6. Ion Channels

Question 1

what are the three broad groups of ion channel

Answer 1

1. voltage-gated
2. neurotransmitter-gated
3. second messenger-gated

Question 2

what does gating refer to

Answer 2

how they open and close

- generally in the presence of a specific molecule that activates the channel

Question 3

describe ion movement during depolarisation

Answer 3

1. Na+ channels open = Na+ influx - membrane becomes more +

2. K+ channels open = K+ efflux - membrane voltage plummets (more -)

Question 4

what is another name for K+ channels involved in depolarisation

Answer 4

delayed rectifiers

Question 5

what does the term delayed rectifiers mean

Answer 5

there is a delay which fits with sodium channel activation and deactivation

Question 6

where are neurotransmitter-gated channels typically found

Answer 6

in the nervous system

Question 7

what happens when neurotransmitters bind to a neurotransmitter-gated channel?

Answer 7

can have either an excitatory or inhibitory effect

- e.g. result in action potential or inhibit it

Question 8

name 2 cations

Answer 8

acetylcholine, glutamate

Question 9

what happens when a cation binds to a neurotransmitter-gated channel?

Answer 9

results in action potential

Question 10

what happens when a anion binds to a neurotransmitter-gated channel?

Answer 10

causes an inhibitory effect

Question 11

name 2 anions

Answer 11

GABA

glycine

Question 12

name 2 examples of second- messengers that can gate channels

Answer 12

cAMP

cGMP

Question 13

give an example of second-messenger gated channels in the human body

Answer 13

in the retina: rod cells
- rod cells absorb photon activating a g-protein

• g-protein binds GTP activating phosphodiesterase
• phosphodiesterase works by hydrolysing cGMP into non-cyclin GMP
• ion channels open in the presence of cGMP, close in its absence

= hyperpolarisation as sodium can no longer enter

Question 14

what is the structure of voltage-gated channels

Answer 14

6 transmembrane domains
1 por domain 
voltage sensor (+ charged AAs)
N terminus forms ball and chain arrangement

Question 15

what makes the transmembrane spanning domain lipid stable

Answer 15

hydrophobic side chains

Question 16

what do the ions move through in voltage gated channels

Answer 16

the pore - also called the selectivity filter

Question 17

what domain in voltage gated channels is the voltage sensor

Answer 17

S4 domain

Question 18

how does the voltage sensor detect voltage change

Answer 18

have charged side chains that can detect voltage change in the membrane

Question 19

what is the purpose of the ball and chain

Answer 19

block the pore causing inactivation

Question 20

what is the structure of the voltage gated channel during deactivation

Answer 20

here the chain is not on but the pore is closed

Question 21

what is the structure of potassium channels

Answer 21

built out of 4 identical transmembrane subunits symmetrically arranged around a central pore

Question 22

what does each potassium transmembrane subunit contain

Answer 22

two membrane spanning alpha helices

Question 23

what is a tetramer

Answer 23

a structure with 4 subunits

Question 24

what do the potassium channel helices form, what does this generate

Answer 24

an inverted cone

• generates a water filled cavity called the vestibule

Question 25

describe how potassium moves through the potassium channel

Answer 25

K+ enters the selectivity filter and loses its 8 waters

this allows it to bind to carbonyl oxygen atoms

the pore only allows dehydrated ions to fit through

potassium binds tightly to carbonyl oxygens, the ion electrostatically repulses the one in front pushing it further down to the next carbonyl oxygen binding site

Question 26

why are potassium channels selective for potassium over sodium

Answer 26

sodium ions bind very strongly to water molecules (= hydration shells)

this makes it too big to fit through the pore

Question 27

describe sodium channel structure (3)

Answer 27

4 domains - each connected (monomer structure)

larger than K+

amino acid motif swings into the pore causing inactivation

Question 28

how does the sodium channel transport ions

Answer 28

transports hydrated ions

pores are wide enough to accommodate sodium and its tightly bound water

Question 29

give an example of a plant that uses voltage gated channels to close leafs

Answer 29

mimosa plant

Question 30

where is the voltage sensor located

Answer 30

in S4

Question 31

what happens when a voltage is sensed

Answer 31

positive charges on S4 react and move stretching the protein open
- pulling S5 and S6 away from interacting on other subunits