L21 - Plants and Microbes Flashcards
Evolution of the ion channel? (Organism/first type etc) (4)
Origins in prokaryotes
Earliest = K+ channels
Excitability depends on VGCC (Ca2+ and Na+)
Na+ channels evolved from Ca2+ channels
Looking at the DNA sequences of different organisms
Selective permeability
Communication between the cell and the environment is through the ligand gated channels
Why do vertebrate have more types of channels?
exploited a lot more niches
Ion channels in prokaryotes
No evidence of excitable
Oscillations in membrane potential due to K+ flux regulate
Paramecium (length, swimming)
- Single celled organism – 100-300 µm long
- Purposeful swimming locomotion (Rapid changes)
- Swims by coordinated beating of cilia
paramecium resting potential and stimulus work?
- Resting membrane potential -40 mV
* Stimulus = chemical, heat, touch, light
Orientation of receptors
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
Mutant para
• Mutants without action potentials can move but show impaired responses to stimuli – locomotion no longer purposeful
The action potential
- 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
How do cilia move?
- Whip-like movements of cilia coordinated into a wave
* Typical ‘9 + 2’ arrangement of microtubules to create axoneme
What are the microtubules are cross linked by?
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
How many genes responsible for Behavioural mutant of paramecium
• Single gene mutations show specific deficits in locomotory responses
Examples of mutations
- 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
Didinium nasutum - Protozoan (Prey and cilia)
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
Mimosa pudica – the ‘sensitive plant’
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
Where are the excitable cells in mimosa?
- Excitable cells located in vascular bundle. RMP -150 mV
* Leaflet rapidly bends downwards
What ion is an AP based in Mimosa?
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
What is the pulvin?
- Pulvinus attaches leaflet to stem
- Cells on upper surface have thick walls and cannot shrink
- Cell on lower surface shrink causing bending of pulvinus
What contains high levels of chlorine?
Vacuole
Dionea muscipula - Venus flytrap
- 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
n the excitable tissues of the Venus flytrap, action potentials are transmitted via:
Plasmodesmata
The acetylcholine binding protein was isolated from:
Lymnaea stagnalis
The subunits of the Gloeobacter violaceus ion channel (GLIC) contain how many transmembrane domains?
4
Acetylcholine binding protein has sequence homology to which part of the nicotinic receptor subunits?
N terminus
which of the following features is present in nicotinic receptor subunits but absent in ELIC?
cys-loop
The Pawn mutant of Paramecium cannot generate voltage gated calcium currents. The consequence of this mutation is that Pawn:
cannot reverse direction of locomotion
