L21 - Plants and Microbes

Question 1

Evolution of the ion channel? (Organism/first type etc) (4)

Answer 1

Origins in prokaryotes

Earliest = K+ channels

Excitability depends on VGCC (Ca2+ and Na+)

Na+ channels evolved from Ca2+ channels

Question 2

Looking at the DNA sequences of different organisms

Answer 2

Selective permeability

Communication between the cell and the environment is through the ligand gated channels

Question 3

Why do vertebrate have more types of channels?

Answer 3

exploited a lot more niches

Question 4

Ion channels in prokaryotes

Answer 4

No evidence of excitable

Oscillations in membrane potential due to K+ flux regulate

Question 5

Paramecium (length, swimming)

Answer 5

• Single celled organism – 100-300 µm long
• Purposeful swimming locomotion (Rapid changes)
• Swims by coordinated beating of cilia

Question 6

paramecium resting potential and stimulus work?

Answer 6

• Resting membrane potential -40 mV

* Stimulus = chemical, heat, touch, light

Question 7

Orientation of receptors

Answer 7

Ca2+-linked mechanoreceptors at front end → backwards swim

K+-linked mechanoreceptors at back → faster forwards swim

Stimulus → receptor potential → Ca2+-based action potential → increased intracellular Ca2+ → reversal of ciliary beat

• Repolarisation → return to forward swimming

Question 8

Mutant para

Answer 8

• Mutants without action potentials can move but show impaired responses to stimuli – locomotion no longer purposeful

Question 9

The action potential

Answer 9

• Fast, regenerative, graded to size of receptor potential
• Voltage-gated Ca2+ channels
• Ca channel inactivation due to Cai-dependent-Ca-inactivation
• Delayed V-gated K+ channels
• Further conductances – eight in total – dictate duration of depolarisation and hence backward swim

Question 10

How do cilia move?

Answer 10

• Whip-like movements of cilia coordinated into a wave

* Typical ‘9 + 2’ arrangement of microtubules to create axoneme

Question 11

What are the microtubules are cross linked by?

Answer 11

Dyne - stabilise the microtubules in the axoneme

Bending caused by crosslinks of dyne ‘walking’ along the mi

Increased intracellular Ca2+ causes reversal of ciliary beat – the cilia bends in the opposite way

Question 12

How many genes responsible for Behavioural mutant of paramecium

Answer 12

• Single gene mutations show specific deficits in locomotory responses

Question 13

Examples of mutations

Answer 13

• Pawn: little or no V-gated Ca current – cannot generate APs and cannot reverse direction of locomotion
  Shows that the action potential is dependent upon the calcium channels
• Dancer: enhanced Ca current – reverses in response to much weaker stimulation
• Pantophobiac: reduced V-gated K current – prolonged depolarisation and therefore swims backwards for longer
  Swing back longer than normal

Question 14

Didinium nasutum - Protozoan (Prey and cilia)

Answer 14

Eats paramecium

show fast, directed movements using beating cilia

• Has a mouth end and 2 rings of cilia – the rings of cilia are finely controlled in order to engulf the paramecium

Question 15

Mimosa pudica – the ‘sensitive plant’

Answer 15

Rapid response to touch, light, vibration, temperature.

Cells respond to touch by generating overshooting AP that propagate from cell to the base of the leaflet

• APs have fast rising phase and prolonged plateau

Question 16

Where are the excitable cells in mimosa?

Answer 16

• Excitable cells located in vascular bundle. RMP -150 mV

* Leaflet rapidly bends downwards

Question 17

What ion is an AP based in Mimosa?

Answer 17

Cl- ion-based action potential causes cell shrinkage
“excitation-turgor loss coupling”

o Fast rising phase - Cl- efflux
o Slower repolarising phase - K+ efflux
o H2O follows by osmosis

Question 18

What is the pulvin?

Answer 18

• Pulvinus attaches leaflet to stem
• Cells on upper surface have thick walls and cannot shrink
• Cell on lower surface shrink causing bending of pulvinus

Question 19

What contains high levels of chlorine?

Answer 19

Vacuole

Question 20

Dionea muscipula - Venus flytrap

Answer 20

• It shuts on itself due to excitability in the cells
• All the cells can generate AP (1-3 SEC DURATION), overshoot and long-lasting – 150 mV
• AP = Ca based
• They are cation based – calcium based
• When it touches the mechanoreceptor ⇒ bending of the cells (deformation) ⇒ AP which is fired and transmitted to the cells through the midrib
• It stays shut for a few days – digestion process

Question 21

n the excitable tissues of the Venus flytrap, action potentials are transmitted via:

Answer 21

Plasmodesmata

Question 22

The acetylcholine binding protein was isolated from:

Answer 22

Lymnaea stagnalis

Question 23

The subunits of the Gloeobacter violaceus ion channel (GLIC) contain how many transmembrane domains?

Answer 23

4

Question 24

Acetylcholine binding protein has sequence homology to which part of the nicotinic receptor subunits?

Answer 24

N terminus

Question 25

which of the following features is present in nicotinic receptor subunits but absent in ELIC?

Answer 25

cys-loop

Question 26

The Pawn mutant of Paramecium cannot generate voltage gated calcium currents. The consequence of this mutation is that Pawn:

Answer 26

cannot reverse direction of locomotion