11.3 Solute Transport Across Membranes

Question 1

summary of transport types

Answer 1

-nonpolar compounds can dissolve in the lipid bilayer and cross a membrane unassisted
-polar compounds and ions require a specific membrane protein carrier

Question 2

simple diffusion

Answer 2

movement of a solute from the region of higher concentration to the region of lower concentration

chemical gradient

Question 3

membrane potential
Vm

Answer 3

a transmembrane electrical gradient that occurs when ions of opposite charge are separated by a permeable membrane

produces a force that:
-opposes ion movements that increase Vm
-drives movements that reduce Vm

electrical gradient

Question 4

electrochemical gradient (electrochemical potential)

Answer 4

determines the direction in which a charged solute moves across a membrane

composed of:
-the chemical gradient
-the electrical gradient (Vm)

Question 5

passive transporters

Answer 5

facilitate movement down a concentration gradient, increasing the transport rate

process is called passive transport or facilitated diffusion

Question 6

active transporters

Answer 6

move substrates across a membrane against a concentration gradient or an electrical potential

process is called active transport

Question 7

primary active transporters

Answer 7

use energy provided DIRECTLY by a chemical reaction

Question 8

secondary active transporters

Answer 8

couple uphill transporter of one substrate with downhilll transport of another

Question 9

transporter proteins reduce the ________ for diffusion by:

Answer 9

energy of activation by:
1. forming noncovalent interactions with the dehydrated solute
2. providing a hydrophilic transmembrane pathway

Question 10

ion channels

Answer 10

provide an aqueous path across the membrane through which inorganic ions can diffuse at very high rates
-most have a “gate” regulated by a biological signal
-typically show some specificity for an ion
-are not saturable with their ion substrate
-flow stops either when the gate is closed or when there is no longer an electrochemical gradient

Question 11

differences between ion channels and transporters

Answer 11

ion channels: single gate while a transporter (pump) ha alternating gates

Question 12

the glucose transporter of erythrocytes mediates

Answer 12

passive transport

Question 13

describe glucose transporter mechanism

Answer 13

enters the erythrocyte by passive transport via GLUT1

analogous with an enzymatic reaction where:
-glucose (“substrate”) outside is Sout
-glucose (“product”) inside in Sin
-transporter (“enzyme”) is T

Question 14

GLUT1

Answer 14

integral membrane protein with 12 hydrophobic segments that form 12 membrane-spanning helices
-helices are amphipathic (residues are nonpolar on one side and polar on the other side)

Question 15

the model of glucose transport into erythrocytes cycles between two extreme conformations:

Answer 15

T1 form
T2 form

Question 16

T1 form

Answer 16

glucose-binding site exposed on the outer membrane surface

Question 17

T2 form

Answer 17

glucose-binding site exposed on the inner surface

Question 18

transport of glucose into a myocyte by GLUT4 is regulated by

Answer 18

insulin

know pathway

Question 19

chloride-bicarbonate exchanger

Answer 19

anion exchanger essential in CO2 transport to the lungs from tissues

-passive transport system
-electroneutral exchange= no net transfer of charge

Question 20

cotransport systems

Answer 20

simultaneously transport two solutes across a membrane
1. antiport
2. symport

Question 21

antiport

Answer 21

moves in opposite directions

Question 22

symport

Answer 22

moves in the same direction

Question 23

uniport systems

Answer 23

carry only one substrate

Question 24

active transport results in

Answer 24

the accumulation of a solute above the equilibrium point

thermodynamically unfavorable (endergonic)
-must be directly or indirectly coupled to an exergonic process

Question 25

primary active transport

Answer 25

solute accumulation is coupled directly to an exergonic chemical reaction

Question 26

secondary active transport

Answer 26

endergonic transport of one solute is coupled to the exergonic flow of a different solute that was originally pumped uphill by primary active transport

Question 27

free-energy change for transport of an uncharged solute

Answer 27

no bonds are broken and delta G is zero

Question 28

free-energy change for transport of an ion

Answer 28

without movement of an accompanying counterion, the process is electrogenic (produces an electrical potential)

Question 29

P-type ATPases

Answer 29

family of cation transporters that are reversibly phosphorylated by ATP as part of the transport cycle
-integral proteins with 8 or 10 predicted membrane-spanning regions
-sensitive to inhibition by the transition-state analog vanadate

Question 30

examples of P-Type ATPases include

Answer 30

Na+K+ATPase= animal cell antiporter for Na+ and K+
H+ATPase= plant and fungi transporter
SERCA pump

Question 31

SERCA pump

Answer 31

sarcoplasmic/ endoplasmic reticulum Ca2+ ATPase pump

uniporter for calcium ions

Question 32

general structure of the P-type ATPases

Answer 32

critical Asp residue in the P domain undergoes phosphorylation and dephosphorylation

Question 33

postulated mechanism of the SERCA pump

Answer 33

each catalytic cycle moves two calcium ions across the membrane and converts an ATP to ADP and Pi

two conformations of the transporter, E1 an E2 interconvert

Question 34

Na+K+ATPase

Answer 34

couples phosphorylation-dephosphorylation of the critical Asp resiue to the movement of Na+ and K+ against their electrochemical gradients
-maintains low sodium concentration and high potassium concentration in the cell
-essential to the conduction of action potentials in neurons

Question 35

V-Type and F-Type ATPases are

Answer 35

ATP-Driven proton pumps

Question 36

V-type ATPases

Answer 36

class of proton-transporting ATPases responsible for acidifying intracellular compartments
-V0 domain serves as a proton channel
-V1 domain contains the ATP-binding site and the ATPase activity

Question 37

F-Type ATPases

Answer 37

catalyze the uphill transmembrane passage of protons, driven by ATP hydrolysis

-F0 integral membrane protein complex provides a pathway for protons
-F1 protein uses energy of ATP to drive proteins uphill

Question 38

F-Type ATPases catalyze reactions in

Answer 38

both direction

a large proton gradient can supply the energy to drive ATP synthesis

when functioning in this direction, F-type ATPases are named ATP synthases

Question 39

ABC transporters

Answer 39

family of ATP-driven transporters that pump substrates across a membrane against a concentration gradient

they have 2 ATP-binding domains (“cassettes”) and two transmembrane domains

example substrates: amino acids, peptides, proteins, metal ions, various lipids, bile salts, and hydrophobic compounds including drugs

Question 40

mechanism of ABC transporters

Answer 40

substrates move across the membrane when two forms of the transporter intercovert
-driven by ATP hydrolysis

Question 41

ion gradients provide the energy for

Answer 41

secondary active transport

Question 42

Na+-glucose symporter

Answer 42

takes up glucose from the intestine in a process driven by the downhill flow of Na+

strong thermodynamic tendency for Na+ to move into the cell provides the energy needed for the transport of glucose into the cell

Question 43

ionophores

Answer 43

“ion bearers”
compounds that shuttle ions across membranes in this way

acts as a shuttle, carrying K= across the membrane down its concentration gradient and deflating that gradient

Question 44

aquaporins (AQPs)

Answer 44

provide channels for movement of water molecules across plasma membranes
-each protein has a specific location and role
-low activation energy suggests that water moves in a continuous stream
-do not allow passage of protons (hydronium ions)

Question 45

ion-selective channels

Answer 45

move inorganic ions across membranes
-the rate of flux can be orders of magnitude greater than the turnover number for a transporter
-not saturable
-gated in response to cellular event

Question 46

ligand-gated channels

Answer 46

binding of an extracellular or intracellular small molecules forces an allosteric transition in the protein, which opens or closes the channel
-generally oligomeric

Question 47

voltage-gated ion channels

Answer 47

a change in transmembrane electrical potential (Vm) causes a charged protein domain to move relative to the membrane, opening or closing the channel

Question 48

patch-clamping

Answer 48

currents are measured through a region of the membrane surface containing only one or a few ion-channel molecules