Excitable cells in plants and microbes Flashcards
What is the order of the evolution of the ion channels?
• Potassium channels are probably the earliest members of the family and calcium channels were next to emerge, followed by sodium channels
What is a Paramecium and give features of it
- Paramecium is a single-celled organism, around 100-300 um long. It moves by coordinated beating of cilia.
- It demonstrates purposeful swimming behaviour and can rapidly change direction to avoid obstacles and predators
Paramecium is an excitable cell. What is it’s resting potential?
-40mV
What does a paramecium fire action potentials in response to and what action potentials does it fire?
can fire Ca2+ based action potentials in response to chemical, heat, touch and light stimuli
How does a Paramecium change direction whilst swimming?
At the front end it has mechanoreceptors linked to Ca2+ channels. When these are activated, a receptor potential occurs which can trigger a Ca2+ based action potential. This action potential increases intracellular Ca2+, which in turn causes the ciliary beat to reverse direction. The outcome is that the animal will swim backwards, away from the obstacle it has encountered. Repolarisation of the membrane leads to a return to forward swimming.
What does repolarisation of the membrane after the depolarisation phase of a Paramecium action potential rely on?
The delayed opening of voltage gated potassium channels.
What do the K+ linked mechanoreceptors at the back end of the Paramecium do?
At the back end of the animal there are K+ linked mechanoreceptors. When these channels are activated, the membrane hyperpolarises and the animal will swim forward at a faster rate.
How do Paramecium cilia move?
Paramecium cilia move in a wave in a whip-like, coordinated fashion. They are formed from a ‘9+2’ arrangement of microtubules termed the axoneme, which is stabilised by protein cross-links via the motor protein dynein. Bending of the cilium is caused by the dynein molecules ‘walking’ along the microtubule. This is similar to how actin and myosin interact in muscle
How does the ‘pawn’ paramecium mutant behave?
Pawn mutants cannot reverse direction. They show little or no calcium current and therefore cannot generate action potential
How does the ‘Dancer’ paramecium mutant behave?
Dancer mutants show an enhanced calcium current. They reverse direction in response to much weaker stimuli than normal cells.
How does the ‘Pantophobia’ mutant behave?
They have a reduced voltage gated potassium channel current and so show prolonged depolarisation. This means they swim backwards for much longer than wild-type.
What has rapid movement in plants evolved to help with?
- Protection from damage (predation)
- Prey capture
- Spreading pollen and seeds
Give features of the Mimosa pudica
- Mimosa is capable of rapid and coordinated movement of its leaves.
- It is thought that this response to touch, vibration, light and temperature, deters predators because it exposes the plant’s thorny stem.
What is the movement of Mimosa’s leaves brought about by?
- Excitation-turgor loss coupling
- a chloride ion-based action potential, which leads to cell shrinkage
How does Excitation-turgor loss coupling work?
- When an action potential is fired the rising phase is due to chloride efflux (chloride ions moving out of the cell). Repolarisation is due to potassium efflux. Therefore, there is a net loss of KCl from the cell wand water follows out of the cell by osmosis
- After the stimulus is finished the ions are pumped back in and turgor is restored.
- This mechanism is used in a specialised piece of tissue known as the pulvinus. The pulvinus attached leaflets to the stem of the Mimosa plant. Only the lower side cells of the pulvinus can lose turgor and shrink – they have thinner walls than the upper surface cells. This means the pulvinus will bend downwards
