3.6.1 stimuli and response

• what are plant responses coordinated by?

  Chemicals called plant growth factors
• what is Indoleacetic acid (IAA)?

  belongs to a class of chemicals called auxins
• How does IAA influence plant growth factors?

  it either stimulates or inhibits cell elongation
• where is IAA synthesised?

  in cells at the tip of the shoot -> growing regions of a plant
• when there is light present, where does it cause IAA to move to?

  the shaded side of the shoot
• What are plant growth factors?

  Plant growth factors are chemicals which regulate plant growth in response to directional stimuli.
• IAA is a chemical
• Explain how plant shoots grow towards a source of light.

  IAA accumulates in cells that are not facing the light. 

  In shoot tips, IAA stimulates cell elongation. 

  This causes the side not facing the light to grow faster than the side facing the light, causing the shoot tip to bend towards the light.

  
• A shoot is illuminated from one side.How does IAA cause the shoot to bend towards the light?

  illumination of the shoot from one side causes IAA to move from the lit side to the shaded side.This means there is a greater concentration of IAA in cells that are not facing the light.In shoots, IAA stimulates cell elongation.This means the shaded side grows faster than the lit side, causing the shoot to bend towards the light.
• Explain how plant roots grow in response to gravity.

  IAA accumulates in cells that are on the lower side of the root.

  In root tips, IAA inhibits cell elongation. 

  This causes the upper side of the root to grow faster than the lower side, causing the root tip to bend downwards.

   
• A high concentration of IAA has a different effect in shoots compared to roots.Describe this difference.

  In positive phototropism in shoots, high concentrations of IAA stimulate cell elongation.In positive geotropism in roots, high concentrations of IAA inhibit cell elongation on the lower side.
• How does IAA cause roots to bend towards the force of gravity?

  IAA accumulates in cells that are on the lower side of the root.In roots, IAA inhibits cell elongation.This means the upper side of the root grows faster than the lower side, causing the root to bend downwards.

  Explanation
• In an axon, what is a nerve impulse?

  a moving area of charge
• a nerve impulse always travels in one direction. What is that?

  away from a receptor, towards an effector.
• The outside of a neurone’s axon is more positive than inside the axon. As a result, when there is a higher concentration of sodium and potassium ions inside the axon, this causes the axon to be more positively charged than the outside of the axon.
• there is a higher conc of sodium ions outside the axon and potassium ions inside the axon

  
• The facilitated diffusion of sodium and potassium ions across the axon membrane is enabled by specific transport proteins called

  ion channels
• what is the difference in concentration and charge across the membrane called ?

  the electrochemical gradient

  because it is concentration gradient (−chemical) combined with an attraction as a result of charge (electro−).
• sodium-potassium pump

  moves two potassium ions into the axon, and three sodium ions out of the axon.

  co-transport and active transport

  carrier protein
• what does the sodium-potassium pump maintain?

  a more positive charge outside the axon, and a less positive charge inside the axon.
• what does a nerve impulse appear as?

  positive charge inside a axon
• what does the resting potential mean?

  the difference between the inside and outside of the axon right now and it is when:

  when a neurone maintains a less positive charge on the inside of the axon.

  It is maintained by a sodium-potassium pump.
• When a neurone is at ‘rest’, this means that that neurone…

  is not exposed to a stimulus but is able to respond to any nerve stimuli that arrive at its axon.
• At resting potential, the inside of the axon is less positively charged than the outside. Why is the establishment of resting potential important for neurones?

  It allows neurones to respond to any nerve impulses arriving at its axon.
• Explain how resting potential is established and maintained in a neurone. Refer to the movement of ions.

  The sodium-potassium pump is involved in the establishment of resting potential. At resting potential, the inside of the axon is less positively charged than the outside. The sodium-potassium pump actively transports 3 sodium ions out of the axon for every 2 potassium ions in, so overall, there is a net movement of positive ions out of the axon. There are also carrier proteins in the membrane that pump sodium ions out of the axon. The axon membrane is relatively impermeable to sodium ions, since gated sodium channels are closed. So there is very little diffusion of sodium ions back into the axon. Also, there is diffusion of potassium ions out of the axon via potassium channels down its electrochemical gradient. Overall, this maintains a higher concentration of sodium ions outside the axon, which maintains resting potential.
• what is a voltage-gated ion channel?

  an ion channel that opens in response to the positive charge of a/an nerve impulse.
• When a nerve impulse appears next to a voltage-gated ion channel, it causes the inside of the axon to become…

  more positively charged and it triggers the channel to open
• what is an action potential

  the state of a neurone in which the inside of axon reaches maximum positive charge
• what is an action potential triggered by?

  the positive charge of a nerve impulse arriving at the axon, which leads to the opening of voltage-gated sodium channels, thus allowing even more sodium

  ions to enter the axon.
• what are the 2 features an action potential is determined by?

  1. the threshold

  Enough sodium ions enter the axon → threshold of positive charge inside the axon compared to the outside of the axon is reached → action potential is triggered

  
  

  1. all-or-nothing principle

  Not enough sodium ions enter the axon → threshold of positive charge inside the axon compared to the outside of the axon is not reached → no action potential is triggered
• what are voltage-gated potassium channels involved in?

  hyperpolarisation
• what happens after an action potential has been reached and the axon re-establishes a resting potential?

  the voltage-gated sodium channels close

  the voltage-gated potassium channels open

  which causes the overall charge inside the axon to become less positive.
• what is hyperpolarisation?

  the state of a neurone in which the inside of an axon reaches its minimum positive charge.
• What transport protein is responsible for establishing a resting potential?

  Sodium-potassium pump
• Which transport protein(s) is/are responsible for establishing an action potential and hyperpolarisation

  action potential: Voltage-gated sodium channel

  hyperpolarisation: Voltage-gated potassium channel
• Explain how a neurone re-establishes resting potential after an action potential.

  The voltage-gated sodium channels close= reduces the number of Na+ diffusing into the axon.The voltage-gated potassium channels open = K+ to diffuse out of the axon = overall charge inside the axon = less +ve.The voltage-gated K+ channels remain open,= more K+ to diffuse out of the axon causing the overall charge inside the axon to become even less +ve than usual which is a state called hyperpolarisation.To bring the axon out of hyperpolarisation, Na+ diffuse into the axon through their regular sodium channels, and down their electrochemical gradient.The sodium-potassium pump restores the electrochemical gradients of Na+ & K+, by actively transporting 3 Na+ out of the axon while, at the same time, transporting K+ back into the axon.As a result, the inside of the axon is again less +ve than the outside of the axon, so resting potential has been restored.
• what is potential difference?

  The difference in charge between two locations
• how is potential difference measured in biology?

  Millivolts (mV)
• When the potential difference across the membrane of a neurone changes from positive to negative, it is referred to as…

  repolarisation
• What is a neurone?

  A specialised animal cell that passes on nerve impulses.