BioPHYSICAL chemistry

18.10

=−12.3 −5.8 kJ mol−^1 =−18.1 kJ mol−^1

=9.6 +5.8 kJ mol−^1 =15.4 kJ mol−^1

ΔGNET=ΔGNa+ΔGK=−2.7 kJ mol−^1

18.11 Transporters move specific molecules across a membrane.

18.12 In the model the opening of a transporter shifts so that the substrate

has access initially to one side and then the other, with simultan-

eous access to both sides forbidden.

18.13 The helices are proposed to flex as the pH increases, causing the

channel to open.

18.14The factors include channel size, electrostatic interactions, and dipole

reorientation.

18.15The channel regions of the proteins are very similar but the voltage-

gated channels contain additional domains that change conformation

in response to a voltage across the membrane; this conformational

change is thought to open and close the channel region.

18.16Potassium channel: a voltage-gated channel has six transmembrane

helices. Only two are found for the bacterial KcsA, which is thought

to resemble the pore region. The organization of the remaining

regions are a S1–S4 voltage sensor, a T1 tetramerization domain,

and a signaling domain.

18.17 A ligand-gated channel is a channel with an opening that is gated

by the binding of a specific substrate.

18.18 The peripherial helices are proposed to shift in response to a

change in tension or membrane polarization, leading to conformation

changes that open or close the channel.

18.19The ion flow is partially controlled by dipoles from the surrounding

protein that favor the flow of a specific change in one direction.

18.20 The cavity allows the ions to enter without losing their soluated

water molecules until they enter the filter region.

=+( .0 0083 −)( 295 ) ln ( )(.

100

2

kJ molK^1 K 1965 kJ mmolV−^1 )( .− 006 V)

ΔGRT

c

c

out zF V

in

K=+∇ln

=+( .0 0083 −)( 295 ) ln ( )(.

1

150

kJ molK^1 K 1965 kJ mmolV−^1 )( .− 006 V)

ΔGRT

c

c

out zF V

in

Na=+∇ln

ANSWERS TO PROBLEMS 483

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