na+ from outside the cell are bound
-Binding of Na+ allows glucose binding and a subsequent conformational change
-Symporter opens to inside
-Na+ are released inside, but are continually extruded to outside by a seperate Na+/K+ pump
-Loss of Na+ is followed by glucose release to inside
-Release of glucose allows the empty symporter to return to initial state

Question 35

Importance of transporters for commercial use

Answer 35

-Drug targets
-Drug resistance mechanisms

Question 36

Aquaporin

Answer 36

Question 37

Aquaporin

Answer 37